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Fetal ultrasound
Diqgnostic Radiology@ Volume 29
Number
Fetal
4 July/August
Ultrasound
Anne Kennedy, MD Department of Radiology University of Utah Medical Center Salt Lake City, Utah
h’il Mosby
2000
Foreword Probably no aspect of obstetrical practice has been more profoundly improved by the advent of ultrasonography than fetal monitoring. For the first time the fetus can be examined critically for maturation, anomalies, and complications of pregnancy with impunity. Fetal ultrasonography has become part of the standard practice of obstetrical care. Although an extensive literature exists on the subject, it seems appropriate to refocus on some of the practical aspects of the problem and the common clinical questions. Dr Kennedy has done this in masterful fashion. Theodore
112
E. Keats,
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Anne Kennedy, MD, is the Director of Women’s Imaging at the University of Utah Medical Center. Her medical school educationtook place at University of Dublin Trinity College, Dublin, Ireland. After this she spent 4 years in internal medicine in the United Kingdom and received the equivalent of board certification in that subspecialty.Her residency training in radiology was conducted at the Hammersmith Hospital and Royal PostgraduateMedical School in London, and she became a Fellow of the Royal College of Radiologists in 1989.Dr Kennedyjoined the University of Utah in June 1992.The combination of a clinical background and an interest in women’s health was a natural introduction to the subspecialty areaof women’s imaging.
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Fetal
Ultrasound
Ultrasound provides an opportunity to visualize the fetus and to assess its response to the intrauterine environment. The topic of fetal ultrasound is broad, and whole textbooks have been devoted to areas such as the diagnosis of anomalies. In an effort to provide current practical information, I have focused on the most common clinical questions asked when pregnant women are referred for obstetric ultrasound. We review the diagnoses of oligohydramnios, polyhydramnios, intrauterine growth retardation (IUGR), macrosomia, multiple pregnancy, and common fetal anomalies. Noninvasive assessment of fetal well-being is addressed, along with the use of sonography to refine risk assessment in patients with abnormal antenatal screening tests.
To make the broad subject of fetal ultrasound practically useful, I have focused on the most common causes of referral to the Obstetric Diagnostic Center at the University of Utah Medical Center. The most common clinical questions asked when pregnant women are referred to our clinic for obstetric ultrasound will be reviewed, and the differential diagnoses and appropriate imaging evaluation for each of these clinical problems will be discussed. The majority of so-called “routine” scans are for size and dates. This brings us to the issue of pregnancies where the clinical findings are at odds with the gestational age based on the mother’s last menstrual period. In covering pregnancies that are too big, we will review polyhydramnios, multiple pregnancy, and macrosomia; in evaluating pregnancies that are too small, we will discuss oligohydramnios, IUGR, and the fetus with anomalies, syndromes, chromosomal defects, and intrauterine infection. Another common indication for referral for detailed obstetric sonography is an abnormal maternal screening test. These tests may indicate an increased risk for fetal aneuploidy (abnormal triple screen) or structural abnormality such as neural tube defect (elevated maternal serum alpha-fetoprotein [AFP]). The sonographic features of trisomies and other common chromosomal abnormalities are outlined in Table I. The causes of elevated AFP are discussed, and examples are provided of neural tube and open abdominal wall defects. The increased use of infertility therapy has resulted doi:10.1067/mdr.2000.107681
114
in an increased number of multiple pregnancies. The risk of spontaneous twins becomes greater with maternal age; many women now delay childbirth until their 30s and multiple pregnancies are a common indication for early sonography and a more detailed assessment throughout the pregnancy. We will review types of twinning and potential complications in multiple pregnancies . The pregnant patient with abnormal bleeding or pain is a source of clinical concern. One of the main issues is differentiation between pregnancy-related causes of pain, such as preterm labor, or other causes. Also key in the evaluation of patients with pain and bleeding is assessment of the cervix and the placental location. Cervical incompetence is an important cause of second trimester pregnancy loss. The features of cervical incompetence will be reviewed even though patients with this condition do not present with pain. The typical presentation is progressive painless dilatation of the cervix, proceeding to second trimester miscarriage if unchecked. The last major topic of discussion will be fetal anomalies. The most common anomalies by body system will be reviewed with emphasis on the practical aspects of differentiating the various subtypes of anomalies and in guiding management of the pregnancy. Obstetric ultrasound is unique in the practice of radiology for several reasons. Our clinic works with the mother, the fetus, and the maternofetal unit. Therefore, multiple specialists may be involved in decision making that maximizes patient care and provides the best outcome for mother and child. We are fortunate at the
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University of Utah to have dedicated radiologists, obstetricians, perinatologists, perinatal geneticists, and pediatricians who work together to manage the pregnancy and ensure the safest possible delivery. We have a close working relationship with an adjacent children’s hospital where expert pediatric surgeons and pediatric cardiologists perform fetal echocardiography and then follow these children after delivery. This orchestrated team approach enables the parents of an abnormal fetus to make informed decisions regarding the management of the pregnancy and subsequent management of their child. Before the birth of an infant, parents also have the opportunity to meet and establish a relationship with the clinical teams that will be taking care of their child after delivery. For completeness, the basic images obtained in a routine antenatal ultrasound are illustrated in Fig 1. Table II describes the clinical utility of each of these images. Size and
for Dates
Common diagnoses in patients who appear large for dates are multiple pregnancy or incorrect dates. Multiple pregnancies are discussed separately later. It is our practice to re-date pregnancies if the sonographic age varies from the menstrual age by more than +lO days in the first 18 weeks. As discussed previously, dating is less accurate as pregnancy progresses, and in the term pregnancy where the mother has had no health care, the issue often becomes not the exact gestational age of the fetus but whether the fetus is content in the intrauterine environment. The clinically important causes of “large for dates” pregnancies include fetal macrosomia and polyhy-
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I Sonographic 21
Trisomy (137)
18
Trisomy ( 154)
13
Turner
findings
syndrome
(65)
Triploidy (50)
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Data from Sniiders defects. 1996.
Frontiers Numbers
in aneuploid
fetuses
26% CVS anomaly 28% Short femur 38% Nuchal thickening 30% Mild caliectasis Also look for clinodactyly and sandal toe 74% IUGR 52% CVS anomaly 47% Choroid plexus cyst 54% “Strawberry” head 54% Single umbilical artery 72% Abnormal extremities: clenched fist, crossed fingers, rocker bottom feet 61% IUGR 43% CVS anomaly 39% Holoprosencephaly 39% Facial cleft 80% Polydactyly 55% IUGR 48% CVS anomaly 80% Nonimmune hydrops 59% Short femur 88% Cystic hygroma 100% IUGR 60% Short femur 75% Syndactyly
Dates
The simple rule in dating a pregnancy is the earlier, the better. Accuracy is far greater in the first trimester; by the third trimester, estimates on gestational age vary by as much as k3 weeks. The clinical assessment of gestational age depends on the patient’s recollection of her last menstrual period and physical findings such as the fundal height. Depending on maternal body habitus, accurate measurement of fundal height can be difficult, and obviously conditions such as uterine fibroids will cause an apparent increase in fundal size that has nothing to do with the contained pregnancy. Differential diagnosis for a pregnancy that is large for dates is listed in Table III. The differential diagnosis for a pregnancy that is small for dates is listed in Table IV. Large
Table Trisomy (155)
RIM, Nicolaides in fetal medicine
in parentheses
KH. Ultrasound series.
London:
refer to number
markers
for fetal chromosomal
Parthenon
Publishing
Group;
of cases with the given diagnosis.
dramnios. If the estimated fetal weight is greater than the 90th percentile for gestational age, then the fetus is large for gestational age and at risk for macrosomia. We return here to the importance of accurate dating in the pregnancy, because the estimated fetal weight varies with gestational age. A fetus that appears to be at risk for macrosomia at 34 weeks may, in fact, be perfectly normal at 38 weeks of gestation. The estimated fetal weight (EFW) is, however, the best parameter for predicting macrosomia. The likelihood is 16% for an EFW <4 kg, 77% for an EFW >4 kg, and 86% for an EFW >4.5 kg? The macrosomic fetus is at risk for shoulder dystocia with vaginal delivery. Facial and brachial plexus injuries and clavicular and extremity fractures result from the mechanical problems of delivering a large fetus. The risk for perinatal asphyxia, meconium aspiration, and neonatal hypoglycemia is also increased. An important cause of fetal macrosomia is maternal diabetes. The pathophysiology is that fetal exposure to high levels of glucose in utero results in relatively high levels of fetal insulin, which in turn cause increased fat deposition in the trunk. Increasing chest and abdominal circumference measurements are seen from approximately 28 weeks of gestation. The biparietal diameter
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Table
II. Clinical
utility
BPD and HC*
Ventricular
atrium
Posterior
fossa
Spine
Four-chamber
Outflow
view
tracts
AC*
Bladder
Cord
insertion
Extremities
Kidneys
site
of images
* BPD and HC, Biporietal diameter
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obtained
in the fetal
survey
Fetal biometry. Also shows the presence of a falx, paired thalami and cavum Septum pellucidum, thus making hotoprosencephaly and agenesis of the corpus callosum unlikely. The shape of the head should be an oval. The cephalic index ” “lemon,” and “cloverleaf” is best apprecican be measured if required. Abnormal head shape such as “strawberry, ated on this view, and encephalocele may be present. For evaluation of mild ventriculomegaly or documentation of progressive hydrocephalus. In early pregnancy the choroid fills the ventricles, Choroid plexus cysts are easily identified in the echogenic choroid. Measurement is Constant throughout pregnancy at 10 mm. The average width is 6.1 f 1.3 mm. Ventriculomegaly is mild at 10 to 12 mm and frank at >13 mm. Displacement of the choroid by more than 3 mm from the medial ventricular Wall is a sign of ventriculomegaly known as the “dangling choroid.” On an axial section with the cerebellum and cavum septum pellucidum both visible, the cerebellar vermis is identified, the normal bilobed shape is appreciated, and the depth of the cisterna magna and the nuchal thickness is measured. The posterior fossa appearance is key in the differential diagnosis of hydrocephalus. If the spine is difficult to evaluate because of fetal position or maternal body habitus, the presence of a normal posterior fossa excludes over 95% of spine defects. Axial views show correct orientation of the posterior elements and confirm the presence of the lower lumbar segments at the level of the iliac blades in fetuses of diabetic mothers who are at risk for caudal regression. Coronal views show the normal tapering at the sacrum and gentle splaying of the canal in the cervical area and are the best view to appreciate scoliosis. Sagittal sections show intact skin over the length of the spine. Face “The face predicts the brain.” Detection of abnormal facies may help to narrow the differential diagnosis of brain anomalies. Also, antenatal diagnosis of facial clefting should prompt careful search for other structural anomalies that if present may prompt genetic amniocentesis. Look at cardiac axis as well as internal structure. Twenty-two percent to 32% of fetuses with a structural cardiac abnormality will be aneuploid. The incidence of aneuploidy increases to 66% in the presence of any other structural defect. The flap of the foramen ovale should be seen to extend into the left atrium. This view allows documentation of the echogenic intracardiac focus, which can be seen in up to 40% of fetuses with trisomy 13 and in 18% of those with trisomy 21. However, up to 5% of normal fetuses have an echogenic intracardiac focus in the left ventricle. Displacement of the cardiac axis is a concern and should prompt full fetal echocardiography, because there is a fetal mortality rate of 51% to 80% in the presence of an abnormal axis. Ectopia cordis is uniformly fatal. If the stomach and the heart are seen on the same image in a true axial section this implies the presence of a diaphragmatic hernia. Increases antenatal diagnosis of congenital heart disease up to 80% as compared with 30% to 60% with 4 chamber only. The pulmonary artery should always be anterior and to the left of the aortic root as the great vessels exit the heart. Fetal biometry and the gastric fundus. Failure to visualize the fundus in the presence of adequate volumes of amniotic fluid suggests that the fetus is unable to swallow or that there is esophageal atresia. In the latter case if a tracheoesophageal fistula is also present then amniotic fluid will reach the GI tract from the lungs, so visualizing the fluid filled stomach does not exclude this diagnosis. Two fluid-filled structures on this view are a clue to presence of duodenal atresia. Intra-abdomnal calcifications are associated with meconium peritonitis and fetal infection and if in the bowel lumen suggest fistula formation between the urinary tract and bowel. Ascites can be cardiac in association with hydrops and urinary secondary to bladder rupture in outlet obstruction, or secondary to bowel perforation. The bladder should be seen to fill and empty during the course of the examination. Absence of the bladder is a concern for kidney agenesis or dysfunction. If normal kidneys are present and the bladder is not seen, normal AFI would be unlikely in chronic hypoxia, and consideration should be given to the possibility of bladder or cloaca1 exstrophy. This anomaly is rare but may be diagnosed by the lack of a normal bladder and an increased soft tissue protuberance in the lower abdominal wall. The use of color Doppler confirms the presence of a three-vessel cord, as the vessels are easily seen adjacent to the bladder. A two-vessel cord is associated with increased risk for chromosomal abnormality, IUGR, and increased perinatal mortality. The cord should insert on the anterior abdominal wall and have intact skin on either side. Gastroschisis is a defect usually to the right of midline through which bowel loops herniate. It is usually not associated with chromosomal abnormalities. In omphalocele the cord inserts onto the apex of a membrane-encased defect that contains bowel with or without liver. Thirty-six percent of fetuses with omphalocele have a chromosomal abnormality. Document the presence of bilateral upper and lower extremities and measure femur length for biometry. Abnormal posturing is Ominous. The fetal limbs are held in flexion but should be seen to undergo flex-extend-flex motion, The hands should be seen to open intermittently and the relationship of the foot to the leg is evaluated to exclude clubfeet. The kidneys are Progressively easier to identify as pregnancy progresses because of increased perirenal fat deposition. Nomograms are available for normal size. The echogenicity, presence of cysts, and pelvicaliectasis are documented. Multiple syndromes and chromosomal disorders are associated with structurally abnormal kidneys. and head circumference;
AC,
abdominal
circumference.
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anatomy. A, Biparietal diameter. This image shows paired thalami (long arrow] and The falx cerebri is outlined by the short arrows. B, Posterior fossa. There is a normal bilobed mal cisterna magna (*) and no nuchal thickening. Note that the cavum septum pellucidum (long arrow) of section for measuremenf of nuchal thickness and cisterna magna. C, Normal longitudinal views of the cate intact skin. D, Normal longitudinal views of the spine in a coronal plane. Arrows indicate the iliac ible between them, excluding caudal regression.
FIG
1. Normal
(arrowhead).
(BPD) and head circumference (HC) grow normally. Serial fetal growth assessment is beneficial in management of the diabetic pregnancy. These women are at risk both for macrosomia and for IUGR; therefore an early scan to accurately document gestational age allows confident assessment of fetal growth and timely intervention for macrosomia or growth retardation. This is particularly important because the incidence of complications of macrosomia is greater in diabetic mothers than in the general population, with 3 1% of macrosomic fetuses of diabetic mothers having this complication, as opposed to 10% when the mother is not diabetic.2
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the normal cavum septum pellucidum cerebellum (short arrow) with a noris visible, indicating the correct plane spine in a sagittal plane. Arrows indiblades. Sacral spine elements are vis-
Polyhydramnios causes uterine enlargement, presenting clinically as “large for dates.” Polyhydramnios is characterized as mild, moderate, or severe on the basis of the depth of the single deepest pocket of amniotic fluid. A depth of 8 cm is mild, 8 to 12 cm is moderate, and greater than 12 cm is considered severe. The fetus can be large, normal-sized, or small. The combination of polyhydramnios and IUGR is ominous and should alert the sonographer to a possible diagnosis of trisomy 18. The combination of polyhydramnios and a large fetus is relatively more benign and may be caused by increased hydration in the larger fetus. The outcome in pregnancies with moderate hydramnios
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FIG 1. (Cont) E, Normal this with the appearance
transverse in Fig 14,
short arrow indicates the upper LA, Left atrium; IV, left ventricle;
section of the lumbar spine shown C. F, Fetal face. This view is used
lip, and the long RA, right atrium;
of the left ventricle in a cranial direction. to curve around the aortic root and then
118
I, Normal bifurcate.
with intact to exclude
skin (short facial and
arrow; palate
long arrow indicates clefting. The asterisks
arrow indicates the lower lip. G, Normal 4-chamber RV, right ventricle. H, Normal left ventricular outflow right
ventricular
outflow
tract.
The aortic
root
iliac wing). Contrast mark the nares, the
view. The fetal spine is indicated by S. tract shows the aorta (A) coursing out
is labeled
A; the pulmonary
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2000
FIG 1. (Cont) J, Cord insertion site. of the cord insertion site. K, Transverse (I). Both lower color Doppler
extremities at this level
The umbilical cord is seen to insert into the abdominal wall. Short arrows show intact skin on either section through the fetal pelvis shows the bladder (*). The iliac blade is identified as an echogenic
are present. The femurs (F) create an echogenic confirms the presence of two umbilical arteries
side line
interface within the lower extremity soft tissue. The application of that will be seen at either side of the bladder. 1, Normal fetal hand. The fingers fluid.
and a large fetus is relatively good.3 If polyhydramnios is severe, there is only a 46% perinatal fetal survival rate. This largely relates to the fact that severe polyhydramnios tends to be associated with major fetal anomalies. Mild polyhydramnios has a much better prognosis, with 81% postnatal fetal survival. Two thirds of mild cases are idiopathic in pathogenesis, and idiopathic polyhydramnios has no adverse effects on traditional
measures of perinatal outcome, although there is an increased incidence of macrosomia and delivery by cesarian section.4 Table V lists the differential diagnoses for polyhydramnios. The presence of polyhydramnios should initiate a careful search for fetal anomalies-in particular, abnormalities of the central nervous system (CNS) and gastrointestinal tracts, which would affect the fetus’s ability to swallow; cardiovascular system for
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do not overlap.
M,
The group of arrows show the fingers; the single arrow shows the thumb. nal OS is well visualized. Calipers mark the cervical length. AF, Amniotic
The maternal
bladder
is full. The inter-
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Table
III. Differential
diagnosis
Wrong dates Multiple pregnancy Polyhydramnios Macrosomia Fibroid Gestational trophoblastic Table
IV. Differential
of large
for dates
disease diagnosis
of small
for dates
Wrong dates Demise Oligohydramnios and all its causes IUGR and all its causes Table
V. Causes
of polyhydramnios
Maternal diabetes Hydrops with all its causes Anomalies Infection Idiopathic
evidence of hydrops; and neck and chest for the presence of soft tissue masses. Individual anomalies are reviewed in more detail later in the relevant section. Before proceeding to discuss the small-for-dates pregnancy, I would like to briefly review the evaluation of amniotic fluid volume. Amniotic fluid volume can be assessed subjectively; this approach requires considerable experience on the part of the examiner and is hard to document for serial assessment. The single vertical pocket technique relies on assessment of the vertical depth of the single deepest pocket of amniotic fluid. It is important that this pocket be free of fetal parts and umbilical cord. Color Doppler can be used to confirm that there is no cord within the pocket being measured. In our practice we use a deepest vertical pocket measurement less than 2 cm to define oligohydramnios, because this is the criterion used in the biophysical profile (BPP) scoring system. For serial assessment of amniotic fluid volume, we prefer to use the amniotic fluid index (AFI). For this measurement the maternal abdomen is divided into quadrants. The single deepest pocket that is free of fetal parts and cord in each quadrant is measured, the depths are added, and the sum is called the AFI. There is a range of normal depending on the gestational age. As a rule, an AFI less than 5 cm is considered oligohydramnios regardless of gestational age, and after 30 weeks the normal range is from 10 to 24 cm. Small for Dates The differential considerations for a pregnancy that is small for dates are listed in Table IV. The most common
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Table
VI. Causes
of oligohydramnios
Fetal demise Maternal drug use Kidney anomalies Intrauterine growth retardation Intrauterine infection Premature rupture of membranes Post dates pregnancy Placental insufficiency Syndromes Chromosomal abnormalities
Table
VII. Causes
of IUGR
Pregnancy-induced hypertension Essential hypertension Preeclamptic toxemia Diabetes mellitus Antiphospholipid syndrome Placental insufficiency Intrauterine infection Chromosomal abnormalities Syndromes Multiple pregnancy
cause is incorrect dates. In the first trimester, sonographic dating is extremely accurate. IUGR is always a concern, but in the general population, inaccuracy of menstrual history is more common than a fetus with intrauterine infection or chromosomal anomalies as the cause for profound early intrauterine growth restriction. Oligohydramnios is a clinically important cause of a “small for dates” uterus, because severe oligohydramnios has a profound impact on the outcome of the pregnancy. In the first trimester, amniotic fluid is largely produced by transudation from the fetal skin and by the placenta and membranes. The kidneys take over amniotic fluid production in the second trimester, and the fetal lungs also contribute to amniotic fluid volume with net movement of fluid out of the lungs. In the presence of severe oligohydramnios, the mother is questioned for a history of leaking amniotic fluid. Premature rupture of membranes occurs in 10% of pregnancies, and the outcome for the fetus depends on the severity of oligohydramnios.5 If there is no history to suggest premature rupture of membranes, the next issue is to demonstrate the presence of kidneys and filling and emptying of the fetal bladder. The fetal bladder should be seen to fill during the course of a normal examination, and the kidneys can usually be identified with careful scanning. Identification becomes easier as pregnancy progresses. However, in the case of lethal anomalies such as Potter’s syndrome, it is beneficial to the parents to establish the diagnosis to allow therapeutic abortion. The flat adrenal sign is useful in diagnosing renal agenesis (Fig 2). We have also found
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FIG 2. arrows. absence
Flat adrenal It is lying
A
0.00
B
0.00
I 5
15
!5
15
I
25
35
45
25
35
1 45
GCI
sign. The adrenal gland is outlined by the long flat against the fetal spine (S) because of the
of the kidney.
