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Imaging Evaluation of Fetal Megacystis: How Can Magnetic Resonance Imaging Help?
Imaging Evaluation of Fetal Megacystis: How Can Magnetic Resonance Imaging Help? Maria A. Calvo-Garcia, MD Evaluation of the kidneys, bladder, and amniotic fluid volume forms part of any standard obstetrical ultrasound. When a fetal genitourinary anomaly is suspected, a more detailed evaluation is necessary. This detailed imaging can be challenging in the setting of decreased or absent amniotic fluid or large maternal body habitus, and in complex malformations. In these situations, magnetic resonance imaging can help to better define the fetal anatomy and provide a more confident and specific prenatal diagnosis. Semin Ultrasound CT MRI ]:]]]-]]] C 2015 Elsevier Inc. All rights reserved.
Introduction
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egacystis is the term used to describe an enlarged bladder at any gestational age. During the second and third trimesters, this diagnosis is based on an overall subjective assessment and visualization of a persistently enlarged bladder. However, in the first trimester, specific size criteria have been defined.1,2 The fetal bladder is first seen at approximately 10-12 weeks and its diameter should be no more than 68 mm. If the bladder is not seen by 15 weeks or if it is persistently enlarged (especially beyond 15 mm), concern for an underlying genitourinary malformation should be raised and further detailed evaluation during the second trimester is warranted.3 This assessment typically includes a detailed ultrasound (US) of the kidneys, bladder, external genitalia, and amniotic fluid volume. The US may be complemented with fetal magnetic resonance imaging (MRI), which improves the evaluation of the kidneys with better detection of renal ectopia and cystic dysplasia. It also helps detect underlying anorectal malformations and allows better prediction of lung hypoplasia through calculation of fetal lung volumes in the third trimester, if needed.4,5
Department of Radiology, Cincinnati Children's Hospital Medical Center, Cincinnati, OH. The objective of this article is to describe the imaging evaluation of fetal megacystis and discuss how a combined approach with US and fetal MRI can be beneficial. Address reprint requests to Maria A. Calvo-Garcia, MD, Department of Radiology, MLC 5031, Cincinnati Children's Hospital Medical Center, 3333 Burnet Ave, Cincinnati, OH. E-mail: [email protected]
http://dx.doi.org/10.1053/j.sult.2015.05.017 0887-2171/& 2015 Elsevier Inc. All rights reserved.
Imaging Checklist for Fetal Megacystis A systematic and organized approach to any imaging analysis enables collection of all the essential imaging clues and identification of the most likely diagnosis or at least the main differential considerations. The most important elements to be assessed in the presence of fetal megacystis are discussed.
Amniotic Fluid Volume US can quantify the amniotic fluid volume. A frequently used method is the evaluation of the single deepest vertical pocket free of fetal parts and umbilical cord. In normal conditions it measures between 2 and 8 cm.6 There is no standardized method for quantification of amniotic fluid volume with fetal MRI, but it allows a subjective overall assessment. After the first trimester, if there is persistent renal dysfunction or lower urinary tract obstruction, the amniotic fluid volume is expected to decrease. Preserved amniotic fluid volume can be seen in less severe forms of lower urinary tract obstruction, in functional megacystis (where no anatomical obstruction is present), and in cases of obstructive megacystis with a combined proximal gastrointestinal obstruction (Fig. 1).2 Alternatively, development of polyhydramnios in the third trimester has been described with megacystismicrocolon-intestinal hypoperistalsis syndrome, due to associated bowel dysmotility.7,8
Bladder US evaluation of the bladder in the setting of massive megacystis, tortuous hydroureters, or extrinsic masses can be 1
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Figure 1 A 29-week female fetus with esophageal atresia and bladder outlet obstruction in the setting of cloacal malformation and VACTERL association. Fetal MRI T2-weighted coronal oblique image shows fluid dilatation of the cervical esophagus (arrow), known as the pouch sign, which is consistent with esophageal atresia; this finding was noted only once during the entire examination and was undetected with US. The stomach was small but the amniotic fluid volume (asterisk) was normal due to an associated bladder outlet obstruction. The bladder is enlarged (b) and there is hydrocolpos (curved arrow). Radial ray deficiency (not shown) and congenital heart defect were also present.