Note
the characteristic
appearance. The short arrows indicate rounded by the hypoechoic cortex.
the
“ice
cream
hyperechoic
sandwich” medulla
sur-I
it helpful to use color Doppler to identify renal arteries coming off the aorta. Image quality is impaired in fetuses with oligohydramnios, and it may be necessary to proceed to magnetic resonance imaging (MRI) to document absolutely the presence or absence of fetal kidneys.6 If fetal kidneys are identified, the bladder is seen to fill, and there is no history of premature rupture of membranes, then oligohydramnios tends to be most strongly associated with chronic asphyxia. Causes of oligohydramnios are listed in Table VI. Fetal demise is an obvious sonographic diagnosis; elucidation of maternal drug usage requires a careful history. Renal agenesis is not the only source of kidney-related oligohydramnios. The kidneys should be evaluated for size, echogenicity, and collecting system dilatation. Oligohydramnios can be associated with chromosomal abnormality or multiple anomalies. Sonographic assessment of fetal anatomy is difficult without the acoustic window normally provided by the amniotic fluid. The use of endovaginal (EV) sonography may provide additional information, especially in early pregnancy. It is also possible to perform amnioinfusion in which warmed normal saline solution is infused into the uterine cavity. This can be used as an acoustic window in addition to which withdrawal of some of the fluid can be used for chromosomal analysis and culture for infectious organisms. IUGR from any cause can be associated with oligohydramnios.7 This is an important observation, because 50% of fetuses with IUGR and oligohydramnios have serious long-term morbidity. The term fetus with oligohydramnios is also at risk. Amniotic fluid volume decreases by approximately one third per week after 40 weeks. Low amniotic fluid volumes correlate with acidosis, low Apgar scores, and poor
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WI
40.00 1 30.00
RC =“20
.cm 10.00
~ -I
C FIG
o-oo 3.
Fetal
51’5 growth
GR patterns.
A,
The X marks
indicate
the growth
of
a normal fetus. Superimposed on this, the black dots show the typical pattern in a small fetus that is growing normally. B, In this case the black dots show the pattern seen in the fetus that starts normally but fails to grow as expected. C, The black dots observed in the small fetus with poor interval abnormal
pattern
and
is often
associated
with
show the growth pattern growth. This is the most intrauterine
demise.
outcome. Therefore, in pregnancies that are at term or post-dates, the presence of oligohydramnios is important and may be a factor in the decision to induce labor.* One of the key issues in the fetus that appears small for dates is the determination of whether this is a small fetus that is faring well in the intrauterine environment or if it is a small fetus with problems. There are several different patterns of fetal “smallness.” One is a small fetus showing normal interval growth. This is the pattern associated with incorrect dating of the pregnancy or perhaps just a physiologically small fetus (Fig 3, A). Another is the fetus that starts out normally but fails to grow as expected. This is the pattern most associated with placental insufficiency (Fig 3, B). Last is the small fetus with poor or no interval growth. This is the most ominous pattern and the one most strongly associated with chromosomal abnormalities or multiple anomalies (Fig 3, C). The differential diagnosis of IUGR is listed in Table VII.
121
;
Y*c .*
FIG with
4. Normal the formula
1”
:
cord and intracranial Doppler. A, S/D is measured S-D/S, where S is the peak systolic velocity and D is
the end diastolic velocity. B, Intracranial circulation flow. The S/D of the cord dle cerebral artery.
The placental circulation is low resistance. in the fetus normally shows high resistance should
never
be greater
than
that of the mid-
Serial assessment of growth will differentiate between these three patterns. Interval scans are suggested at no more often than 3-week intervals. At follow-up, the usual measurements are obtained to document fetal growth. Amniotic fluid volume provides an independent assessment of fetal well-being, and Doppler studies of the umbilical cord and fetal cerebral circulation provide an insight into the fetal response to the intrauterine environment. Doppler of the umbilical vessels provides a noninvasive method of assessing resistance in the placental circulation. As a rule of thumb, in a fetus greater than 30 weeks of gestational age, the umbilical artery systolicto-diastolic ratio (S/D) should be less than 3. To improve accuracy, it is suggested that three measurements be acquired and an average obtained of the results.9 In the fetus suffering from hypoxia, blood flow is
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selectively diverted to the brain. Therefore, the normally high-resistance flow seen in the middle cerebral artery changes and becomes low resistance. The normal S/D ratio in the middle cerebral artery is greater than 4 with little diastolic flow. The middle cerebral S/D ratio should always be greater than the umbilical artery S/D ratios (Fig 4). When the balance changes and the umbilical artery S/D ratio is greater than that seen in the middle cerebral artery, this is an indication of fetal compromise. lo In the small fetus, cord Doppler does not diagnose IUGR but may predict outcome in fetuses suffering from it. In the presence of reversed end diastolic flow, there is a strong association with perinatal mortality, and delivery is strongly considered. 11 If the gestational age is such that the risks of preterm delivery are high, then the fetus is monitored extremely closely in utero with daily BPPs. A study by Mandruzzato in 199112 showed that patients with reversed diastolic flow all delivered fetuses less than the 5th percentile in weight for gestational age. There was a higher incidence of neonatal intensive care time, and 7 of 11 fetuses died in the perinatal period. The same group showed that absent end diastolic flow was associated with low birth rate, with 19 of 21 fetuses weighing less than the 5th percentile for gestational age at delivery. All deliveries were by cesarean section, but there were no perinatal deaths in the group with absent end diastolic flow. If there is a marked reduction in diastolic flow in the umbilical arteries, BPPs and non-stress tests (NSTs) are performed twice a week. More recently work has been directed at assessing the fetal venous circulation in an attempt to provide further evidence of fetal compromise. If placental vascular resistance is increased, higher pressures must be generated to maintain flow. Eventually increased myocardial work is required to maintain pressures; thus there is increased end diastolic pressure, causing tricuspid regurgitation. Tricuspid regurgitation results in a “kickback” with atria1 systole, which is manifested on Doppler assessment of the fetus and cord as flow reversal in the inferior vena cava and venous pulsations in the ductus venosus and umbilical veins.13 It must be emphasized, however, that management decisions should not be based on Doppler alone. The information from Doppler studies is only part of the picture, and other factors such as maternal blood pressure, fetal anomalies, and gestational age are always factored into the decision to deliver early.
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Biophysical
Profile
Table
After 25 weeks of gestation, when potential viability has been reached, performance of a BPP allows a noninvasive assessment of fetal well-being and can provide invaluable information to guide management of the pregnancy. Normal fetal movement is a powerful indicator that the fetal nervous system is intact and functioning normally. Absence of normal movement may be caused by primary nervous system anomalies or flexion contractures, but these are usually obvious sonographically. The other cause of impaired fetal movement is hypoxia. The fetus has variable patterns of activity during the day, and therefore it is essential that the BPP be conducted over an extended time period, up to 30 minutes if necessary. This is to compensate for periods of fetal inactivity. Four parameters are assessed sonographically. First is the volume of amniotic fluid. This is the most important variable, as will be shown later. Fetal breathing movements are evaluated by watching the fetal diaphragm. Gross body movements are assessed by watching for discrete episodes of motion; the score is normal if there are more than three episodes in the 30-minute scan time. Fetal tone is assessed by watching for active extension with return to flexion of the trunk, limbs, or hands. Each variable is given a score of 2 if present and 0 if absent. If the BPP is scored as 8 out of 8, then an NST or fetal cardiotocography is not required, as it adds no additional information. If the score if 6 out of 8, an NST is performed. If the NST is reactive, the total score is 8 out of 10. If the amniotic fluid is normal, a score of 8 out of 10 allows further close monitoring of the fetus. If the amniotic fluid is abnormal, a score of 8 out of 10 is an indication for delivery if the gestational age is greater than 36 weeks. A fetus scoring 6 out of 10 with normal fluid may have acute asphyxia; a decision to deliver in this situation will be based largely on gestational age. A score of 6 out of 10 where the fluid is low is an indication for delivery in a fetus greater than 26 weeks of gestation. This emphasizes the importance of the amniotic fluid volume in the overall assessment of fetal well-being. Perinatal mortality can be correlated with the BPP score. A score of 4 out of 10 corresponds to a perinatal mortality rate of 91 per 1000, 2 out of 10 is 125 per 1000, and 0 out of 10 is associated with a perinatal mortality rate of 600 per 1000 deliveries.14 Monitoring of the high-risk pregnancy is validated by studies correlating neonatal outcome with BPP score in pregnancy. Manning et al l5 have shown a significant
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VIII.
Causes
of abnormal
High
maternal
serum
AFP
Fetal demise Bleeding Open neural tube defect Open abdominal wall Congenital Finnish nephrosis Incorrect dates Multiple pregnancy Wrong dates Trisomy 21
Low
reduction in the cerebral palsy rate when pregnancies were managed according to the BPP score. Other studies strongly support the hypothesis that the perinatal mortality rate can be reduced by the application of the BPP scoring system to the high-risk population.16J7 It is interesting to note that an abnormal Doppler tracing may precede abnormalities in the non-stress test by up to 24 hours. There are also situations in which the BPP is abnormal but the cord Doppler is unremarkable. This occurs either in situations where the cord is abnormal, such as when it is short or there is a knot in the cord, or when there is a problem with the fetus impairing motion, such as a neurologic abnormality or viral infection of the fetus. It is known that progressive reduction of amniotic fluid volume in the post-dates pregnancy is associated with fetal acidosis, low Apgar scores, and poor outcome. Therefore monitoring of the post-dates pregnancy may be most simply performed by amniotic fluid evaluation and NSTs; however, if serial measurements of umbilical cord Doppler show a trend of increasing resistance, this may be an indication to deliver. Assessment of the fetal aortic waveforms may be more beneficial in this circumstance. Abnormal
Antenatal
Screening
When antenatal screening tests suggest increased risk for chromosomal or structural abnormality, a detailed sonographic assessment of the fetus provides additional information and can modify the risk based on the serum screening and maternal age. Maternal serum AFP can be elevated for a number of reasons (Table VIII). The most important of these are neural tube defects and anterior abdominal wall defects. The sonographic diagnosis of these conditions is reviewed in more detail in the anomaly section. Fetal demise and multiple pregnancies are obvious. In the absence of an obvious cause, it is often helpful to look for echogenic material in the amniotic fluid or to examine the placen-
123
Table
IX. Sonographic
Mild ventriculomegaly Echogenic intracardiac Single umbilical artery Echogenic bowel Choroid plexus cysts Short humerus/femur Nuchal thickening Fetal pyelectasis ~......
FIG 5. Incompetent cervix. The cervix is dilated, membranes (*), despite the application of both arrow) and Shirodkar (short arrow) sutures.
with funneling of McDonald (/ong
tal implantation site to look for clinically occult bleeding. The maternal serum AFP test is relatively low in specificity but when combined with ultrasound is very sensitive for neural tube defects. In cases of clinical concern, amniocentesis can be performed to assess the amniotic fluid acetyl cholinesterase level for open neural tube defects. In cases where the ultrasound result is considered to be structurally normal, the risk estimate on the basis of the AFP is revised before consideration for amniocentesis. The triple screen was developed in an effort to identify a group of pregnant women at increased risk for the presence of fetal trisomies, particularly trisomy 21. Maternal serum AFP measurements are correlated with measurements of human chorionic gonadotrophin and unconjugated estriol. Triple screen results, maternal age, and gestational age are used to calculate a risk for fetal trisomy. Ultrasound is performed to confirm that gestational age is accurate and to survey fetal anatomy. Detection of Down syndrome is improved over the detection rate when maternal age alone is used. Detection of Turner syndrome antenatally is approximately 50%, as compared with 8% detection on the basis of maternal age alone. Sixty percent to 80% of trisomy 18 cases are detected, with a false-positive rate of less than 1%. The study does not detect trisomy 13 or triploid fetuses. However, these conditions usually manifest sonographically early in pregnancy. Maternal triploidy typically causes early severe IUGR. Paternal triploidy results in
124
“soft
signs”
of fetal
aneuploidy
focus
_
-
a molar pregnancy, and trisomy 13 is associated with significant craniofacial anomalies and IUGR. Multiple non-specific signs have been described in association with chromosomal anomalies. These are usually referred to as “soft signs.” When multiple soft signs are present, the likelihood of a chromosomal abnormality is increased relative to the situation when only a single soft sign is identified. Table IX lists the sonographic soft signs for fetal chromosomal abnormality. Table I lists some instances of sonographic findings other than the soft signs in association with different chromosomal abnormalities. Pain and
Bleeding
In early pregnancy, the clinical issue when the patient presents with pain and bleeding is ectopic pregnancy versus threatened abortion. As pregnancy progresses, bleeding may be caused by placenta previa or placental abruption. Abruption is associated with pain, while placenta previa typically is not. There are multiple other causes of abdominal pain in the pregnant patient, but a discussion of this topic is beyond the scope of this article. There are several useful articles on the subject to which the interested reader is referred.18T19 The evaluation of preterm labor and cervical incompetence is included, because both conditions affect fetal outcome and because the assessment of the cervix is an integral part of any obstetric scan. Dilatation of the cervix in the first trimester generally heralds an inevitable abortion. In the second trimester, painless dilatation of the cervix is associated with the incompetent cervix (Fig 5). This is an important cause of second trimester pregnancy loss. In the first and second trimesters, the cervix can usually be seen transabdominally, either by using the maternal bladder as an acoustic window or by angling through the amniotic fluid from a position just inferior to the umbilicus. In the third trimester, as many as 30% of patients cannot be adequately assessed transabdominally, and either the EV approach or transperineal sonog-
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2000
raphy will be required. In patients with premature rupture of membranes, EV sonography is contraindicated because of the risk of infection. Transperineal sonography can still be performed in this clinical situation. Fig 1, M, shows the normal appearance of the cervix. On occasion, the external OSis obscured by rectal gas during transperineal sonography. If this is the case, it can be helpful to have the patient tilt her pelvis forward or place her hips on a cushion and repeat the study. EV sonography allows accurate assessment of the cervix. Many patients prefer EV to transabdominal sonography, and many patients prefer EV to the transperineal approach. Theoretic risks of the EV approach to cervical assessment include stimulation of contractions and preterm labor, precipitation of bleeding in patients with placenta previa, and introduction of infection in patients with premature rupture of membranes. The first two relative contraindications can be avoided with careful technique. Timor-Tritsch and Yunis showed quite elegantly that it is highly unlikely that the EV transducer can be inserted into the cervix because of the normal anatomic relationships of the cervix and vagina.20 The probe is only inserted 3 to 4 cm so that the cervix will be in the focal zone, and because the insertion is monitored by the sonographer, direct contact with the cervix is highly unlikely. The length of the normal cervix is greater than 3 cm. The range is considerable, with up to 5 cm being normal. It is important in patients in whom the cervix is being assessed that the bladder is not overfilled, as marked bladder distention can cause compression of the lower uterine segment and create a pseudo placenta previa (Fig 6). Normal effacement and shortening begin at approximately 30 weeks of gestation. On EV sonography, 25 mm is the 10th percentile for cervical length, a length of 15 mm corresponds to 50% effacement, and 10 mm corresponds to approximately 75% effacement. Sonography, however, can demonstrate funneling of the membranes into the internal OS.This is an early sign of cervical incompetence and may be seen sonographically before there are any detectable changes in the clinical examination of the cervix. The cervix is dynamic in pregnancy, and therefore progressive shortening is more important than a single short measurement. Also, funneling is of more significance if progressive or marked. It is considered abnormal for the membranes to protrude more than 3 mm beyond the internal OS, and it is thought that the risk of preterm labor is higher if there is more than 6 mm of funneling. Significant levels are not well defined, and
Curr Probl
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FIG 6. Pseudo placenta previa. A, When the maternal bladder is overfilled it can compress the lower uterine segment, causing an apparent low placental insertion site (arrow). B, After the bladder has been emptied it is clear that the placental margin (arrow) is well clear of the internal OS (*).
prospective clinical trials will be necessary to establish levels at which intervention is merited. It is advisable to measure the length and width of the funneled portion as well as the closed portion. While scanning, application of mild pressure to the fundus of the uterus acts effectively as a stress test for the cervix and may make funneling and dilatation apparent. The cervix should be assessed repeatedly during the study in patients at risk for preterm labor or cervical incompetence, and the reported measurements should be those at the time of maximum abnormality. A carefully conducted EV ultrasound has less risk of inducing premature rupture of membranes or stimulating labor than a speculum or digital examination.21 Preterm labor is defined as the onset of labor before 37 weeks of gestation. Fifty percent of patients with painful contractions in pregnancy are actually not in labor. Therefore, preterm labor is defined as the pres-
125
Table
X. Outcome
of twin
pregnancy
at 6 weeks
and 12 weeks
of gestation 12 weeks
6 weeks Probability
of:
Dichorionic
Monochorionic
With normal ultrasound Two live infants One live infant No live infants With abnormal ultrasound Two live infants One live infant No live infants Data from Benson CB, Doubilet
39.8% 30.4% 29.8%
75.8% 17.1% 7.1%
74.4% 0.5% 25.1%
95.8% 0.2% 4.3%
11.3% 45.4% 43.6%
37.4% 44.7% 17.9%
36% 1.3% 62.7%
81.4% 0.9% 17.7%
PM, David V. Prognosis
of first trimester
twin pregnancies:
ence of painful contraction in association with cervical changes. Multiple
Pregnancies
The incidence of multiple pregnancy is increasing in part because of increased success in the treatment of infertility. Twins are more common than higher-order multiple pregnancies. We will review the specific complications associated with twin pregnancies and discuss the determination of chorionicity. These principles can be applied to higher-order multiple pregnancies. Approximately 70% of twins are dizygotic; in this situation the physiology mimics that of two singleton pregnancies. The remaining 30% of pregnancies are monozygotic; these can then be subdivided on the basis of chorionicity and amnionicity. Approximately 30% are dichorionic, diamniotic; the majority are monochorionic, diamniotic; a small proportion (approximately 4% to 10%) are monochorionic, monoamniotic; and very rarely conjoined twins occur. The determination of chorionicity is important, because the outcome of monochorionic twins is poorer than that of dichorionic twins. This is because monochorionic twins are at risk for specific complications caused by the presence of vascular connections between the fetuses within the single placental mass. In dichorionic pregnancies or dizygotic pregnancies, the two placental masses may be fused, but because they arose from separate chorions, there are no vascular connections between them. Monochorionic twins are at risk for twin-twin transfusion syndrome, twin embolization syndrome, and acardiac parabiotic twinning. Monochorionic monoamniotic twins are at additional risk for cord entanglement, particularly after 34 weeks, and may also be locked at delivery. Conjoined twins are rare, comprising less than 1%
126
Dichorionic
Monochorionic
polychotomous
logistic
regression
analysis.
Radiology
1994;192:765-8.
of all twin pregnancies. The prognosis and outcome in conjoined twins largely depend on the degree of fusion and the presence of shared organs. Table X shows the likelihood of successful outcome in the different types of twin pregnancies. Determination
of Chorionicity
The best time to determine chorionicity is in early pregnancy. The earliest visible sign of intrauterine pregnancy is decidual reaction, called the double decidual sac sign, which is created by the echogenic chorionic tissue. In early pregnancy the chorionic sacs are counted. Then the number of amnions in each chorion is identified, plus the number of embryos. It may be difficult to resolve amnion in very early scans, and it is sometimes useful to count the yolk sacs, because the number of yolk sacs usually parallels the number of amnions (Fig 7). The membrane between fetuses will be thicker in a dichorionic, diamniotic pregnancy than in a monochorionic, diamniotic pregnancy. In the former there are four layers comprising the membrane; in the latter there are only two. In later pregnancy, if fetal sex can be identified and the fetuses are of different gender, then the pregnancy must be dizygotic, therefore dichorionic. The membrane can be evaluated for thickness and for the presence of a chorionic peak (Fig 8). If echogenic chorion extends into the peak leading to the intertwin membrane, this is evidence of dichorionic twinning, but its absence does not necessarily imply monochorionic twinning. If no membrane can be resolved between the fetuses, it can be useful to use color Doppler to evaluate the umbilical cords. If both cords can be followed to a common cord mass, it is likely that the cords are entangled, and this would be monochorionic, monoamniotic twinning (Fig 9). If there is difficulty in perception of a membrane, it
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2000
FIG
8.
membrane
FIG
7.
with
two
Early
monochorionic
embryos
and
twins.
two yolk
A single
sacs
(arrows].
gestation
sac
will
form, was sacs
of a diamniotic
twin
Diagn
Radiol,
July/August
Echogenic indicating
(*)
extends
a dichorionic
tissue
twin
into
the
intertwin
pregnancy.
pregnancy.
may be that there is a stuck twin, and this should be carefully evaluated. A stuck twin is usually smaller with oligohydramnios, and therefore the membrane separating the fetuses is closely applied to the smaller twin’s surface and not readily resolved (Fig 10). This is most frequently seen in monochorionic twins with twin-twin transfusion syndrome; however, a stuck twin can be seen in dichorionic twins. It may be useful to change maternal position during scanning, as the stuck twin may be held in an antigravitational position by the closely applied separating membrane. All twins are at increased risk for IUGR when compared with single pregnancies. The twin growth rate slows after 30 to 34 weeks of gestation, but in general, the singleton charts can be used to assess fetal growth. In very early pregnancy, if there is more than a 5-day difference between the crown-rump lengths, this is cause for concern regarding the possibility of major anomalies in the smaller twin, and close monitoring of the pregnancy will be required to evaluate for this. Discordant twin growth is defined as a 20% to 25% difference in birth weights between the twins. This is important, because the perinatal mortality rate in discordant twins is 2.5 times that of twins that grow in a concordant manner. Twin growth is best assessed by
Curr Probl
peak.
(arrows),
is present
from the chorion frondosum (CFJ. At this early stage a membrane not visualized between the embryos, but the presence of two yolk is suggestive
The placenta
Twin
2000
FIG 9. Monochorionic, monoamniotic twins. No membrane was identified, and the umbilical cords of both fetuses could be followed to this tangle of vessels (arrows). P, Placenta.
the use of the abdominal circumference measurement or the estimated fetal weight. An abdominal circumference difference of more than 20 mm between the twins is reason for concern, and if an estimated fetal weight difference of 15% is used as a criterion to suggest discordant growth, this will improve sensitivity. Some authorities suggest using the term discordant growth only if one twin meets the criteria for the diagnosis of IUGR. This is because the outcome is not as poor in twins with differing birth weights as long as the small twin weighs more than 2500 g. In the presence of differing growth rates between the fetuses, cord Doppler may be useful, because abnormal S/D ratios have a positive predictive value of 70% to 90% for discordant twin growth. Twin pregnancies with concern for discordant growth are carefully monitored with twice-weekly BPPs after viability.
127
cythemic, may be normal sized or can be macrosomic, and is often edematous. The donor twin has oligohydramnios secondary to hypovolemia and poor kidney perfusion; the recipient twin, on the other hand, has a hyperdynamic circulation, is often hypertensive, and has polyhydramnios. Because of the oligohydramnios, the small donor twin is often seen as a stuck twin. Cord Dopplers are abnormal in the donor. Absent end diastolic flow and reverse diastolic flow can occur in the donor twin but are not seen in the recipient. It is also useful to evaluate the site of cord insertion into the placenta. In a series of 100 cases of twin-twin transfusion at the University of Utah, there was velamentous insertion or marginal insertion of the donor cord in 98 patients, and the cord was peripheral on the smaller placental mass in the remaining 2 cases.22 The velamentous/marginal insertion may have some importance in the pathogenesis of twintwin transfusion syndrome. This can be demonstrated by color Doppler in up to 13 % of monochorionic twins.23 When identified, it may indicate the subset at greatest risk for development of twin-twin transfusion. Twin
FIG 10. Twin-twin transfusion syndrome. A, The arrows indicate the inter-twin membrane closely applied to the smaller donor fetus. The asterisk is in a small pool of amniotic fluid adjacent to the fetal hand (H). 6, (arrows)
The recipient twin is floating in ascites (*).
Twin-twin
Transfusion
hydropic,
with
central
bowel
loops
Syndrome
This abnormality occurs because of vascular connections between the two fetal circulations through the single placental mass; one twin is the donor, the other the recipient. This syndrome is seen in 5% to 30% of monochorionic twins, even though vascular anastomoses occur in 85% to 100%. The donor twin is small, hypovolemic, and anemic. The recipient twin is poly-
128
Embolization
Syndrome
In a dizygotic or dichorionic pregnancy, demise of one twin results in resorption of that fetus. There are usually no adverse sequelae for the remaining live twin. In monochorionic pregnancies, because of the vascular connections between the fetuses, vascular events occur in the surviving twin as a result of the death of the co-twin. The pathogenesis of these is uncertain, but the two most popular hypotheses are that there is embolization of material from the dead twin to the live twin, or that there is acute exsanguination of the live twin into the dead twin’s circulation. The results of this phenomenon are as would be expected from any physiology causing hypoperfusion, whether it is embolic or hypotensive. In the central nervous system, porencephaly, ventriculomegaly, cerebral atrophy, and microcephaly occur. Other systems are affected, including the gastrointestinal system, with liver and spleen infarcts and bowel atresias; the genitourinary system, with renal cortical necrosis; and the cardiorespiratory system, where lung infarcts may occur. A variety of other facial and extremity anomalies have been described in relation to this rare complication of monochorionic twinning.24
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Acardiac
Parabiotic
Sequence
This condition is also described as the twin reversed arterial perfusion sequence (TRAP). The normal placental circulation is for oxygenated blood from the placenta to travel via the umbilical vein to the heart, where it is shunted across the foramen ovale to the left atrium, left ventricle, and then to the aorta to preferentially supply the brain. Deoxygenated blood returns to the placenta via the umbilical arteries. In the TRAP sequence, there are artery-to-artery and vein-to-vein anastomoses within the placenta. This is rare, occurring in approximately 1% of monochorionic pregnancies. One twin returns deoxygenated blood to the placenta via the umbilical arteries. Artery-to-artery anastomoses occur within the placental mass, and this deoxygenated blood is shunted to the second twin. The second twin, therefore, receives poorly oxygenated blood from the placenta via the umbilical arteries; in other words, flow direction in the umbilical arteries is reversed. This blood, because it enters the fetus via the umbilical arterial circulation, perfuses the lower extremities selectively, and as a result there is marked limitation of growth in the upper half of the body. The lack of normal circulation impairs normal cardiac development, resulting in the acardiac feature. Poor perfusion of the upper half of the body results in lack of development of the head and upper extremities. This anomaly is commonly associated with a two-vessel cord.25 Conjoined
Anomalies
Whole textbooks have been devoted to antenatal detection and diagnosis of fetal anomalies. For the sake of
Curr
Probl
Diagn
section
at the the of
twin
11.
Thoracomphalopagus
1 is labeled
conjoined
twins.
S.
Twins
Conjoined twins are rare, occurring only in monoamniotic pregnancies, in 1 of 50,000 to 100,000 deliveries. The nomenclature depends on the site of fusion: thorucopagus denotes twins joined at the chest; omphalopagus denotes twins joined at the abdomen; thoracomphalopagus twins are joined at the chest and abdomen (Fig 11). Together these three categories comprise 70% of conjoined twins. Twins joined at the pelvis are referred to as ischiopagus, those joined at the sacrum are referred to as pygopagus, and those joined at the head are referred to as cmniopagus. The prognosis for conjoined twins depends on the extent of joining, the amount of shared organs, and the presence of associated anomalies.26 Monochorionic, monoamniotic twins are also at risk for cord accidents, because the cords may become entangled or knotted. Fetal
FIG
A, Cross
the level of the fetal chest shows the hearts (H) in continuity across midline. 6, Cross section at the level of the abdomen shows that twins are (oined from the chest through the abdomen. The stomach
Radiol,
July/August
2000
brevity we will cover just the most common anomalies by anatomic system and emphasize practical points for differentiating one from another. Central
Nervous
System
The most common anomalies seen within the central nervous system are choroid plexus cysts, hydrocephalus or lesions mimicking hydrocephalus, destructive processes, neural tube defects, and caudal regression. Choroid plexus cysts are associated with trisomy 18; therefore, visualization should prompt a careful fetal analysis for other structural defects (Fig 12). The fetus with trisomy 18 will usually have multiple anomalies, and it is our practice if choroid plexus cysts are seen as an isolated anomaly to not recommend genetic amniocentesis. The key things to observe in the presence of choroid plexus cysts to assure structural normality of the fetus are a normal heart, a normal facial profile without evidence of micrognathia, and a normal hand without evidence of overlapping fingers. The features associated with various chromosomal syndromes are outlined in Table I.
129
FIG 12. Trisomy 18. A, Transverse appearance caused by brachycephaly.
image through the thalami This is associated with
(T) at the level of the cavum septum pellucidum (*) shows the “strawberry” trisomy 18, as listed in Table I. 8, Transverse image through the ventricles
a choroid plexus cyst (C) and an interhemispheric arachnoid cyst (A). sistent flexion. Contrast this with the normal appearance in Fig 1, 1.
Hydrocephalus occurs at a rate of 0.5 to 3 per 1000 live births and is caused by increased formation of cerebrospinal fluid or more commonly by impaired drainage. The key to diagnosing the cause of hydrocephalus is the posterior fossa anatomy (Fig 1, B). If the posterior fossa is normal with a bilobed cerebellum and a normal cisterna magna, then the most likely diag-
130
C, The fetal
fingers
(arrows)
are overlapped
and
the fingers
held
head shows in per-
nosis is aqueductal stenosis (Fig 13). In the X-linked recessive form of this disease, the fetus very often has an abducted thumb, and this is something that may be useful to look for in terms of identifying cause. If the cerebellum is abnormal with a rounded configuration typically described as the “banana” cerebellum with small or absent cisterna magna, then the diagnosis is
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Arnold-Chiari malformation (Fig 14), the prognosis of which largely depends on the extent of the associated neural tube defect. If the posterior fossa shows an enlarged cisterna magna and absence of the cerebellar vermis, this is the Dandy-Walker syndrome (Fig 15). An important point in the diagnosis of the DandyWalker syndrome is that the cerebellum develops sequentially from superior to inferior. Inferior vermian agenesis may result in an apparently normal bilobed cerebellum with the superior portion of the vermis being seen on axial sections. Coronal imaging will help to identify fetuses with inferior vermian agenesis as a cause for the hydrocephalus. The normal vermis is seen to extend to the level of the bottom of the cerebellar lobes on coronal sections. Borderline ventriculomegaly is occasionally documented and can be associated with chromosomal anomalies such as trisomy 21. It may be a sign of very early hydrocephalus, agenesis of the corpus callosum, or a sequel to intrauterine infection. The measurement of the atria of the lateral ventricle is relatively constant throughout pregnancy. The upper limit of normal is measured as 10 mm, although the average measurement is closer to 6 mm. Subjectively, ventricular dilatation is assessed by the presence of a “dangling” choroid. If the choroid is displaced more than 3 mm from the medial ventricular wall, this is supportive of abnormal ventricular dilatation (Fig 13, A). Agenesis of the corpus callosum can be excluded if there is a normal cavum septum pellucidurn (Fig 1, A). In the fetus with mild ventriculomegaly, careful evaluation may reveal other “soft signs” of Down syndrome, which would prompt genetic amniocentesis. Fetuses suffering from intrauterine infection very often have IUGR and may have other features such as the presence of intracranial and intrahepatic calcifications:The presence of hepatosplenomegaly in the fetus with clinical suspicion for intrauterine infection suggests cytomegalovirus. Conditions that mimic hydrocephalus include holoprosencephaly and hydranencephaly. Hydranencephaly is thought to be secondary to a vascular insult, and in this condition the cerebral cortex is destroyed. There is usually a midline falx present. The facial development is normal, and typically the fetus is otherwise structurally normal. The vertebrobasilar circulation is intact and therefore the cerebellum and brain stem usually develop normally. Holoprosencephaly is a disorder of diverticulation of the fetal brain and is classified as lobar, semi-lobar, or alobar. In this condition there is a monoventricle.
Curr Probl
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2000
! 1 ’
FIG
13.
Aqueductal
stenosis.
A,
At 18 weeks
of gestation,
the atria
of
the lateral ventricle measured 9 mm (calipers). This is within the normal range, but there was a subjective impression of a dangling choroid (arrow). The posterior fossa was normal, excluding Arnold-Chiari and Dandy-Walker was normal.
malformation, and the remainder of the structural survey Because of the subjective ventriculomegaly, the patient was
asked to return hydrocephalus.
for a follow-up scan. 6, At follow-up Calipers demarcate the dilated third
gling choroid (arrow) is clearly (IV). The male fetus was noted with X-linked recessive aqueduct
there is now frank ventricle. The dan-
seen within the dilated lateral ventricle to have an abducted thumb consistent stenosis.
Therefore no falx is present and the thalami are fused (Fig 16). Holoprosencephaly is commonly associated with facial anomalies (Fig 17). The face predicts the brain but the brain does not predict the face; therefore the presence of an intracranial anomaly with normal
131
FIG 14. Arnold-Chiari malformation. to the reduction in intracranial pressure
A,
cerebellum
the brainstem
(arrow)
wrapped
around
this image with Fig 1, C, which shows diverging (arrow) and the meningocele meningocele sac (orrow).
Cross caused
section of the head shows the “lemon” by the spinal defect. Ventriculomegaly in the “banana”
the normal appearance. sac (*). D, Sagittal
configuration.
Tube
Defects
One of the most devastating of the neural tube defects is anencephaly. In this condition, the fetal brain does not develop and a small amount of material called angiomatous stroma is seen in the base of the skull. The skull base and part of the occiput develop in cartilage, but there is no cranial vault and no normal cerebral tissue. The characteristic appearance of this is illustrated in Fig 18. This anomaly is incompatible with life and occurs in approximately 1 out of 1000 pregnancies. An encephalocele is a skull defect with associated focal herniation of brain through the defect (Fig 19). These
132
Note
also
the obliteration
bones (arrow) that is due fossa view showing the
of the cisterna
C, Transverse image of the lower lumbar spine view of the lower lumbar spine shows intact skin
facies should not preclude the diagnosis of holoprosencephaly. Neural
head appearance of the frontal B, Posterior (*) IS also present. shows
the
(arrowheads)
magna.
Compare
posterior elements superior to the
defects are commonly occipital or nasal and are associated with certain syndromes such as Meckel-Gruber. The prognosis for the fetus depends on the presence of associated abnormalities and the extent of brain involvement. Spinal
Cord
Defects
The spectrum of abnormality runs from spina bifida occulta through meningocele, myelomeningocele, and myeloschisis to rachischisis. The normal coronal view of the spine shows smooth tapering of the interpedicular distance to a point at the sacrum and mild widening at the cervical end of the spine. The spine should be completely covered by skin; this is best visualized on sag&al section. Transverse sections of the spine show
Curr Probl
Diagn
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2000
FIG
15.
cephalus, magna
Dandy-Walker
malformation.
views of the posterior [CM) with cerebrospinal
In
this
fetus
with
hydro-
fossa show an enlarged cisterna fluid (arrow) extending to the brain-
stem because of the absence of the demarcate the cerebellar lobes.
cerebellar
vermis.
The
asterisks
FIG
16.
Holoprosencephaly.
fused thalami (T). The dorsal the absence of the falx.
Coronal
section
cyst comprises
of the fetal
head
the rest of the brain.
shows Note
the “teepee” appearance of the posterior elements converging. It is abnormal for the posterior elements to diverge (Fig 14). Spinal defects are commonly associated with the Arnold-Chiari malformation described in the section on CNS anomalies. Caudal
Regression
Caudal regression occurs in approximately 1 in 60,000 births and is described most commonly in the fetuses of diabetic mothers. The sequence consists of partial or complete sacral agenesis and anomalies or absence of parts of the lumbar spine. The spine can actually be absent below the level of TlO. As a consequence of the spinal anomalies, the lower extremities are often held in abnormal position and can be flexed, externally rotated, or fused. The syndrome is associated with a single umbilical artery and may be associated with agenesis of part of the urinary tract. The syndrome will be excluded if normal sagittal and coronal sections of the spine are seen. It is our practice in fetuses of diabetic mothers to obtain a cross section of the lower spine at the level of the iliac blades to confirm that sacral elements are present (Fig 1, D and E). Gastrointestinal
Anomalies
Gastrointestinal (GI) anomalies include bowel atresia, gastroschisis, omphalocele, and echogenic bowel.
Curr
Probl
Diagn
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2000
FIG One
17. Facial cleft. Oblique axial image looking up at the fetal orbit (0) and the outline of the nose (N) are seen. Arrows
cate the amniotic
cheeks, and the fluid extends.
asterisk
identifies
the facial
cleft
face. indi-
into which
Bowel atresias manifest with distended loops of bowel and polyhydramnids caused by inability of the fetus to swallow amniotic fluid. The best recognized is duode-
133
FIG (0). There
18. Anenecephaly. A, There is no skull vault 8, The cervical spine extends to normal occipital is no skull
vault
superior
above bones
the orbits (arrows).
to this.
nal atresia, which is important because of its association with trisomy 21. The characteristic appearance is the sonographic equivalent of the double-bubble described on plain films, the equivalent in the fetus being a fluid-filled stomach in continuity with a fluidfilled duodenum (Fig 20).
134
ing, it generally does not require further evaluation with amniocentesis. Echogenic bowel may, however, also be associated with cystic fibrosis, and evaluation of the neonate is merited in addition to genetic assessment of the parents. Gastroschisis is a defect in the anterior abdominal wall, usually to the right of midline. The umbilical cord inserts normally, and bowel loops hemiate through the defect, floating freely in amniotic fluid. This tends to be an isolated sporadic abnormality and is not associated with chromosomal syndromes. The other
Curr Probl
Diagn
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2000
_
FIG 21. Omphalocele. The umbilical cord (arrow) inserts on the apex of a mass (*) protruding from the abdominal wall. A membrane limits the mass. The skin of the lateral abdominal wall is seen clearly as an echogenic interface (arrowheads).
common anterior wall defect is omphalocele, in which there is defect in the abdominal wall covered by a membrane. The umbilical cord inserts on top of the membrane covering the protruding organs (Fig 21). Either liver, bowel, or liver and bowel can herniate through the defect. Bowel-only omphaloceles have a worse prognosis than those with bowel and liver, and omphalocele is associated with chromosomal disorders, particularly trisomy 18. Genitourinary
System
Anomalies
Genitourinary (GU) system anomalies primarily involve the kidneys and can be classified as abnormalities of kidney development versus obstructive processes. Bilateral renal agenesis is known as Potter’s syndrome. This is a uniformly fatal anomaly and presents in early pregnancy with profound oligohydramnios. Fetal visualization is impaired because of the absence of amniotic fluid, but indications include the flat adrenal sign, in which the adrenals are seen to lay along the spine in the renal bed. The adrenals have a typical “ice cream sandwich” appearance, with a hypoechoic outer component and echogenic inner component (Fig 2). This is different from the appearance of normal kidneys and should not be mistaken as normal renal parenchyma. Use of color Doppler can be helpful in determining whether there is flow into renal arteries from the abdominal aorta. In particularly difficult cases, amnio-infusion may help to facilitate adequate imaging, or MRI can be performed to confirm the absence of fetal kidneys.
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_..
._--_____
FIG 22. Fetal hydronephrosis. The fetal kidney is outlined by asterisks. The dilated calyces (Cj are shown in continuity with the pelvis (i’), confirming hydronephrosis. Simple pelvicaliectasis on a transverse scan is often of no significance if mild. Calyceal involvement is best demonstrated on coronal images such as this. Caliectasis implies more significant collecting system dilatation.
Unilateral renal agenesis is usually not associated with oligohydramnios and may prove to be a difficult antenatal diagnosis. Renal obstruction or renal collecting system dilatation presents as fetal hydronephrosis (Fig 22). Mild pelvicaliectasis is not uncommon, and in our facility we use the guidelines that a collecting system greater than 4 mm in anteroposterior (AP) diameter before 30 weeks should be reevaluated after 30 weeks. If after 30 weeks the collecting system AP diameter is 7 mm or less, no further evaluation is required. In those fetuses in which the AP diameter is greater than 7 mm, additional postnatal assessment is indicated, and it is essential that this be performed when the infant is at least 3 days old. Newborn infants are dehydrated, and therefore collecting system dilatation may be spuriously absent if the infant is scanned within the first 3 days of life. The most common causes of obstruction are ureteropelvic junction (UPJ) obstruction and distal ureteral obstruction secondary to ureterocele or ectopic implantation in the duplex kidney. In male fetuses, posterior urethral valves cause bladder distention and bilateral obstruction and may result in bladder rupture with urinary ascites. Cystic diseases include autosomal dominant polycystic kidney disease, autosomal recessive polycystic kidney disease, and multicystic dysplastic kidney syndrome. Autosomal recessive polycystic kidney disease presents as enlarged uniformly echogenic kidneys in which indi-
135
FIG 23. Multicystic dysplastic kidney. Coronal shows bilateral enlarged echogenic kidneys with cated
section multiple
of the fetus cysts demar-
by calipers.
vidual cysts are not resolved but rather the echogenicity is thought to be caused by multiple spectral reflectors from cyst walls. Autosomal dominant polycystic kidney disease is a rare antenatal diagnosis; if this is suspected, it is worth scanning the kidneys of the parents. Multicystic dysplastic kidney is thought to arise because of failure of the ureteric bud to unite with the metanephric blastema. Ureters are present, but they do not connect with the developing collecting system, and therefore tubule formation does not occur. These are typically large kidneys with multiple cysts and no identifiable renal parenchyma. If bilateral, the condition is fatal and is associated with profound oligohydramnios (Fig 23). If unilateral, the prognosis is better; however, multicystic dysplastic kidney is associated with contralateral UPJ obstruction, and therefore careful monitoring of the pregnancy and the child after delivery is indicated to preserve kidney function. Thoracic
and
Cardiovascular
Anomalies
There are three important differential diagnoses for an echogenic mass in the fetal chest. Cystic adenomatoid malformation presents as an intrathoracic mass. The appearance varies with the internal architecture. Type 1 has multiple large cysts of various sizes, type 2 has uniform cysts that are usually multiple and less than or equal to 2 cm in diameter, and type 3 has tiny cysts that are not resolved individually; therefore, this lesion appears as a solid parenchymal mass (Fig 24). All of these occur within the thorax, so the gastric fundus will be in a normal position, and with careful evaluation the cysts will not be confused with bowel loops. The blood supply is from the pulmonary artery; this feature may be best resolved with the use of power Doppler. Pulmonary sequestration is an uncommon developmental anomaly. It can be intra-lobar-that is, have
136
pulmonary venous drainage-or be extra-lobar-that is, have systemic venous drainage. Both types have a systemic arterial supply, and it is this feature that allows a confident diagnosis, because the feeding vessel can be seen arising from the aorta with careful color or power Doppler interrogation. Extralobar sequestration often has a triangular configuration, and up to 5% may be sub-diaphragmatic. Intralobar sequestration is more commonly round in shape, and all occur in the lower lobes (Fig 25). Congenital diaphragmatic hernia occurs in approximately 1 of 2000 to 1 of 3000 births. The prognosis for the fetus depends on the degree of pulmonary hypoplasia and the presence of associated anomalies. The diagnosis is usually obvious, because the gastric fundus will be seen on the same axial plane as the heart (Fig 26). Left-sided hernias are more common than rightsided, and the fetal heart is displaced to the right hemithorax by the herniated bowel. Coronal sections of the fetus also demonstrate displacement of the liver superiorly, and color Doppler will show that the echogenie “mass” contains portal and hepatic veins. The full scope of fetal echocardiography is beyond this article. The accuracy of prenatal cardiac diagnosis is poor (from 33% to 63%) when only the 4-chamber view is obtained. It is our practice to try to image the outflow tracts in all fetuses even though this is not included in the AIUM guidelines (Fig 1, G, H, and Z). By doing so the accuracy can be increased to as much as 85%.27,28 During assessment of the fetal heart the sonographer should attempt to answer the following six questions29: (1) Is the cardiac axis normal? (2) Is the heart size normal? (3) Are the ventricles symmetric? (4) Is there a septal defect? (5) Are the atrioventricular (AV) valves normally positioned? (6) Is there any abnormality in the endocardium, pericardium, or myocardium? Table XI lists the conditions associated with an increased risk of congenital heart disease. We routinely have formal fetal echocardiography performed in all fetuses of diabetic mothers and in all fetuses with a diaphragmatic hernia. Identifiable anomalies include endocardial cushion defect in association with trisomy 21, tetralogy of Fallot (Fig 27), double outlet right ventricle, hypoplastic left heart, and coarctation of the aorta. The sonographic features of these conditions are listed in Table XII. The issue of hydrops is important and not uncommon. Common causes are listed in Table XIII. Hydrops is present when there is fetal ascites, pleural effusion
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FIG 24. fetal thorax rior to the Doppler. above the
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Cystic adenomatoid malformation. A, Cross section of the shows an echogenic mass demarcated by calipers posteheart [H). No aortic feeding vessels were identified on color B, Sagittal section of the fetus shows that the mass (M) is diaphragm (arrow].
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with or without pericardial effusion, and skin thickening. By definition there must be fluid accumulation at more than one site. The most common cause used to be Rhesus iso-immunization; the routine use of Rhogam
137
FIG 27. Tetralogy septal defect (*),
0.4 4% 9% 12%
of Fallot.
The aorta
LV, Left ventricle;
(A) is overriding
a ventricular
RV, right ventricle.
to 0.8% of all pregnancies if one sibling affected if father affected if mother affected
increased in: Non-gestational diabetics Drugs: anticonvulsant, lithium, Alcohol Lupus Rubella Fetuses with other anomalies
retinoic
acid
makes this now the cause of less than 1% of cases. Hydrops can be categorized as immune or non-immune. The non-immune may be secondary to cardiac structural defects or arrythmia. Other structural abnormalities cause hydrops by diminishing venous return, by compressing the heart, or by causing high output failure. Fig 28 demonstrates a fetus with hydrops resulting from a cystic hygroma. The presence of hydrops should prompt a careful search for cardiac and other structural defects as well as assessment of the heart rate and rhythm. Isolated pleural fluid is somewhat less ominous than full-blown hydrops and may be the result of a chylothorax. Fluid within the peritoneal space may be cardisc ascites, but consideration must also be given to urinary ascites from bladder rupture or bowel perforation with meconium peritonitis. The presence of a distended bladder in a male fetus with abnormal kidneys strongly
138
I
FIG 28. Fetal hydrops in association with cystic hygroma. A, Sagittal section through the fetus shows subcutaneous edema (S), ascites [A), and pleural effusion (F). B, Transverse section through the fetal head shows a large septated fluid collection (arrows) surrounding the cranium.
suggests outlet obstruction resulting from posterior urethral valves. In a fetus with meconium peritonitis, consideration should be given to bowel atresia and cystic fibrosis. The latter is associated with echogenic bowel, and the former will often be associated with dilated loops of bowel and polyhydramnios. If there is calcified material within the lumen, this suggests a fistula between the urinary and GI tracts, because meconium will not calcify within normal bowel loops. This is most often seen with anal atresia.30 Skeletal
Anomalies
More than 200 skeletal dysplasias have been described, and the process of diagnosis is lengthy and complex. Plain films of the long bones, spine, and hands and feet are often required. In the antenatal setting the important issue is whether there is a lethal
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Table
XII. Sonographic
Atrioventricular
Coarctation
of congenital
defect
of Fallot (Fig 27)
outlet
Hypoplastic
right
ventricle
left heart
Transposition
of the great
Truncus
arteriosus
Ebsteins
anomaly
Data from EA
vessels
Stomm, JA Drose. The
heart
disease
Ventricular septal defect (ensure that the septum is evaluated with a perpendicular beam not just parallel). Atrial septum may be absent. AV valve abnormal. The tricuspid valve has three cusps, the mitral two. In a complete atrioventricular septal defect there is a single valve with bridging leaflets; in the partial type the valves are separated but abnormal. Large right ventricle, small left ventricle (RV:LV diameter ratio of 2:l is suggestive) Pulmonary artery:aorta diameter 22 SD above the mean (1.18 i: 0.06) Small aorta Small pulmonary artery secondary to atresia/stenosis Or large pulmonary artery secondary to absent pulmonary valve. Large aorta overriding ventricular septal defect. If associated with an open abdominal wall consider Pentalogy of Cantrell. If more than 50% of each of the great vessels arises from the right ventricle. Ventricular septal defect Related to maternal diabetes and alcohol abuse. Small left ventricle: use charts to compare ventricular size to that expected for the BPD to determine which are small and which are large. Hypoplastic or atretic mitral and aortic valves 80% associated with coarctation. Difficult antenatal diagnosis. Consider it if the great vessels are not seen to cross as they exit the heart. Single outflow vessel overrides the septum. Almost always associated with a ventricular septal defect. Large right atrium and small right ventricle secondary to apical displacement of the tricuspid valve. Maternal lithium exposure.
of the aorta
Tetralogy
Double
features
septal
fetal heart in diagnostic
ultrasound.
2nd ed. Rumack
skeletal dysplasia present. Timely antenatal diagnosis of these conditions allows the family the option of termination as well as providing information regarding the risk of recurrence in subsequent pregnancies. As a general rule, 50% of skeletal dysplasias affecting the fetus are lethal, and severe limb shortening is associated with a bad prognosis.31 In osteogenses imperfecta type 2, the bones are so poorly mineralized that they appear thick, because the un-ossified matrix does not reflect sound in the same way that normally mineralized bone does. The presence of multiple fractures causes angulation of long bones and “beading” of the ribs. The skull vault will be compressible with the application of mild transducer pressure. This disease manifests early and is excluded if the scan is normal after 17 weeks of gestational age. Congenital hypophosphatasia shows poor mineralization and angulation involving only the femur and tibia and is associated with hypoplasia of the fibula and scapula. Thanatophoric dysplasia manifests with severe rhizomelic limb shortening. It is associated with the “cloverleaf’ deformity of the skull, but this is seen in only about 14% of cases. Hydrocephalus, redundant skin, and polyhydramnios can be identified sonographically. The characteristic appearance of the spine with
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CM, Wilson
Table
XIII.
SR, Charbonneau
Causes
Immune: ABO and Rhesus Non-immune: Structural:
Placental/vascular:
Maternal
causes
of fetal
JW, editors.
St Louis: Mosby.
hydrops
alloimmunization Cystic hygroma Diaphragmatic hernia and other chest masses Cardiac defects and arrythmias Tumors: neuroblastoma, sacrococcygeal teratoma Chromosomal/genetic abnormalities Intrauterine infection Twin-twin transfusion Chorioangioma Vein of Galen aneurysm Severe diabetes Severe anemia
flattened vertebral bodies is best appreciated on film. We use fluoroscopy to position the mother so that the fetal spine is projected off the maternal spine in as close as possible to a lateral projection. Achondrogenesis causes the most severe limb shortening, mineralization is poor or absent, and the pubis and sacrum may be absent. It is associated with hydrops and polyhyramnios and is usually diagnosed by 19 weeks of gestation. Homozygous achondroplasia is lethal. This diagnosis
139
is easilv confirmed bv looking at the parents, at least one of whom will have to have heterozygous achondroplasia. Non-lethal but important skeletal findings include clubfoot, rocker bottom foot, overlapping fingers, and syndactyly and polydactyly. These features are rarely seen in isolation and when found should prompt careful evaluation because they are associated with syndromes and anomalies. For examnle. rocker bottom feet are seen in trisomy 18 and 13, syndactly is associated with Apert syndrome, and polydactyly is associated with Meckel-Gruber syndrome. d
d
A
Conclusion Ultrasound affords us a unique opportunity to evaluate fetal anatomy and to assess fetal well-being noninvasively. Rigorous attention to detail and dedicated experienced staff are required to perform high-quality antenatal sonography. An integrated team approach with open communication between sonologists, perinatologists, and pediatric specialists provides the best possible care for the family unit. When a fetus is abnormal, the multi-disciplinary approach allows time for development of a relationship between the parents and the health care professionals who will care for their child after delivery. I am fortunate to work in such an institution, and I encourage others to look beyond “turf battles” and strive to develop such a system, because it enhances both patient care and the educational process for all of the care providers involved. REFERENCES 1. Benson CB, Doubilet PM, Saltzmann DH. Sonographic determination of fetal weight in diabetic pregnancies. Am J Obstet Gynecol 1987;156:441-4. 2. Acker DP, Sachs BP, Friedman EA. Risk factors for shoulder dystocia. Obstet Gynecol 1985;66:762-8. 3. Sohaey R, Nyberg DA, Sickler GK. Idiopathic polyhydramnios: association with fetal macrosomia. Radiology 1994; 190:393-6. 4. Panting Kemp A, Nguyen T, Chang E, Quillen E, Castro L. Idiopathic polyhydramnios and perinatal outcome. Am J Obstet Gynecol 1999;181:1079-82. 5. Vintzileos AM, Campbell WA, Nochimson DJ, Weinbaum PJ. Degree of oligohydramnios and pregnancy outcome in patients with premature rupture of the membranes. Obstet Gynecol 1985;66: 162-7. 6. Huppert BJ, Brandt KR, Ramin KD, King BF..Single shot fast spin echo MR of the fetus: a pictorial essay. Radiographics 1999;19:S215-27. 7. Piepert JF, Donnenfeld AE. Oligohydramnios: a review. Obstet Gynecol Surv 1991;46:325-9. 8. Clement D, Schifirin BS, Kates RB. Acute oligohydramnios in post date pregnancy. Am J Obstet Gynecol 1987;157:884-6. 9. Grant EG. Maternal-fetal Doppler sonography: potential or reality? Semin Roentgen01 1991;26:75-86.
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10. Arbeille P, Body G, Saliba E. Fetal cerebral circulation assessment by Doppler ultrasound in normal and pathological pregnancies. Eur J Obstet Gynecol Reprod Biol 1998;29:261-73. Il. Karsdorp VH, van Vugt JM, van Geijn HP, et al. Clinical significance of absent or reversed end diastolic velocity waveforms in umbilical artery. Lancet 1994,344:1664-g. 12. Mandruzzato GP, Bogati P, Fischer L, Gigli C. The clinical significance of absent or reverse end-diastolic flow in the fetal aorta or umbilical artery. Ultrasound Obstet Gynecol 1991;1:192-6. 13. Mernagh JR, Mohide P, Cacoc F. Doppler ultrasound in obstetrics: a pictorial review. Scientific Exhibit. RSNA 1999. 14. Sohaey R. Amniotic fluid and fetal well-being. In: Zwiebel WJ, Sohaey R, editors. Introduction to ultrasound. 1st ed. Philadelphia: WB Saunders; 1998. p. 359-71. 15. Manning F, Harman C, Menticoglou S. Fetal biophysical profile score and cerebral palsy at age 3 years. Am J Obstet Gynecol 1996;174:319. 16. Chamberlain PF. Later fetal death: has ultrasound a role to play in its prevention? Irish J Med Sci 1991;161:251-4. 17. Baskett TF, Allen AC, Gray JH, et al. Fetal biophysical profile and perinatal death. Obstet Gynecol 1987;70:357-60. 18. Kennedy A. Assessment of acute abdominal pain in the pregnant patient. Semin Ultrasound CT MR 2000;21:64-77. 19. Boridy IC, Maklad N, Sandler CM. Suspected urolithiasis in pregnant women. Imaging algorithm and literature review. Am J Roentgen01 1996;167:869-75. 20. Timor-Tritsch IE, Yunis RA. Confirming the safety of transvaginal ultrasound in patients suspected of placenta previa. Obstet Gynecol 1993;81:742-4. 21. Hertzberg BS, Kliewer MA, Farrell TA, DeLong DM. Spontaneously changing gravid cervix. Clinical implications and prognostic features. Radiology 1995;196:721-4. 22. Ward K. Unpublished data. 23. Abrahim-Zadeh R, Coleman BG. The role of color Doppler in ultrasonography in obstetrics. Semin Roentgen01 1998; 33:351-9. 24. Patten RM, Mack LA, Nyberg DA, et al. Twin embolization syndrome: prenatal sonographic detection and significance. Radiology 1989;173:685-9. 25. Levi CS, Lyons EA, Mattel M, Dashefsky SM, Holt SC. Sonography in the diagnosis and management of multi-fetal pregnancy. In: Rumack C, Wilson SR, Charbonneau JW, editors. Diagnostic ultrasound. 2nd ed. St Louis: Mosby; 1998. p. 1043-66. 26. Filly RA, Goldstein RB, Callen PW. Monochorionic twinning: sonographic assessment. Am J Roentgen01 1990;154: 459-69. 27. Frates MC. Sonography of the normal fetal heart: a practical approach. Am J Roentgen01 1999;173:1363-70. 28. Bromley B, Estroff JA, Sanders SP. Fetal echocardiography: accuracy and limitations in a population at high and low risk for heart defects. Am J Obstet Gynecol 1992;166:1473-81. 29. McGahan Jl? Sonography of the fetal heart: findings on the 4 chamber view. Am J Roentgen01 1991;156:547-53. 30. Harris RD, Nyberg DA, Mack LA, et al. Anorectal atresia: prenatal sonographic diagnosis. Am J Roentgen01 1987;149: 395-400. 31. Sohaey R. Fetal musculoskeletal diagnosis. In: Zwiebel WJ, Sohaey R. Introduction to ultrasound. 1st ed. Philadelphia: WB Saunders; 1998. p. 476-89.
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Number
Fetal
4 July/August
Ultrasound
Anne Kennedy, MD Department of Radiology University of Utah Medical Center Salt Lake City, Utah
h’il Mosby
2000
Foreword Probably no aspect of obstetrical practice has been more profoundly improved by the advent of ultrasonography than fetal monitoring. For the first time the fetus can be examined critically for maturation, anomalies, and complications of pregnancy with impunity. Fetal ultrasonography has become part of the standard practice of obstetrical care. Although an extensive literature exists on the subject, it seems appropriate to refocus on some of the practical aspects of the problem and the common clinical questions. Dr Kennedy has done this in masterful fashion. Theodore
112
E. Keats,
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Anne Kennedy, MD, is the Director of Women’s Imaging at the University of Utah Medical Center. Her medical school educationtook place at University of Dublin Trinity College, Dublin, Ireland. After this she spent 4 years in internal medicine in the United Kingdom and received the equivalent of board certification in that subspecialty.Her residency training in radiology was conducted at the Hammersmith Hospital and Royal PostgraduateMedical School in London, and she became a Fellow of the Royal College of Radiologists in 1989.Dr Kennedyjoined the University of Utah in June 1992.The combination of a clinical background and an interest in women’s health was a natural introduction to the subspecialty areaof women’s imaging.
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113
Fetal
Ultrasound
Ultrasound provides an opportunity to visualize the fetus and to assess its response to the intrauterine environment. The topic of fetal ultrasound is broad, and whole textbooks have been devoted to areas such as the diagnosis of anomalies. In an effort to provide current practical information, I have focused on the most common clinical questions asked when pregnant women are referred for obstetric ultrasound. We review the diagnoses of oligohydramnios, polyhydramnios, intrauterine growth retardation (IUGR), macrosomia, multiple pregnancy, and common fetal anomalies. Noninvasive assessment of fetal well-being is addressed, along with the use of sonography to refine risk assessment in patients with abnormal antenatal screening tests.
To make the broad subject of fetal ultrasound practically useful, I have focused on the most common causes of referral to the Obstetric Diagnostic Center at the University of Utah Medical Center. The most common clinical questions asked when pregnant women are referred to our clinic for obstetric ultrasound will be reviewed, and the differential diagnoses and appropriate imaging evaluation for each of these clinical problems will be discussed. The majority of so-called “routine” scans are for size and dates. This brings us to the issue of pregnancies where the clinical findings are at odds with the gestational age based on the mother’s last menstrual period. In covering pregnancies that are too big, we will review polyhydramnios, multiple pregnancy, and macrosomia; in evaluating pregnancies that are too small, we will discuss oligohydramnios, IUGR, and the fetus with anomalies, syndromes, chromosomal defects, and intrauterine infection. Another common indication for referral for detailed obstetric sonography is an abnormal maternal screening test. These tests may indicate an increased risk for fetal aneuploidy (abnormal triple screen) or structural abnormality such as neural tube defect (elevated maternal serum alpha-fetoprotein [AFP]). The sonographic features of trisomies and other common chromosomal abnormalities are outlined in Table I. The causes of elevated AFP are discussed, and examples are provided of neural tube and open abdominal wall defects. The increased use of infertility therapy has resulted doi:10.1067/mdr.2000.107681
114
in an increased number of multiple pregnancies. The risk of spontaneous twins becomes greater with maternal age; many women now delay childbirth until their 30s and multiple pregnancies are a common indication for early sonography and a more detailed assessment throughout the pregnancy. We will review types of twinning and potential complications in multiple pregnancies . The pregnant patient with abnormal bleeding or pain is a source of clinical concern. One of the main issues is differentiation between pregnancy-related causes of pain, such as preterm labor, or other causes. Also key in the evaluation of patients with pain and bleeding is assessment of the cervix and the placental location. Cervical incompetence is an important cause of second trimester pregnancy loss. The features of cervical incompetence will be reviewed even though patients with this condition do not present with pain. The typical presentation is progressive painless dilatation of the cervix, proceeding to second trimester miscarriage if unchecked. The last major topic of discussion will be fetal anomalies. The most common anomalies by body system will be reviewed with emphasis on the practical aspects of differentiating the various subtypes of anomalies and in guiding management of the pregnancy. Obstetric ultrasound is unique in the practice of radiology for several reasons. Our clinic works with the mother, the fetus, and the maternofetal unit. Therefore, multiple specialists may be involved in decision making that maximizes patient care and provides the best outcome for mother and child. We are fortunate at the
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University of Utah to have dedicated radiologists, obstetricians, perinatologists, perinatal geneticists, and pediatricians who work together to manage the pregnancy and ensure the safest possible delivery. We have a close working relationship with an adjacent children’s hospital where expert pediatric surgeons and pediatric cardiologists perform fetal echocardiography and then follow these children after delivery. This orchestrated team approach enables the parents of an abnormal fetus to make informed decisions regarding the management of the pregnancy and subsequent management of their child. Before the birth of an infant, parents also have the opportunity to meet and establish a relationship with the clinical teams that will be taking care of their child after delivery. For completeness, the basic images obtained in a routine antenatal ultrasound are illustrated in Fig 1. Table II describes the clinical utility of each of these images. Size and
for Dates
Common diagnoses in patients who appear large for dates are multiple pregnancy or incorrect dates. Multiple pregnancies are discussed separately later. It is our practice to re-date pregnancies if the sonographic age varies from the menstrual age by more than +lO days in the first 18 weeks. As discussed previously, dating is less accurate as pregnancy progresses, and in the term pregnancy where the mother has had no health care, the issue often becomes not the exact gestational age of the fetus but whether the fetus is content in the intrauterine environment. The clinically important causes of “large for dates” pregnancies include fetal macrosomia and polyhy-
Curr Probl
I Sonographic 21
Trisomy (137)
18
Trisomy ( 154)
13
Turner
findings
syndrome
(65)
Triploidy (50)
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Data from Sniiders defects. 1996.
Frontiers Numbers
in aneuploid
fetuses
26% CVS anomaly 28% Short femur 38% Nuchal thickening 30% Mild caliectasis Also look for clinodactyly and sandal toe 74% IUGR 52% CVS anomaly 47% Choroid plexus cyst 54% “Strawberry” head 54% Single umbilical artery 72% Abnormal extremities: clenched fist, crossed fingers, rocker bottom feet 61% IUGR 43% CVS anomaly 39% Holoprosencephaly 39% Facial cleft 80% Polydactyly 55% IUGR 48% CVS anomaly 80% Nonimmune hydrops 59% Short femur 88% Cystic hygroma 100% IUGR 60% Short femur 75% Syndactyly
Dates
The simple rule in dating a pregnancy is the earlier, the better. Accuracy is far greater in the first trimester; by the third trimester, estimates on gestational age vary by as much as k3 weeks. The clinical assessment of gestational age depends on the patient’s recollection of her last menstrual period and physical findings such as the fundal height. Depending on maternal body habitus, accurate measurement of fundal height can be difficult, and obviously conditions such as uterine fibroids will cause an apparent increase in fundal size that has nothing to do with the contained pregnancy. Differential diagnosis for a pregnancy that is large for dates is listed in Table III. The differential diagnosis for a pregnancy that is small for dates is listed in Table IV. Large
Table Trisomy (155)
RIM, Nicolaides in fetal medicine
in parentheses
KH. Ultrasound series.
London:
refer to number
markers
for fetal chromosomal
Parthenon
Publishing
Group;
of cases with the given diagnosis.
dramnios. If the estimated fetal weight is greater than the 90th percentile for gestational age, then the fetus is large for gestational age and at risk for macrosomia. We return here to the importance of accurate dating in the pregnancy, because the estimated fetal weight varies with gestational age. A fetus that appears to be at risk for macrosomia at 34 weeks may, in fact, be perfectly normal at 38 weeks of gestation. The estimated fetal weight (EFW) is, however, the best parameter for predicting macrosomia. The likelihood is 16% for an EFW <4 kg, 77% for an EFW >4 kg, and 86% for an EFW >4.5 kg? The macrosomic fetus is at risk for shoulder dystocia with vaginal delivery. Facial and brachial plexus injuries and clavicular and extremity fractures result from the mechanical problems of delivering a large fetus. The risk for perinatal asphyxia, meconium aspiration, and neonatal hypoglycemia is also increased. An important cause of fetal macrosomia is maternal diabetes. The pathophysiology is that fetal exposure to high levels of glucose in utero results in relatively high levels of fetal insulin, which in turn cause increased fat deposition in the trunk. Increasing chest and abdominal circumference measurements are seen from approximately 28 weeks of gestation. The biparietal diameter
115
Table
II. Clinical
utility
BPD and HC*
Ventricular
atrium
Posterior
fossa
Spine
Four-chamber
Outflow
view
tracts
AC*
Bladder
Cord
insertion
Extremities
Kidneys
site
of images
* BPD and HC, Biporietal diameter
116
obtained
in the fetal
survey
Fetal biometry. Also shows the presence of a falx, paired thalami and cavum Septum pellucidum, thus making hotoprosencephaly and agenesis of the corpus callosum unlikely. The shape of the head should be an oval. The cephalic index ” “lemon,” and “cloverleaf” is best apprecican be measured if required. Abnormal head shape such as “strawberry, ated on this view, and encephalocele may be present. For evaluation of mild ventriculomegaly or documentation of progressive hydrocephalus. In early pregnancy the choroid fills the ventricles, Choroid plexus cysts are easily identified in the echogenic choroid. Measurement is Constant throughout pregnancy at 10 mm. The average width is 6.1 f 1.3 mm. Ventriculomegaly is mild at 10 to 12 mm and frank at >13 mm. Displacement of the choroid by more than 3 mm from the medial ventricular Wall is a sign of ventriculomegaly known as the “dangling choroid.” On an axial section with the cerebellum and cavum septum pellucidum both visible, the cerebellar vermis is identified, the normal bilobed shape is appreciated, and the depth of the cisterna magna and the nuchal thickness is measured. The posterior fossa appearance is key in the differential diagnosis of hydrocephalus. If the spine is difficult to evaluate because of fetal position or maternal body habitus, the presence of a normal posterior fossa excludes over 95% of spine defects. Axial views show correct orientation of the posterior elements and confirm the presence of the lower lumbar segments at the level of the iliac blades in fetuses of diabetic mothers who are at risk for caudal regression. Coronal views show the normal tapering at the sacrum and gentle splaying of the canal in the cervical area and are the best view to appreciate scoliosis. Sagittal sections show intact skin over the length of the spine. Face “The face predicts the brain.” Detection of abnormal facies may help to narrow the differential diagnosis of brain anomalies. Also, antenatal diagnosis of facial clefting should prompt careful search for other structural anomalies that if present may prompt genetic amniocentesis. Look at cardiac axis as well as internal structure. Twenty-two percent to 32% of fetuses with a structural cardiac abnormality will be aneuploid. The incidence of aneuploidy increases to 66% in the presence of any other structural defect. The flap of the foramen ovale should be seen to extend into the left atrium. This view allows documentation of the echogenic intracardiac focus, which can be seen in up to 40% of fetuses with trisomy 13 and in 18% of those with trisomy 21. However, up to 5% of normal fetuses have an echogenic intracardiac focus in the left ventricle. Displacement of the cardiac axis is a concern and should prompt full fetal echocardiography, because there is a fetal mortality rate of 51% to 80% in the presence of an abnormal axis. Ectopia cordis is uniformly fatal. If the stomach and the heart are seen on the same image in a true axial section this implies the presence of a diaphragmatic hernia. Increases antenatal diagnosis of congenital heart disease up to 80% as compared with 30% to 60% with 4 chamber only. The pulmonary artery should always be anterior and to the left of the aortic root as the great vessels exit the heart. Fetal biometry and the gastric fundus. Failure to visualize the fundus in the presence of adequate volumes of amniotic fluid suggests that the fetus is unable to swallow or that there is esophageal atresia. In the latter case if a tracheoesophageal fistula is also present then amniotic fluid will reach the GI tract from the lungs, so visualizing the fluid filled stomach does not exclude this diagnosis. Two fluid-filled structures on this view are a clue to presence of duodenal atresia. Intra-abdomnal calcifications are associated with meconium peritonitis and fetal infection and if in the bowel lumen suggest fistula formation between the urinary tract and bowel. Ascites can be cardiac in association with hydrops and urinary secondary to bladder rupture in outlet obstruction, or secondary to bowel perforation. The bladder should be seen to fill and empty during the course of the examination. Absence of the bladder is a concern for kidney agenesis or dysfunction. If normal kidneys are present and the bladder is not seen, normal AFI would be unlikely in chronic hypoxia, and consideration should be given to the possibility of bladder or cloaca1 exstrophy. This anomaly is rare but may be diagnosed by the lack of a normal bladder and an increased soft tissue protuberance in the lower abdominal wall. The use of color Doppler confirms the presence of a three-vessel cord, as the vessels are easily seen adjacent to the bladder. A two-vessel cord is associated with increased risk for chromosomal abnormality, IUGR, and increased perinatal mortality. The cord should insert on the anterior abdominal wall and have intact skin on either side. Gastroschisis is a defect usually to the right of midline through which bowel loops herniate. It is usually not associated with chromosomal abnormalities. In omphalocele the cord inserts onto the apex of a membrane-encased defect that contains bowel with or without liver. Thirty-six percent of fetuses with omphalocele have a chromosomal abnormality. Document the presence of bilateral upper and lower extremities and measure femur length for biometry. Abnormal posturing is Ominous. The fetal limbs are held in flexion but should be seen to undergo flex-extend-flex motion, The hands should be seen to open intermittently and the relationship of the foot to the leg is evaluated to exclude clubfeet. The kidneys are Progressively easier to identify as pregnancy progresses because of increased perirenal fat deposition. Nomograms are available for normal size. The echogenicity, presence of cysts, and pelvicaliectasis are documented. Multiple syndromes and chromosomal disorders are associated with structurally abnormal kidneys. and head circumference;
AC,
abdominal
circumference.
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anatomy. A, Biparietal diameter. This image shows paired thalami (long arrow] and The falx cerebri is outlined by the short arrows. B, Posterior fossa. There is a normal bilobed mal cisterna magna (*) and no nuchal thickening. Note that the cavum septum pellucidum (long arrow) of section for measuremenf of nuchal thickness and cisterna magna. C, Normal longitudinal views of the cate intact skin. D, Normal longitudinal views of the spine in a coronal plane. Arrows indicate the iliac ible between them, excluding caudal regression.
FIG
1. Normal
(arrowhead).
(BPD) and head circumference (HC) grow normally. Serial fetal growth assessment is beneficial in management of the diabetic pregnancy. These women are at risk both for macrosomia and for IUGR; therefore an early scan to accurately document gestational age allows confident assessment of fetal growth and timely intervention for macrosomia or growth retardation. This is particularly important because the incidence of complications of macrosomia is greater in diabetic mothers than in the general population, with 3 1% of macrosomic fetuses of diabetic mothers having this complication, as opposed to 10% when the mother is not diabetic.2
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the normal cavum septum pellucidum cerebellum (short arrow) with a noris visible, indicating the correct plane spine in a sagittal plane. Arrows indiblades. Sacral spine elements are vis-
Polyhydramnios causes uterine enlargement, presenting clinically as “large for dates.” Polyhydramnios is characterized as mild, moderate, or severe on the basis of the depth of the single deepest pocket of amniotic fluid. A depth of 8 cm is mild, 8 to 12 cm is moderate, and greater than 12 cm is considered severe. The fetus can be large, normal-sized, or small. The combination of polyhydramnios and IUGR is ominous and should alert the sonographer to a possible diagnosis of trisomy 18. The combination of polyhydramnios and a large fetus is relatively more benign and may be caused by increased hydration in the larger fetus. The outcome in pregnancies with moderate hydramnios
117
FIG 1. (Cont) E, Normal this with the appearance
transverse in Fig 14,
short arrow indicates the upper LA, Left atrium; IV, left ventricle;
section of the lumbar spine shown C. F, Fetal face. This view is used
lip, and the long RA, right atrium;
of the left ventricle in a cranial direction. to curve around the aortic root and then
118
I, Normal bifurcate.
with intact to exclude
skin (short facial and
arrow; palate
long arrow indicates clefting. The asterisks
arrow indicates the lower lip. G, Normal 4-chamber RV, right ventricle. H, Normal left ventricular outflow right
ventricular
outflow
tract.
The aortic
root
iliac wing). Contrast mark the nares, the
view. The fetal spine is indicated by S. tract shows the aorta (A) coursing out
is labeled
A; the pulmonary
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July/August
(p) is seen
2000
FIG 1. (Cont) J, Cord insertion site. of the cord insertion site. K, Transverse (I). Both lower color Doppler
extremities at this level
The umbilical cord is seen to insert into the abdominal wall. Short arrows show intact skin on either section through the fetal pelvis shows the bladder (*). The iliac blade is identified as an echogenic
are present. The femurs (F) create an echogenic confirms the presence of two umbilical arteries
side line
interface within the lower extremity soft tissue. The application of that will be seen at either side of the bladder. 1, Normal fetal hand. The fingers fluid.
and a large fetus is relatively good.3 If polyhydramnios is severe, there is only a 46% perinatal fetal survival rate. This largely relates to the fact that severe polyhydramnios tends to be associated with major fetal anomalies. Mild polyhydramnios has a much better prognosis, with 81% postnatal fetal survival. Two thirds of mild cases are idiopathic in pathogenesis, and idiopathic polyhydramnios has no adverse effects on traditional
measures of perinatal outcome, although there is an increased incidence of macrosomia and delivery by cesarian section.4 Table V lists the differential diagnoses for polyhydramnios. The presence of polyhydramnios should initiate a careful search for fetal anomalies-in particular, abnormalities of the central nervous system (CNS) and gastrointestinal tracts, which would affect the fetus’s ability to swallow; cardiovascular system for
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2000
do not overlap.
M,
The group of arrows show the fingers; the single arrow shows the thumb. nal OS is well visualized. Calipers mark the cervical length. AF, Amniotic
The maternal
bladder
is full. The inter-
119
Table
III. Differential
diagnosis
Wrong dates Multiple pregnancy Polyhydramnios Macrosomia Fibroid Gestational trophoblastic Table
IV. Differential
of large
for dates
disease diagnosis
of small
for dates
Wrong dates Demise Oligohydramnios and all its causes IUGR and all its causes Table
V. Causes
of polyhydramnios
Maternal diabetes Hydrops with all its causes Anomalies Infection Idiopathic
evidence of hydrops; and neck and chest for the presence of soft tissue masses. Individual anomalies are reviewed in more detail later in the relevant section. Before proceeding to discuss the small-for-dates pregnancy, I would like to briefly review the evaluation of amniotic fluid volume. Amniotic fluid volume can be assessed subjectively; this approach requires considerable experience on the part of the examiner and is hard to document for serial assessment. The single vertical pocket technique relies on assessment of the vertical depth of the single deepest pocket of amniotic fluid. It is important that this pocket be free of fetal parts and umbilical cord. Color Doppler can be used to confirm that there is no cord within the pocket being measured. In our practice we use a deepest vertical pocket measurement less than 2 cm to define oligohydramnios, because this is the criterion used in the biophysical profile (BPP) scoring system. For serial assessment of amniotic fluid volume, we prefer to use the amniotic fluid index (AFI). For this measurement the maternal abdomen is divided into quadrants. The single deepest pocket that is free of fetal parts and cord in each quadrant is measured, the depths are added, and the sum is called the AFI. There is a range of normal depending on the gestational age. As a rule, an AFI less than 5 cm is considered oligohydramnios regardless of gestational age, and after 30 weeks the normal range is from 10 to 24 cm. Small for Dates The differential considerations for a pregnancy that is small for dates are listed in Table IV. The most common
120
Table
VI. Causes
of oligohydramnios
Fetal demise Maternal drug use Kidney anomalies Intrauterine growth retardation Intrauterine infection Premature rupture of membranes Post dates pregnancy Placental insufficiency Syndromes Chromosomal abnormalities
Table
VII. Causes
of IUGR
Pregnancy-induced hypertension Essential hypertension Preeclamptic toxemia Diabetes mellitus Antiphospholipid syndrome Placental insufficiency Intrauterine infection Chromosomal abnormalities Syndromes Multiple pregnancy
cause is incorrect dates. In the first trimester, sonographic dating is extremely accurate. IUGR is always a concern, but in the general population, inaccuracy of menstrual history is more common than a fetus with intrauterine infection or chromosomal anomalies as the cause for profound early intrauterine growth restriction. Oligohydramnios is a clinically important cause of a “small for dates” uterus, because severe oligohydramnios has a profound impact on the outcome of the pregnancy. In the first trimester, amniotic fluid is largely produced by transudation from the fetal skin and by the placenta and membranes. The kidneys take over amniotic fluid production in the second trimester, and the fetal lungs also contribute to amniotic fluid volume with net movement of fluid out of the lungs. In the presence of severe oligohydramnios, the mother is questioned for a history of leaking amniotic fluid. Premature rupture of membranes occurs in 10% of pregnancies, and the outcome for the fetus depends on the severity of oligohydramnios.5 If there is no history to suggest premature rupture of membranes, the next issue is to demonstrate the presence of kidneys and filling and emptying of the fetal bladder. The fetal bladder should be seen to fill during the course of a normal examination, and the kidneys can usually be identified with careful scanning. Identification becomes easier as pregnancy progresses. However, in the case of lethal anomalies such as Potter’s syndrome, it is beneficial to the parents to establish the diagnosis to allow therapeutic abortion. The flat adrenal sign is useful in diagnosing renal agenesis (Fig 2). We have also found
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2000
FIG 2. arrows. absence
Flat adrenal It is lying
A
0.00
B
0.00
I 5
15
!5
15
I
25
35
45
25
35
1 45
GCI
sign. The adrenal gland is outlined by the long flat against the fetal spine (S) because of the
of the kidney.
Note
the characteristic
appearance. The short arrows indicate rounded by the hypoechoic cortex.
the
“ice
cream
hyperechoic
sandwich” medulla
sur-I
it helpful to use color Doppler to identify renal arteries coming off the aorta. Image quality is impaired in fetuses with oligohydramnios, and it may be necessary to proceed to magnetic resonance imaging (MRI) to document absolutely the presence or absence of fetal kidneys.6 If fetal kidneys are identified, the bladder is seen to fill, and there is no history of premature rupture of membranes, then oligohydramnios tends to be most strongly associated with chronic asphyxia. Causes of oligohydramnios are listed in Table VI. Fetal demise is an obvious sonographic diagnosis; elucidation of maternal drug usage requires a careful history. Renal agenesis is not the only source of kidney-related oligohydramnios. The kidneys should be evaluated for size, echogenicity, and collecting system dilatation. Oligohydramnios can be associated with chromosomal abnormality or multiple anomalies. Sonographic assessment of fetal anatomy is difficult without the acoustic window normally provided by the amniotic fluid. The use of endovaginal (EV) sonography may provide additional information, especially in early pregnancy. It is also possible to perform amnioinfusion in which warmed normal saline solution is infused into the uterine cavity. This can be used as an acoustic window in addition to which withdrawal of some of the fluid can be used for chromosomal analysis and culture for infectious organisms. IUGR from any cause can be associated with oligohydramnios.7 This is an important observation, because 50% of fetuses with IUGR and oligohydramnios have serious long-term morbidity. The term fetus with oligohydramnios is also at risk. Amniotic fluid volume decreases by approximately one third per week after 40 weeks. Low amniotic fluid volumes correlate with acidosis, low Apgar scores, and poor
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WI
40.00 1 30.00
RC =“20
.cm 10.00
~ -I
C FIG
o-oo 3.
Fetal
51’5 growth
GR patterns.
A,
The X marks
indicate
the growth
of
a normal fetus. Superimposed on this, the black dots show the typical pattern in a small fetus that is growing normally. B, In this case the black dots show the pattern seen in the fetus that starts normally but fails to grow as expected. C, The black dots observed in the small fetus with poor interval abnormal
pattern
and
is often
associated
with
show the growth pattern growth. This is the most intrauterine
demise.
outcome. Therefore, in pregnancies that are at term or post-dates, the presence of oligohydramnios is important and may be a factor in the decision to induce labor.* One of the key issues in the fetus that appears small for dates is the determination of whether this is a small fetus that is faring well in the intrauterine environment or if it is a small fetus with problems. There are several different patterns of fetal “smallness.” One is a small fetus showing normal interval growth. This is the pattern associated with incorrect dating of the pregnancy or perhaps just a physiologically small fetus (Fig 3, A). Another is the fetus that starts out normally but fails to grow as expected. This is the pattern most associated with placental insufficiency (Fig 3, B). Last is the small fetus with poor or no interval growth. This is the most ominous pattern and the one most strongly associated with chromosomal abnormalities or multiple anomalies (Fig 3, C). The differential diagnosis of IUGR is listed in Table VII.
121
;
Y*c .*
FIG with
4. Normal the formula
1”
:
cord and intracranial Doppler. A, S/D is measured S-D/S, where S is the peak systolic velocity and D is
the end diastolic velocity. B, Intracranial circulation flow. The S/D of the cord dle cerebral artery.
The placental circulation is low resistance. in the fetus normally shows high resistance should
never
be greater
than
that of the mid-
Serial assessment of growth will differentiate between these three patterns. Interval scans are suggested at no more often than 3-week intervals. At follow-up, the usual measurements are obtained to document fetal growth. Amniotic fluid volume provides an independent assessment of fetal well-being, and Doppler studies of the umbilical cord and fetal cerebral circulation provide an insight into the fetal response to the intrauterine environment. Doppler of the umbilical vessels provides a noninvasive method of assessing resistance in the placental circulation. As a rule of thumb, in a fetus greater than 30 weeks of gestational age, the umbilical artery systolicto-diastolic ratio (S/D) should be less than 3. To improve accuracy, it is suggested that three measurements be acquired and an average obtained of the results.9 In the fetus suffering from hypoxia, blood flow is
122
selectively diverted to the brain. Therefore, the normally high-resistance flow seen in the middle cerebral artery changes and becomes low resistance. The normal S/D ratio in the middle cerebral artery is greater than 4 with little diastolic flow. The middle cerebral S/D ratio should always be greater than the umbilical artery S/D ratios (Fig 4). When the balance changes and the umbilical artery S/D ratio is greater than that seen in the middle cerebral artery, this is an indication of fetal compromise. lo In the small fetus, cord Doppler does not diagnose IUGR but may predict outcome in fetuses suffering from it. In the presence of reversed end diastolic flow, there is a strong association with perinatal mortality, and delivery is strongly considered. 11 If the gestational age is such that the risks of preterm delivery are high, then the fetus is monitored extremely closely in utero with daily BPPs. A study by Mandruzzato in 199112 showed that patients with reversed diastolic flow all delivered fetuses less than the 5th percentile in weight for gestational age. There was a higher incidence of neonatal intensive care time, and 7 of 11 fetuses died in the perinatal period. The same group showed that absent end diastolic flow was associated with low birth rate, with 19 of 21 fetuses weighing less than the 5th percentile for gestational age at delivery. All deliveries were by cesarean section, but there were no perinatal deaths in the group with absent end diastolic flow. If there is a marked reduction in diastolic flow in the umbilical arteries, BPPs and non-stress tests (NSTs) are performed twice a week. More recently work has been directed at assessing the fetal venous circulation in an attempt to provide further evidence of fetal compromise. If placental vascular resistance is increased, higher pressures must be generated to maintain flow. Eventually increased myocardial work is required to maintain pressures; thus there is increased end diastolic pressure, causing tricuspid regurgitation. Tricuspid regurgitation results in a “kickback” with atria1 systole, which is manifested on Doppler assessment of the fetus and cord as flow reversal in the inferior vena cava and venous pulsations in the ductus venosus and umbilical veins.13 It must be emphasized, however, that management decisions should not be based on Doppler alone. The information from Doppler studies is only part of the picture, and other factors such as maternal blood pressure, fetal anomalies, and gestational age are always factored into the decision to deliver early.
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Biophysical
Profile
Table
After 25 weeks of gestation, when potential viability has been reached, performance of a BPP allows a noninvasive assessment of fetal well-being and can provide invaluable information to guide management of the pregnancy. Normal fetal movement is a powerful indicator that the fetal nervous system is intact and functioning normally. Absence of normal movement may be caused by primary nervous system anomalies or flexion contractures, but these are usually obvious sonographically. The other cause of impaired fetal movement is hypoxia. The fetus has variable patterns of activity during the day, and therefore it is essential that the BPP be conducted over an extended time period, up to 30 minutes if necessary. This is to compensate for periods of fetal inactivity. Four parameters are assessed sonographically. First is the volume of amniotic fluid. This is the most important variable, as will be shown later. Fetal breathing movements are evaluated by watching the fetal diaphragm. Gross body movements are assessed by watching for discrete episodes of motion; the score is normal if there are more than three episodes in the 30-minute scan time. Fetal tone is assessed by watching for active extension with return to flexion of the trunk, limbs, or hands. Each variable is given a score of 2 if present and 0 if absent. If the BPP is scored as 8 out of 8, then an NST or fetal cardiotocography is not required, as it adds no additional information. If the score if 6 out of 8, an NST is performed. If the NST is reactive, the total score is 8 out of 10. If the amniotic fluid is normal, a score of 8 out of 10 allows further close monitoring of the fetus. If the amniotic fluid is abnormal, a score of 8 out of 10 is an indication for delivery if the gestational age is greater than 36 weeks. A fetus scoring 6 out of 10 with normal fluid may have acute asphyxia; a decision to deliver in this situation will be based largely on gestational age. A score of 6 out of 10 where the fluid is low is an indication for delivery in a fetus greater than 26 weeks of gestation. This emphasizes the importance of the amniotic fluid volume in the overall assessment of fetal well-being. Perinatal mortality can be correlated with the BPP score. A score of 4 out of 10 corresponds to a perinatal mortality rate of 91 per 1000, 2 out of 10 is 125 per 1000, and 0 out of 10 is associated with a perinatal mortality rate of 600 per 1000 deliveries.14 Monitoring of the high-risk pregnancy is validated by studies correlating neonatal outcome with BPP score in pregnancy. Manning et al l5 have shown a significant
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2000
VIII.
Causes
of abnormal
High
maternal
serum
AFP
Fetal demise Bleeding Open neural tube defect Open abdominal wall Congenital Finnish nephrosis Incorrect dates Multiple pregnancy Wrong dates Trisomy 21
Low
reduction in the cerebral palsy rate when pregnancies were managed according to the BPP score. Other studies strongly support the hypothesis that the perinatal mortality rate can be reduced by the application of the BPP scoring system to the high-risk population.16J7 It is interesting to note that an abnormal Doppler tracing may precede abnormalities in the non-stress test by up to 24 hours. There are also situations in which the BPP is abnormal but the cord Doppler is unremarkable. This occurs either in situations where the cord is abnormal, such as when it is short or there is a knot in the cord, or when there is a problem with the fetus impairing motion, such as a neurologic abnormality or viral infection of the fetus. It is known that progressive reduction of amniotic fluid volume in the post-dates pregnancy is associated with fetal acidosis, low Apgar scores, and poor outcome. Therefore monitoring of the post-dates pregnancy may be most simply performed by amniotic fluid evaluation and NSTs; however, if serial measurements of umbilical cord Doppler show a trend of increasing resistance, this may be an indication to deliver. Assessment of the fetal aortic waveforms may be more beneficial in this circumstance. Abnormal
Antenatal
Screening
When antenatal screening tests suggest increased risk for chromosomal or structural abnormality, a detailed sonographic assessment of the fetus provides additional information and can modify the risk based on the serum screening and maternal age. Maternal serum AFP can be elevated for a number of reasons (Table VIII). The most important of these are neural tube defects and anterior abdominal wall defects. The sonographic diagnosis of these conditions is reviewed in more detail in the anomaly section. Fetal demise and multiple pregnancies are obvious. In the absence of an obvious cause, it is often helpful to look for echogenic material in the amniotic fluid or to examine the placen-
123
Table
IX. Sonographic
Mild ventriculomegaly Echogenic intracardiac Single umbilical artery Echogenic bowel Choroid plexus cysts Short humerus/femur Nuchal thickening Fetal pyelectasis ~......
FIG 5. Incompetent cervix. The cervix is dilated, membranes (*), despite the application of both arrow) and Shirodkar (short arrow) sutures.
with funneling of McDonald (/ong
tal implantation site to look for clinically occult bleeding. The maternal serum AFP test is relatively low in specificity but when combined with ultrasound is very sensitive for neural tube defects. In cases of clinical concern, amniocentesis can be performed to assess the amniotic fluid acetyl cholinesterase level for open neural tube defects. In cases where the ultrasound result is considered to be structurally normal, the risk estimate on the basis of the AFP is revised before consideration for amniocentesis. The triple screen was developed in an effort to identify a group of pregnant women at increased risk for the presence of fetal trisomies, particularly trisomy 21. Maternal serum AFP measurements are correlated with measurements of human chorionic gonadotrophin and unconjugated estriol. Triple screen results, maternal age, and gestational age are used to calculate a risk for fetal trisomy. Ultrasound is performed to confirm that gestational age is accurate and to survey fetal anatomy. Detection of Down syndrome is improved over the detection rate when maternal age alone is used. Detection of Turner syndrome antenatally is approximately 50%, as compared with 8% detection on the basis of maternal age alone. Sixty percent to 80% of trisomy 18 cases are detected, with a false-positive rate of less than 1%. The study does not detect trisomy 13 or triploid fetuses. However, these conditions usually manifest sonographically early in pregnancy. Maternal triploidy typically causes early severe IUGR. Paternal triploidy results in
124
“soft
signs”
of fetal
aneuploidy
focus
_
-
a molar pregnancy, and trisomy 13 is associated with significant craniofacial anomalies and IUGR. Multiple non-specific signs have been described in association with chromosomal anomalies. These are usually referred to as “soft signs.” When multiple soft signs are present, the likelihood of a chromosomal abnormality is increased relative to the situation when only a single soft sign is identified. Table IX lists the sonographic soft signs for fetal chromosomal abnormality. Table I lists some instances of sonographic findings other than the soft signs in association with different chromosomal abnormalities. Pain and
Bleeding
In early pregnancy, the clinical issue when the patient presents with pain and bleeding is ectopic pregnancy versus threatened abortion. As pregnancy progresses, bleeding may be caused by placenta previa or placental abruption. Abruption is associated with pain, while placenta previa typically is not. There are multiple other causes of abdominal pain in the pregnant patient, but a discussion of this topic is beyond the scope of this article. There are several useful articles on the subject to which the interested reader is referred.18T19 The evaluation of preterm labor and cervical incompetence is included, because both conditions affect fetal outcome and because the assessment of the cervix is an integral part of any obstetric scan. Dilatation of the cervix in the first trimester generally heralds an inevitable abortion. In the second trimester, painless dilatation of the cervix is associated with the incompetent cervix (Fig 5). This is an important cause of second trimester pregnancy loss. In the first and second trimesters, the cervix can usually be seen transabdominally, either by using the maternal bladder as an acoustic window or by angling through the amniotic fluid from a position just inferior to the umbilicus. In the third trimester, as many as 30% of patients cannot be adequately assessed transabdominally, and either the EV approach or transperineal sonog-
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2000
raphy will be required. In patients with premature rupture of membranes, EV sonography is contraindicated because of the risk of infection. Transperineal sonography can still be performed in this clinical situation. Fig 1, M, shows the normal appearance of the cervix. On occasion, the external OSis obscured by rectal gas during transperineal sonography. If this is the case, it can be helpful to have the patient tilt her pelvis forward or place her hips on a cushion and repeat the study. EV sonography allows accurate assessment of the cervix. Many patients prefer EV to transabdominal sonography, and many patients prefer EV to the transperineal approach. Theoretic risks of the EV approach to cervical assessment include stimulation of contractions and preterm labor, precipitation of bleeding in patients with placenta previa, and introduction of infection in patients with premature rupture of membranes. The first two relative contraindications can be avoided with careful technique. Timor-Tritsch and Yunis showed quite elegantly that it is highly unlikely that the EV transducer can be inserted into the cervix because of the normal anatomic relationships of the cervix and vagina.20 The probe is only inserted 3 to 4 cm so that the cervix will be in the focal zone, and because the insertion is monitored by the sonographer, direct contact with the cervix is highly unlikely. The length of the normal cervix is greater than 3 cm. The range is considerable, with up to 5 cm being normal. It is important in patients in whom the cervix is being assessed that the bladder is not overfilled, as marked bladder distention can cause compression of the lower uterine segment and create a pseudo placenta previa (Fig 6). Normal effacement and shortening begin at approximately 30 weeks of gestation. On EV sonography, 25 mm is the 10th percentile for cervical length, a length of 15 mm corresponds to 50% effacement, and 10 mm corresponds to approximately 75% effacement. Sonography, however, can demonstrate funneling of the membranes into the internal OS.This is an early sign of cervical incompetence and may be seen sonographically before there are any detectable changes in the clinical examination of the cervix. The cervix is dynamic in pregnancy, and therefore progressive shortening is more important than a single short measurement. Also, funneling is of more significance if progressive or marked. It is considered abnormal for the membranes to protrude more than 3 mm beyond the internal OS, and it is thought that the risk of preterm labor is higher if there is more than 6 mm of funneling. Significant levels are not well defined, and
Curr Probl
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2000
FIG 6. Pseudo placenta previa. A, When the maternal bladder is overfilled it can compress the lower uterine segment, causing an apparent low placental insertion site (arrow). B, After the bladder has been emptied it is clear that the placental margin (arrow) is well clear of the internal OS (*).
prospective clinical trials will be necessary to establish levels at which intervention is merited. It is advisable to measure the length and width of the funneled portion as well as the closed portion. While scanning, application of mild pressure to the fundus of the uterus acts effectively as a stress test for the cervix and may make funneling and dilatation apparent. The cervix should be assessed repeatedly during the study in patients at risk for preterm labor or cervical incompetence, and the reported measurements should be those at the time of maximum abnormality. A carefully conducted EV ultrasound has less risk of inducing premature rupture of membranes or stimulating labor than a speculum or digital examination.21 Preterm labor is defined as the onset of labor before 37 weeks of gestation. Fifty percent of patients with painful contractions in pregnancy are actually not in labor. Therefore, preterm labor is defined as the pres-
125
Table
X. Outcome
of twin
pregnancy
at 6 weeks
and 12 weeks
of gestation 12 weeks
6 weeks Probability
of:
Dichorionic
Monochorionic
With normal ultrasound Two live infants One live infant No live infants With abnormal ultrasound Two live infants One live infant No live infants Data from Benson CB, Doubilet
39.8% 30.4% 29.8%
75.8% 17.1% 7.1%
74.4% 0.5% 25.1%
95.8% 0.2% 4.3%
11.3% 45.4% 43.6%
37.4% 44.7% 17.9%
36% 1.3% 62.7%
81.4% 0.9% 17.7%
PM, David V. Prognosis
of first trimester
twin pregnancies:
ence of painful contraction in association with cervical changes. Multiple
Pregnancies
The incidence of multiple pregnancy is increasing in part because of increased success in the treatment of infertility. Twins are more common than higher-order multiple pregnancies. We will review the specific complications associated with twin pregnancies and discuss the determination of chorionicity. These principles can be applied to higher-order multiple pregnancies. Approximately 70% of twins are dizygotic; in this situation the physiology mimics that of two singleton pregnancies. The remaining 30% of pregnancies are monozygotic; these can then be subdivided on the basis of chorionicity and amnionicity. Approximately 30% are dichorionic, diamniotic; the majority are monochorionic, diamniotic; a small proportion (approximately 4% to 10%) are monochorionic, monoamniotic; and very rarely conjoined twins occur. The determination of chorionicity is important, because the outcome of monochorionic twins is poorer than that of dichorionic twins. This is because monochorionic twins are at risk for specific complications caused by the presence of vascular connections between the fetuses within the single placental mass. In dichorionic pregnancies or dizygotic pregnancies, the two placental masses may be fused, but because they arose from separate chorions, there are no vascular connections between them. Monochorionic twins are at risk for twin-twin transfusion syndrome, twin embolization syndrome, and acardiac parabiotic twinning. Monochorionic monoamniotic twins are at additional risk for cord entanglement, particularly after 34 weeks, and may also be locked at delivery. Conjoined twins are rare, comprising less than 1%
126
Dichorionic
Monochorionic
polychotomous
logistic
regression
analysis.
Radiology
1994;192:765-8.
of all twin pregnancies. The prognosis and outcome in conjoined twins largely depend on the degree of fusion and the presence of shared organs. Table X shows the likelihood of successful outcome in the different types of twin pregnancies. Determination
of Chorionicity
The best time to determine chorionicity is in early pregnancy. The earliest visible sign of intrauterine pregnancy is decidual reaction, called the double decidual sac sign, which is created by the echogenic chorionic tissue. In early pregnancy the chorionic sacs are counted. Then the number of amnions in each chorion is identified, plus the number of embryos. It may be difficult to resolve amnion in very early scans, and it is sometimes useful to count the yolk sacs, because the number of yolk sacs usually parallels the number of amnions (Fig 7). The membrane between fetuses will be thicker in a dichorionic, diamniotic pregnancy than in a monochorionic, diamniotic pregnancy. In the former there are four layers comprising the membrane; in the latter there are only two. In later pregnancy, if fetal sex can be identified and the fetuses are of different gender, then the pregnancy must be dizygotic, therefore dichorionic. The membrane can be evaluated for thickness and for the presence of a chorionic peak (Fig 8). If echogenic chorion extends into the peak leading to the intertwin membrane, this is evidence of dichorionic twinning, but its absence does not necessarily imply monochorionic twinning. If no membrane can be resolved between the fetuses, it can be useful to use color Doppler to evaluate the umbilical cords. If both cords can be followed to a common cord mass, it is likely that the cords are entangled, and this would be monochorionic, monoamniotic twinning (Fig 9). If there is difficulty in perception of a membrane, it
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Diagn
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2000
FIG
8.
membrane
FIG
7.
with
two
Early
monochorionic
embryos
and
twins.
two yolk
A single
sacs
(arrows].
gestation
sac
will
form, was sacs
of a diamniotic
twin
Diagn
Radiol,
July/August
Echogenic indicating
(*)
extends
a dichorionic
tissue
twin
into
the
intertwin
pregnancy.
pregnancy.
may be that there is a stuck twin, and this should be carefully evaluated. A stuck twin is usually smaller with oligohydramnios, and therefore the membrane separating the fetuses is closely applied to the smaller twin’s surface and not readily resolved (Fig 10). This is most frequently seen in monochorionic twins with twin-twin transfusion syndrome; however, a stuck twin can be seen in dichorionic twins. It may be useful to change maternal position during scanning, as the stuck twin may be held in an antigravitational position by the closely applied separating membrane. All twins are at increased risk for IUGR when compared with single pregnancies. The twin growth rate slows after 30 to 34 weeks of gestation, but in general, the singleton charts can be used to assess fetal growth. In very early pregnancy, if there is more than a 5-day difference between the crown-rump lengths, this is cause for concern regarding the possibility of major anomalies in the smaller twin, and close monitoring of the pregnancy will be required to evaluate for this. Discordant twin growth is defined as a 20% to 25% difference in birth weights between the twins. This is important, because the perinatal mortality rate in discordant twins is 2.5 times that of twins that grow in a concordant manner. Twin growth is best assessed by
Curr Probl
peak.
(arrows),
is present
from the chorion frondosum (CFJ. At this early stage a membrane not visualized between the embryos, but the presence of two yolk is suggestive
The placenta
Twin
2000
FIG 9. Monochorionic, monoamniotic twins. No membrane was identified, and the umbilical cords of both fetuses could be followed to this tangle of vessels (arrows). P, Placenta.
the use of the abdominal circumference measurement or the estimated fetal weight. An abdominal circumference difference of more than 20 mm between the twins is reason for concern, and if an estimated fetal weight difference of 15% is used as a criterion to suggest discordant growth, this will improve sensitivity. Some authorities suggest using the term discordant growth only if one twin meets the criteria for the diagnosis of IUGR. This is because the outcome is not as poor in twins with differing birth weights as long as the small twin weighs more than 2500 g. In the presence of differing growth rates between the fetuses, cord Doppler may be useful, because abnormal S/D ratios have a positive predictive value of 70% to 90% for discordant twin growth. Twin pregnancies with concern for discordant growth are carefully monitored with twice-weekly BPPs after viability.
127
cythemic, may be normal sized or can be macrosomic, and is often edematous. The donor twin has oligohydramnios secondary to hypovolemia and poor kidney perfusion; the recipient twin, on the other hand, has a hyperdynamic circulation, is often hypertensive, and has polyhydramnios. Because of the oligohydramnios, the small donor twin is often seen as a stuck twin. Cord Dopplers are abnormal in the donor. Absent end diastolic flow and reverse diastolic flow can occur in the donor twin but are not seen in the recipient. It is also useful to evaluate the site of cord insertion into the placenta. In a series of 100 cases of twin-twin transfusion at the University of Utah, there was velamentous insertion or marginal insertion of the donor cord in 98 patients, and the cord was peripheral on the smaller placental mass in the remaining 2 cases.22 The velamentous/marginal insertion may have some importance in the pathogenesis of twintwin transfusion syndrome. This can be demonstrated by color Doppler in up to 13 % of monochorionic twins.23 When identified, it may indicate the subset at greatest risk for development of twin-twin transfusion. Twin
FIG 10. Twin-twin transfusion syndrome. A, The arrows indicate the inter-twin membrane closely applied to the smaller donor fetus. The asterisk is in a small pool of amniotic fluid adjacent to the fetal hand (H). 6, (arrows)
The recipient twin is floating in ascites (*).
Twin-twin
Transfusion
hydropic,
with
central
bowel
loops
Syndrome
This abnormality occurs because of vascular connections between the two fetal circulations through the single placental mass; one twin is the donor, the other the recipient. This syndrome is seen in 5% to 30% of monochorionic twins, even though vascular anastomoses occur in 85% to 100%. The donor twin is small, hypovolemic, and anemic. The recipient twin is poly-
128
Embolization
Syndrome
In a dizygotic or dichorionic pregnancy, demise of one twin results in resorption of that fetus. There are usually no adverse sequelae for the remaining live twin. In monochorionic pregnancies, because of the vascular connections between the fetuses, vascular events occur in the surviving twin as a result of the death of the co-twin. The pathogenesis of these is uncertain, but the two most popular hypotheses are that there is embolization of material from the dead twin to the live twin, or that there is acute exsanguination of the live twin into the dead twin’s circulation. The results of this phenomenon are as would be expected from any physiology causing hypoperfusion, whether it is embolic or hypotensive. In the central nervous system, porencephaly, ventriculomegaly, cerebral atrophy, and microcephaly occur. Other systems are affected, including the gastrointestinal system, with liver and spleen infarcts and bowel atresias; the genitourinary system, with renal cortical necrosis; and the cardiorespiratory system, where lung infarcts may occur. A variety of other facial and extremity anomalies have been described in relation to this rare complication of monochorionic twinning.24
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Acardiac
Parabiotic
Sequence
This condition is also described as the twin reversed arterial perfusion sequence (TRAP). The normal placental circulation is for oxygenated blood from the placenta to travel via the umbilical vein to the heart, where it is shunted across the foramen ovale to the left atrium, left ventricle, and then to the aorta to preferentially supply the brain. Deoxygenated blood returns to the placenta via the umbilical arteries. In the TRAP sequence, there are artery-to-artery and vein-to-vein anastomoses within the placenta. This is rare, occurring in approximately 1% of monochorionic pregnancies. One twin returns deoxygenated blood to the placenta via the umbilical arteries. Artery-to-artery anastomoses occur within the placental mass, and this deoxygenated blood is shunted to the second twin. The second twin, therefore, receives poorly oxygenated blood from the placenta via the umbilical arteries; in other words, flow direction in the umbilical arteries is reversed. This blood, because it enters the fetus via the umbilical arterial circulation, perfuses the lower extremities selectively, and as a result there is marked limitation of growth in the upper half of the body. The lack of normal circulation impairs normal cardiac development, resulting in the acardiac feature. Poor perfusion of the upper half of the body results in lack of development of the head and upper extremities. This anomaly is commonly associated with a two-vessel cord.25 Conjoined
Anomalies
Whole textbooks have been devoted to antenatal detection and diagnosis of fetal anomalies. For the sake of
Curr
Probl
Diagn
section
at the the of
twin
11.
Thoracomphalopagus
1 is labeled
conjoined
twins.
S.
Twins
Conjoined twins are rare, occurring only in monoamniotic pregnancies, in 1 of 50,000 to 100,000 deliveries. The nomenclature depends on the site of fusion: thorucopagus denotes twins joined at the chest; omphalopagus denotes twins joined at the abdomen; thoracomphalopagus twins are joined at the chest and abdomen (Fig 11). Together these three categories comprise 70% of conjoined twins. Twins joined at the pelvis are referred to as ischiopagus, those joined at the sacrum are referred to as pygopagus, and those joined at the head are referred to as cmniopagus. The prognosis for conjoined twins depends on the extent of joining, the amount of shared organs, and the presence of associated anomalies.26 Monochorionic, monoamniotic twins are also at risk for cord accidents, because the cords may become entangled or knotted. Fetal
FIG
A, Cross
the level of the fetal chest shows the hearts (H) in continuity across midline. 6, Cross section at the level of the abdomen shows that twins are (oined from the chest through the abdomen. The stomach
Radiol,
July/August
2000
brevity we will cover just the most common anomalies by anatomic system and emphasize practical points for differentiating one from another. Central
Nervous
System
The most common anomalies seen within the central nervous system are choroid plexus cysts, hydrocephalus or lesions mimicking hydrocephalus, destructive processes, neural tube defects, and caudal regression. Choroid plexus cysts are associated with trisomy 18; therefore, visualization should prompt a careful fetal analysis for other structural defects (Fig 12). The fetus with trisomy 18 will usually have multiple anomalies, and it is our practice if choroid plexus cysts are seen as an isolated anomaly to not recommend genetic amniocentesis. The key things to observe in the presence of choroid plexus cysts to assure structural normality of the fetus are a normal heart, a normal facial profile without evidence of micrognathia, and a normal hand without evidence of overlapping fingers. The features associated with various chromosomal syndromes are outlined in Table I.
129
FIG 12. Trisomy 18. A, Transverse appearance caused by brachycephaly.
image through the thalami This is associated with
(T) at the level of the cavum septum pellucidum (*) shows the “strawberry” trisomy 18, as listed in Table I. 8, Transverse image through the ventricles
a choroid plexus cyst (C) and an interhemispheric arachnoid cyst (A). sistent flexion. Contrast this with the normal appearance in Fig 1, 1.
Hydrocephalus occurs at a rate of 0.5 to 3 per 1000 live births and is caused by increased formation of cerebrospinal fluid or more commonly by impaired drainage. The key to diagnosing the cause of hydrocephalus is the posterior fossa anatomy (Fig 1, B). If the posterior fossa is normal with a bilobed cerebellum and a normal cisterna magna, then the most likely diag-
130
C, The fetal
fingers
(arrows)
are overlapped
and
the fingers
held
head shows in per-
nosis is aqueductal stenosis (Fig 13). In the X-linked recessive form of this disease, the fetus very often has an abducted thumb, and this is something that may be useful to look for in terms of identifying cause. If the cerebellum is abnormal with a rounded configuration typically described as the “banana” cerebellum with small or absent cisterna magna, then the diagnosis is
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Arnold-Chiari malformation (Fig 14), the prognosis of which largely depends on the extent of the associated neural tube defect. If the posterior fossa shows an enlarged cisterna magna and absence of the cerebellar vermis, this is the Dandy-Walker syndrome (Fig 15). An important point in the diagnosis of the DandyWalker syndrome is that the cerebellum develops sequentially from superior to inferior. Inferior vermian agenesis may result in an apparently normal bilobed cerebellum with the superior portion of the vermis being seen on axial sections. Coronal imaging will help to identify fetuses with inferior vermian agenesis as a cause for the hydrocephalus. The normal vermis is seen to extend to the level of the bottom of the cerebellar lobes on coronal sections. Borderline ventriculomegaly is occasionally documented and can be associated with chromosomal anomalies such as trisomy 21. It may be a sign of very early hydrocephalus, agenesis of the corpus callosum, or a sequel to intrauterine infection. The measurement of the atria of the lateral ventricle is relatively constant throughout pregnancy. The upper limit of normal is measured as 10 mm, although the average measurement is closer to 6 mm. Subjectively, ventricular dilatation is assessed by the presence of a “dangling” choroid. If the choroid is displaced more than 3 mm from the medial ventricular wall, this is supportive of abnormal ventricular dilatation (Fig 13, A). Agenesis of the corpus callosum can be excluded if there is a normal cavum septum pellucidurn (Fig 1, A). In the fetus with mild ventriculomegaly, careful evaluation may reveal other “soft signs” of Down syndrome, which would prompt genetic amniocentesis. Fetuses suffering from intrauterine infection very often have IUGR and may have other features such as the presence of intracranial and intrahepatic calcifications:The presence of hepatosplenomegaly in the fetus with clinical suspicion for intrauterine infection suggests cytomegalovirus. Conditions that mimic hydrocephalus include holoprosencephaly and hydranencephaly. Hydranencephaly is thought to be secondary to a vascular insult, and in this condition the cerebral cortex is destroyed. There is usually a midline falx present. The facial development is normal, and typically the fetus is otherwise structurally normal. The vertebrobasilar circulation is intact and therefore the cerebellum and brain stem usually develop normally. Holoprosencephaly is a disorder of diverticulation of the fetal brain and is classified as lobar, semi-lobar, or alobar. In this condition there is a monoventricle.
Curr Probl
Diagn
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2000
! 1 ’
FIG
13.
Aqueductal
stenosis.
A,
At 18 weeks
of gestation,
the atria
of
the lateral ventricle measured 9 mm (calipers). This is within the normal range, but there was a subjective impression of a dangling choroid (arrow). The posterior fossa was normal, excluding Arnold-Chiari and Dandy-Walker was normal.
malformation, and the remainder of the structural survey Because of the subjective ventriculomegaly, the patient was
asked to return hydrocephalus.
for a follow-up scan. 6, At follow-up Calipers demarcate the dilated third
gling choroid (arrow) is clearly (IV). The male fetus was noted with X-linked recessive aqueduct
there is now frank ventricle. The dan-
seen within the dilated lateral ventricle to have an abducted thumb consistent stenosis.
Therefore no falx is present and the thalami are fused (Fig 16). Holoprosencephaly is commonly associated with facial anomalies (Fig 17). The face predicts the brain but the brain does not predict the face; therefore the presence of an intracranial anomaly with normal
131
FIG 14. Arnold-Chiari malformation. to the reduction in intracranial pressure
A,
cerebellum
the brainstem
(arrow)
wrapped
around
this image with Fig 1, C, which shows diverging (arrow) and the meningocele meningocele sac (orrow).
Cross caused
section of the head shows the “lemon” by the spinal defect. Ventriculomegaly in the “banana”
the normal appearance. sac (*). D, Sagittal
configuration.
Tube
Defects
One of the most devastating of the neural tube defects is anencephaly. In this condition, the fetal brain does not develop and a small amount of material called angiomatous stroma is seen in the base of the skull. The skull base and part of the occiput develop in cartilage, but there is no cranial vault and no normal cerebral tissue. The characteristic appearance of this is illustrated in Fig 18. This anomaly is incompatible with life and occurs in approximately 1 out of 1000 pregnancies. An encephalocele is a skull defect with associated focal herniation of brain through the defect (Fig 19). These
132
Note
also
the obliteration
bones (arrow) that is due fossa view showing the
of the cisterna
C, Transverse image of the lower lumbar spine view of the lower lumbar spine shows intact skin
facies should not preclude the diagnosis of holoprosencephaly. Neural
head appearance of the frontal B, Posterior (*) IS also present. shows
the
(arrowheads)
magna.
Compare
posterior elements superior to the
defects are commonly occipital or nasal and are associated with certain syndromes such as Meckel-Gruber. The prognosis for the fetus depends on the presence of associated abnormalities and the extent of brain involvement. Spinal
Cord
Defects
The spectrum of abnormality runs from spina bifida occulta through meningocele, myelomeningocele, and myeloschisis to rachischisis. The normal coronal view of the spine shows smooth tapering of the interpedicular distance to a point at the sacrum and mild widening at the cervical end of the spine. The spine should be completely covered by skin; this is best visualized on sag&al section. Transverse sections of the spine show
Curr Probl
Diagn
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July/August
2000
FIG
15.
cephalus, magna
Dandy-Walker
malformation.
views of the posterior [CM) with cerebrospinal
In
this
fetus
with
hydro-
fossa show an enlarged cisterna fluid (arrow) extending to the brain-
stem because of the absence of the demarcate the cerebellar lobes.
cerebellar
vermis.
The
asterisks
FIG
16.
Holoprosencephaly.
fused thalami (T). The dorsal the absence of the falx.
Coronal
section
cyst comprises
of the fetal
head
the rest of the brain.
shows Note
the “teepee” appearance of the posterior elements converging. It is abnormal for the posterior elements to diverge (Fig 14). Spinal defects are commonly associated with the Arnold-Chiari malformation described in the section on CNS anomalies. Caudal
Regression
Caudal regression occurs in approximately 1 in 60,000 births and is described most commonly in the fetuses of diabetic mothers. The sequence consists of partial or complete sacral agenesis and anomalies or absence of parts of the lumbar spine. The spine can actually be absent below the level of TlO. As a consequence of the spinal anomalies, the lower extremities are often held in abnormal position and can be flexed, externally rotated, or fused. The syndrome is associated with a single umbilical artery and may be associated with agenesis of part of the urinary tract. The syndrome will be excluded if normal sagittal and coronal sections of the spine are seen. It is our practice in fetuses of diabetic mothers to obtain a cross section of the lower spine at the level of the iliac blades to confirm that sacral elements are present (Fig 1, D and E). Gastrointestinal
Anomalies
Gastrointestinal (GI) anomalies include bowel atresia, gastroschisis, omphalocele, and echogenic bowel.
Curr
Probl
Diagn
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2000
FIG One
17. Facial cleft. Oblique axial image looking up at the fetal orbit (0) and the outline of the nose (N) are seen. Arrows
cate the amniotic
cheeks, and the fluid extends.
asterisk
identifies
the facial
cleft
face. indi-
into which
Bowel atresias manifest with distended loops of bowel and polyhydramnids caused by inability of the fetus to swallow amniotic fluid. The best recognized is duode-
133
FIG (0). There
18. Anenecephaly. A, There is no skull vault 8, The cervical spine extends to normal occipital is no skull
vault
superior
above bones
the orbits (arrows).
to this.
nal atresia, which is important because of its association with trisomy 21. The characteristic appearance is the sonographic equivalent of the double-bubble described on plain films, the equivalent in the fetus being a fluid-filled stomach in continuity with a fluidfilled duodenum (Fig 20).
134
ing, it generally does not require further evaluation with amniocentesis. Echogenic bowel may, however, also be associated with cystic fibrosis, and evaluation of the neonate is merited in addition to genetic assessment of the parents. Gastroschisis is a defect in the anterior abdominal wall, usually to the right of midline. The umbilical cord inserts normally, and bowel loops hemiate through the defect, floating freely in amniotic fluid. This tends to be an isolated sporadic abnormality and is not associated with chromosomal syndromes. The other
Curr Probl
Diagn
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2000
_
FIG 21. Omphalocele. The umbilical cord (arrow) inserts on the apex of a mass (*) protruding from the abdominal wall. A membrane limits the mass. The skin of the lateral abdominal wall is seen clearly as an echogenic interface (arrowheads).
common anterior wall defect is omphalocele, in which there is defect in the abdominal wall covered by a membrane. The umbilical cord inserts on top of the membrane covering the protruding organs (Fig 21). Either liver, bowel, or liver and bowel can herniate through the defect. Bowel-only omphaloceles have a worse prognosis than those with bowel and liver, and omphalocele is associated with chromosomal disorders, particularly trisomy 18. Genitourinary
System
Anomalies
Genitourinary (GU) system anomalies primarily involve the kidneys and can be classified as abnormalities of kidney development versus obstructive processes. Bilateral renal agenesis is known as Potter’s syndrome. This is a uniformly fatal anomaly and presents in early pregnancy with profound oligohydramnios. Fetal visualization is impaired because of the absence of amniotic fluid, but indications include the flat adrenal sign, in which the adrenals are seen to lay along the spine in the renal bed. The adrenals have a typical “ice cream sandwich” appearance, with a hypoechoic outer component and echogenic inner component (Fig 2). This is different from the appearance of normal kidneys and should not be mistaken as normal renal parenchyma. Use of color Doppler can be helpful in determining whether there is flow into renal arteries from the abdominal aorta. In particularly difficult cases, amnio-infusion may help to facilitate adequate imaging, or MRI can be performed to confirm the absence of fetal kidneys.
Curr Probl
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2000
_..
._--_____
FIG 22. Fetal hydronephrosis. The fetal kidney is outlined by asterisks. The dilated calyces (Cj are shown in continuity with the pelvis (i’), confirming hydronephrosis. Simple pelvicaliectasis on a transverse scan is often of no significance if mild. Calyceal involvement is best demonstrated on coronal images such as this. Caliectasis implies more significant collecting system dilatation.
Unilateral renal agenesis is usually not associated with oligohydramnios and may prove to be a difficult antenatal diagnosis. Renal obstruction or renal collecting system dilatation presents as fetal hydronephrosis (Fig 22). Mild pelvicaliectasis is not uncommon, and in our facility we use the guidelines that a collecting system greater than 4 mm in anteroposterior (AP) diameter before 30 weeks should be reevaluated after 30 weeks. If after 30 weeks the collecting system AP diameter is 7 mm or less, no further evaluation is required. In those fetuses in which the AP diameter is greater than 7 mm, additional postnatal assessment is indicated, and it is essential that this be performed when the infant is at least 3 days old. Newborn infants are dehydrated, and therefore collecting system dilatation may be spuriously absent if the infant is scanned within the first 3 days of life. The most common causes of obstruction are ureteropelvic junction (UPJ) obstruction and distal ureteral obstruction secondary to ureterocele or ectopic implantation in the duplex kidney. In male fetuses, posterior urethral valves cause bladder distention and bilateral obstruction and may result in bladder rupture with urinary ascites. Cystic diseases include autosomal dominant polycystic kidney disease, autosomal recessive polycystic kidney disease, and multicystic dysplastic kidney syndrome. Autosomal recessive polycystic kidney disease presents as enlarged uniformly echogenic kidneys in which indi-
135
FIG 23. Multicystic dysplastic kidney. Coronal shows bilateral enlarged echogenic kidneys with cated
section multiple
of the fetus cysts demar-
by calipers.
vidual cysts are not resolved but rather the echogenicity is thought to be caused by multiple spectral reflectors from cyst walls. Autosomal dominant polycystic kidney disease is a rare antenatal diagnosis; if this is suspected, it is worth scanning the kidneys of the parents. Multicystic dysplastic kidney is thought to arise because of failure of the ureteric bud to unite with the metanephric blastema. Ureters are present, but they do not connect with the developing collecting system, and therefore tubule formation does not occur. These are typically large kidneys with multiple cysts and no identifiable renal parenchyma. If bilateral, the condition is fatal and is associated with profound oligohydramnios (Fig 23). If unilateral, the prognosis is better; however, multicystic dysplastic kidney is associated with contralateral UPJ obstruction, and therefore careful monitoring of the pregnancy and the child after delivery is indicated to preserve kidney function. Thoracic
and
Cardiovascular
Anomalies
There are three important differential diagnoses for an echogenic mass in the fetal chest. Cystic adenomatoid malformation presents as an intrathoracic mass. The appearance varies with the internal architecture. Type 1 has multiple large cysts of various sizes, type 2 has uniform cysts that are usually multiple and less than or equal to 2 cm in diameter, and type 3 has tiny cysts that are not resolved individually; therefore, this lesion appears as a solid parenchymal mass (Fig 24). All of these occur within the thorax, so the gastric fundus will be in a normal position, and with careful evaluation the cysts will not be confused with bowel loops. The blood supply is from the pulmonary artery; this feature may be best resolved with the use of power Doppler. Pulmonary sequestration is an uncommon developmental anomaly. It can be intra-lobar-that is, have
136
pulmonary venous drainage-or be extra-lobar-that is, have systemic venous drainage. Both types have a systemic arterial supply, and it is this feature that allows a confident diagnosis, because the feeding vessel can be seen arising from the aorta with careful color or power Doppler interrogation. Extralobar sequestration often has a triangular configuration, and up to 5% may be sub-diaphragmatic. Intralobar sequestration is more commonly round in shape, and all occur in the lower lobes (Fig 25). Congenital diaphragmatic hernia occurs in approximately 1 of 2000 to 1 of 3000 births. The prognosis for the fetus depends on the degree of pulmonary hypoplasia and the presence of associated anomalies. The diagnosis is usually obvious, because the gastric fundus will be seen on the same axial plane as the heart (Fig 26). Left-sided hernias are more common than rightsided, and the fetal heart is displaced to the right hemithorax by the herniated bowel. Coronal sections of the fetus also demonstrate displacement of the liver superiorly, and color Doppler will show that the echogenie “mass” contains portal and hepatic veins. The full scope of fetal echocardiography is beyond this article. The accuracy of prenatal cardiac diagnosis is poor (from 33% to 63%) when only the 4-chamber view is obtained. It is our practice to try to image the outflow tracts in all fetuses even though this is not included in the AIUM guidelines (Fig 1, G, H, and Z). By doing so the accuracy can be increased to as much as 85%.27,28 During assessment of the fetal heart the sonographer should attempt to answer the following six questions29: (1) Is the cardiac axis normal? (2) Is the heart size normal? (3) Are the ventricles symmetric? (4) Is there a septal defect? (5) Are the atrioventricular (AV) valves normally positioned? (6) Is there any abnormality in the endocardium, pericardium, or myocardium? Table XI lists the conditions associated with an increased risk of congenital heart disease. We routinely have formal fetal echocardiography performed in all fetuses of diabetic mothers and in all fetuses with a diaphragmatic hernia. Identifiable anomalies include endocardial cushion defect in association with trisomy 21, tetralogy of Fallot (Fig 27), double outlet right ventricle, hypoplastic left heart, and coarctation of the aorta. The sonographic features of these conditions are listed in Table XII. The issue of hydrops is important and not uncommon. Common causes are listed in Table XIII. Hydrops is present when there is fetal ascites, pleural effusion
Curr Probl
Diagn
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2000
FIG 24. fetal thorax rior to the Doppler. above the
Curr Probl
Cystic adenomatoid malformation. A, Cross section of the shows an echogenic mass demarcated by calipers posteheart [H). No aortic feeding vessels were identified on color B, Sagittal section of the fetus shows that the mass (M) is diaphragm (arrow].
Diagn
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2000
with or without pericardial effusion, and skin thickening. By definition there must be fluid accumulation at more than one site. The most common cause used to be Rhesus iso-immunization; the routine use of Rhogam
137
FIG 27. Tetralogy septal defect (*),
0.4 4% 9% 12%
of Fallot.
The aorta
LV, Left ventricle;
(A) is overriding
a ventricular
RV, right ventricle.
to 0.8% of all pregnancies if one sibling affected if father affected if mother affected
increased in: Non-gestational diabetics Drugs: anticonvulsant, lithium, Alcohol Lupus Rubella Fetuses with other anomalies
retinoic
acid
makes this now the cause of less than 1% of cases. Hydrops can be categorized as immune or non-immune. The non-immune may be secondary to cardiac structural defects or arrythmia. Other structural abnormalities cause hydrops by diminishing venous return, by compressing the heart, or by causing high output failure. Fig 28 demonstrates a fetus with hydrops resulting from a cystic hygroma. The presence of hydrops should prompt a careful search for cardiac and other structural defects as well as assessment of the heart rate and rhythm. Isolated pleural fluid is somewhat less ominous than full-blown hydrops and may be the result of a chylothorax. Fluid within the peritoneal space may be cardisc ascites, but consideration must also be given to urinary ascites from bladder rupture or bowel perforation with meconium peritonitis. The presence of a distended bladder in a male fetus with abnormal kidneys strongly
138
I
FIG 28. Fetal hydrops in association with cystic hygroma. A, Sagittal section through the fetus shows subcutaneous edema (S), ascites [A), and pleural effusion (F). B, Transverse section through the fetal head shows a large septated fluid collection (arrows) surrounding the cranium.
suggests outlet obstruction resulting from posterior urethral valves. In a fetus with meconium peritonitis, consideration should be given to bowel atresia and cystic fibrosis. The latter is associated with echogenic bowel, and the former will often be associated with dilated loops of bowel and polyhydramnios. If there is calcified material within the lumen, this suggests a fistula between the urinary and GI tracts, because meconium will not calcify within normal bowel loops. This is most often seen with anal atresia.30 Skeletal
Anomalies
More than 200 skeletal dysplasias have been described, and the process of diagnosis is lengthy and complex. Plain films of the long bones, spine, and hands and feet are often required. In the antenatal setting the important issue is whether there is a lethal
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Table
XII. Sonographic
Atrioventricular
Coarctation
of congenital
defect
of Fallot (Fig 27)
outlet
Hypoplastic
right
ventricle
left heart
Transposition
of the great
Truncus
arteriosus
Ebsteins
anomaly
Data from EA
vessels
Stomm, JA Drose. The
heart
disease
Ventricular septal defect (ensure that the septum is evaluated with a perpendicular beam not just parallel). Atrial septum may be absent. AV valve abnormal. The tricuspid valve has three cusps, the mitral two. In a complete atrioventricular septal defect there is a single valve with bridging leaflets; in the partial type the valves are separated but abnormal. Large right ventricle, small left ventricle (RV:LV diameter ratio of 2:l is suggestive) Pulmonary artery:aorta diameter 22 SD above the mean (1.18 i: 0.06) Small aorta Small pulmonary artery secondary to atresia/stenosis Or large pulmonary artery secondary to absent pulmonary valve. Large aorta overriding ventricular septal defect. If associated with an open abdominal wall consider Pentalogy of Cantrell. If more than 50% of each of the great vessels arises from the right ventricle. Ventricular septal defect Related to maternal diabetes and alcohol abuse. Small left ventricle: use charts to compare ventricular size to that expected for the BPD to determine which are small and which are large. Hypoplastic or atretic mitral and aortic valves 80% associated with coarctation. Difficult antenatal diagnosis. Consider it if the great vessels are not seen to cross as they exit the heart. Single outflow vessel overrides the septum. Almost always associated with a ventricular septal defect. Large right atrium and small right ventricle secondary to apical displacement of the tricuspid valve. Maternal lithium exposure.
of the aorta
Tetralogy
Double
features
septal
fetal heart in diagnostic
ultrasound.
2nd ed. Rumack
skeletal dysplasia present. Timely antenatal diagnosis of these conditions allows the family the option of termination as well as providing information regarding the risk of recurrence in subsequent pregnancies. As a general rule, 50% of skeletal dysplasias affecting the fetus are lethal, and severe limb shortening is associated with a bad prognosis.31 In osteogenses imperfecta type 2, the bones are so poorly mineralized that they appear thick, because the un-ossified matrix does not reflect sound in the same way that normally mineralized bone does. The presence of multiple fractures causes angulation of long bones and “beading” of the ribs. The skull vault will be compressible with the application of mild transducer pressure. This disease manifests early and is excluded if the scan is normal after 17 weeks of gestational age. Congenital hypophosphatasia shows poor mineralization and angulation involving only the femur and tibia and is associated with hypoplasia of the fibula and scapula. Thanatophoric dysplasia manifests with severe rhizomelic limb shortening. It is associated with the “cloverleaf’ deformity of the skull, but this is seen in only about 14% of cases. Hydrocephalus, redundant skin, and polyhydramnios can be identified sonographically. The characteristic appearance of the spine with
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2000
CM, Wilson
Table
XIII.
SR, Charbonneau
Causes
Immune: ABO and Rhesus Non-immune: Structural:
Placental/vascular:
Maternal
causes
of fetal
JW, editors.
St Louis: Mosby.
hydrops
alloimmunization Cystic hygroma Diaphragmatic hernia and other chest masses Cardiac defects and arrythmias Tumors: neuroblastoma, sacrococcygeal teratoma Chromosomal/genetic abnormalities Intrauterine infection Twin-twin transfusion Chorioangioma Vein of Galen aneurysm Severe diabetes Severe anemia
flattened vertebral bodies is best appreciated on film. We use fluoroscopy to position the mother so that the fetal spine is projected off the maternal spine in as close as possible to a lateral projection. Achondrogenesis causes the most severe limb shortening, mineralization is poor or absent, and the pubis and sacrum may be absent. It is associated with hydrops and polyhyramnios and is usually diagnosed by 19 weeks of gestation. Homozygous achondroplasia is lethal. This diagnosis
139
is easilv confirmed bv looking at the parents, at least one of whom will have to have heterozygous achondroplasia. Non-lethal but important skeletal findings include clubfoot, rocker bottom foot, overlapping fingers, and syndactyly and polydactyly. These features are rarely seen in isolation and when found should prompt careful evaluation because they are associated with syndromes and anomalies. For examnle. rocker bottom feet are seen in trisomy 18 and 13, syndactly is associated with Apert syndrome, and polydactyly is associated with Meckel-Gruber syndrome. d
d
A
Conclusion Ultrasound affords us a unique opportunity to evaluate fetal anatomy and to assess fetal well-being noninvasively. Rigorous attention to detail and dedicated experienced staff are required to perform high-quality antenatal sonography. An integrated team approach with open communication between sonologists, perinatologists, and pediatric specialists provides the best possible care for the family unit. When a fetus is abnormal, the multi-disciplinary approach allows time for development of a relationship between the parents and the health care professionals who will care for their child after delivery. I am fortunate to work in such an institution, and I encourage others to look beyond “turf battles” and strive to develop such a system, because it enhances both patient care and the educational process for all of the care providers involved. REFERENCES 1. Benson CB, Doubilet PM, Saltzmann DH. Sonographic determination of fetal weight in diabetic pregnancies. Am J Obstet Gynecol 1987;156:441-4. 2. Acker DP, Sachs BP, Friedman EA. Risk factors for shoulder dystocia. Obstet Gynecol 1985;66:762-8. 3. Sohaey R, Nyberg DA, Sickler GK. Idiopathic polyhydramnios: association with fetal macrosomia. Radiology 1994; 190:393-6. 4. Panting Kemp A, Nguyen T, Chang E, Quillen E, Castro L. Idiopathic polyhydramnios and perinatal outcome. Am J Obstet Gynecol 1999;181:1079-82. 5. Vintzileos AM, Campbell WA, Nochimson DJ, Weinbaum PJ. Degree of oligohydramnios and pregnancy outcome in patients with premature rupture of the membranes. Obstet Gynecol 1985;66: 162-7. 6. Huppert BJ, Brandt KR, Ramin KD, King BF..Single shot fast spin echo MR of the fetus: a pictorial essay. Radiographics 1999;19:S215-27. 7. Piepert JF, Donnenfeld AE. Oligohydramnios: a review. Obstet Gynecol Surv 1991;46:325-9. 8. Clement D, Schifirin BS, Kates RB. Acute oligohydramnios in post date pregnancy. Am J Obstet Gynecol 1987;157:884-6. 9. Grant EG. Maternal-fetal Doppler sonography: potential or reality? Semin Roentgen01 1991;26:75-86.
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10. Arbeille P, Body G, Saliba E. Fetal cerebral circulation assessment by Doppler ultrasound in normal and pathological pregnancies. Eur J Obstet Gynecol Reprod Biol 1998;29:261-73. Il. Karsdorp VH, van Vugt JM, van Geijn HP, et al. Clinical significance of absent or reversed end diastolic velocity waveforms in umbilical artery. Lancet 1994,344:1664-g. 12. Mandruzzato GP, Bogati P, Fischer L, Gigli C. The clinical significance of absent or reverse end-diastolic flow in the fetal aorta or umbilical artery. Ultrasound Obstet Gynecol 1991;1:192-6. 13. Mernagh JR, Mohide P, Cacoc F. Doppler ultrasound in obstetrics: a pictorial review. Scientific Exhibit. RSNA 1999. 14. Sohaey R. Amniotic fluid and fetal well-being. In: Zwiebel WJ, Sohaey R, editors. Introduction to ultrasound. 1st ed. Philadelphia: WB Saunders; 1998. p. 359-71. 15. Manning F, Harman C, Menticoglou S. Fetal biophysical profile score and cerebral palsy at age 3 years. Am J Obstet Gynecol 1996;174:319. 16. Chamberlain PF. Later fetal death: has ultrasound a role to play in its prevention? Irish J Med Sci 1991;161:251-4. 17. Baskett TF, Allen AC, Gray JH, et al. Fetal biophysical profile and perinatal death. Obstet Gynecol 1987;70:357-60. 18. Kennedy A. Assessment of acute abdominal pain in the pregnant patient. Semin Ultrasound CT MR 2000;21:64-77. 19. Boridy IC, Maklad N, Sandler CM. Suspected urolithiasis in pregnant women. Imaging algorithm and literature review. Am J Roentgen01 1996;167:869-75. 20. Timor-Tritsch IE, Yunis RA. Confirming the safety of transvaginal ultrasound in patients suspected of placenta previa. Obstet Gynecol 1993;81:742-4. 21. Hertzberg BS, Kliewer MA, Farrell TA, DeLong DM. Spontaneously changing gravid cervix. Clinical implications and prognostic features. Radiology 1995;196:721-4. 22. Ward K. Unpublished data. 23. Abrahim-Zadeh R, Coleman BG. The role of color Doppler in ultrasonography in obstetrics. Semin Roentgen01 1998; 33:351-9. 24. Patten RM, Mack LA, Nyberg DA, et al. Twin embolization syndrome: prenatal sonographic detection and significance. Radiology 1989;173:685-9. 25. Levi CS, Lyons EA, Mattel M, Dashefsky SM, Holt SC. Sonography in the diagnosis and management of multi-fetal pregnancy. In: Rumack C, Wilson SR, Charbonneau JW, editors. Diagnostic ultrasound. 2nd ed. St Louis: Mosby; 1998. p. 1043-66. 26. Filly RA, Goldstein RB, Callen PW. Monochorionic twinning: sonographic assessment. Am J Roentgen01 1990;154: 459-69. 27. Frates MC. Sonography of the normal fetal heart: a practical approach. Am J Roentgen01 1999;173:1363-70. 28. Bromley B, Estroff JA, Sanders SP. Fetal echocardiography: accuracy and limitations in a population at high and low risk for heart defects. Am J Obstet Gynecol 1992;166:1473-81. 29. McGahan Jl? Sonography of the fetal heart: findings on the 4 chamber view. Am J Roentgen01 1991;156:547-53. 30. Harris RD, Nyberg DA, Mack LA, et al. Anorectal atresia: prenatal sonographic diagnosis. Am J Roentgen01 1987;149: 395-400. 31. Sohaey R. Fetal musculoskeletal diagnosis. In: Zwiebel WJ, Sohaey R. Introduction to ultrasound. 1st ed. Philadelphia: WB Saunders; 1998. p. 476-89.
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