challenging owing to anatomical distortion and limitations in differentiating adjacent bowel. Fetal MRI allows a better understanding of the fetal anatomy, the level of obstruction, and associated findings. The bladder is adjacent to the anterior abdominal wall and bordered on each side by the umbilical arteries (Fig. 2).2 If a pelvic or abdominopelvic cystic structure keeps this anatomical
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relationship with the umbilical arteries and the abdominal wall, it truly represents the bladder. Otherwise, the cystic lesion could represent a different structure, such as hydrocolpos (Fig. 3), enteric duplication cyst, bowel dilatation, meconium pseudocyst, lymphatic malformation, ovarian cyst, anterior sacral meningocele, or a type IV sacrococcygeal teratoma.9 The bladder configuration and content can provide diagnostic clues. Long-standing obstructive bladders may eventually develop wall thickening and trabeculation. In the presence of a dilated bladder and posterior urethra (known as the keyhole sign), posterior urethral obstruction should be considered (Fig. 4). Fetal MRI can help define more subtle or intermittent posterior urethral dilatation, as the examination allows visualization of the same anatomical area over a longer period of time (Fig. 5). Elongation of the bladder dome toward the abdominal cord insertion supports a patent urachus, potentially seen as a decompression pathway in the setting of severe bladder outlet obstruction (Fig. 6).10 Enlarged bladders with lobulated contours or with large diverticula have been reported in cloacal malformations, and in some cases they may have associated layering debris, which represents meconium and supports an abnormal fistulous connection with the colon (Fig. 7).11-13
Evaluation of the Large Bowel for Potential Underlying Anorectal Malformations or Microcolon Assessment of the bowel provides important clues in the context of genitourinary anomalies. Compared with US, fetal MRI is advantageous because it is able to easily track the course of the entire colon and rectum once it is distended with meconium. In normal circumstances, meconium displays a dark T2 and bright T1-weighted signal, starts accumulating in the distal rectum at 20-21 weeks, and fills the entire colon after 25 weeks.14-16 Subsequently, the content and caliber of the large bowel increase homogeneously during the third trimester.17 On a sagittal view, the rectum should be seen
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Figure 2 Normal fetal bladder with a 3-vessel cord. Axial color-Doppler US (A) and fetal MRI, balanced steady-state freeprecession (B) images. The umbilical arteries (arrows) run from the abdominal cord insertion (ACI) parallel to the lateral walls of the bladder (asterisk) to join the iliac arteries. (Color version of figure is available online.)
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Figure 3 A 18-week female fetus with cloacal malformation, 2-vessel cord, and absent amniotic fluid. (A) Coronal US image shows a hydrocolpos (arrow) simulating an enlarged bladder. The heart (H) has been labeled for anatomical reference. (B) Axial color-Doppler US image shows, in fact, a very small bladder (asterisk) anterior to a large hydrocolpos (arrow). The first and more anterior abdominal structure that the umbilical artery (curved arrow) outlines from the abdominal cord insertion (ACI) is the bladder. (Color version of figure is available online.)
immediately posterior and caudal to the bladder, and its length should be at least 10 mm from the bladder base to the perineum (Fig. 8).14 If the fetal rectal anatomy does not follow this pattern, it should immediately raise concern for an underlying bowel abnormality, such as an anorectal malformation or an impaired transfer of meconium to this region (as in a bowel obstruction).
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Anorectal malformations include a wide spectrum of anomalies in male and female fetuses affecting the anus, rectum, and genitourinary tract (Table 1).18 Two of these malformations, long common channel cloacas (in female fetus) and cloacal dysgenesis (in both genders), can present in utero with megacystis and lower urinary tract obstruction. In the case of long common channel cloacas, the urinary tract, vagina, and
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Figure 4 A 16-week male fetus with megacystis and severe oligohydramnios. (A) US coronal and (B) fetal MRI sagittal T2weighted images. There is dilatation of the bladder (asterisk) and posterior urethra (curved arrow), also called the keyhole sign. The main consideration at the time of the workup was posterior urethral valves, although urethral stenosis from other etiology or even atresia could not be excluded, given the severity of dilatation and early oligohydramnios. The family opted for no intervention and there was intrauterine fetal demise 3 weeks later.
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Figure 5 Posterior urethral valves with intermittent keyhole configuration of the fetal bladder. (A and B) Fetal MRI coronal T2-weighted images in a 28-week male fetus with megacystis. The posterior urethra (curved arrow) was barely detectable at the beginning of the examination (A) but eventually became dilated (B). (C) Sagittal T1-weighted image. Note normal meconium-filled rectum (arrow), posterior to the enlarged bladder (asterisk). (D) Lateral voiding cystourethrogram on the first day of life with a urethral catheter in place. The bladder (asterisk) is elongated and irregular in contour. The posterior urethra is dilated (curved arrow) and there is a linear filling defect consistent with urethral valves (dashed arrow) as well as incidental faint opacification of the prostatic utricle (arrowhead). Note also the presence of severe vesicoureteral reflux (black arrow).
rectum converge in a common channel longer than 3 cm that opens to the perineum. In cloacal dysgenesis, the bladder, rectum, and vagina (in female fetus) are connected or end blindly, there is no perineal opening, and the external genitalia are incompletely formed or abnormal.11,12,19 The depiction of these anomalies remains challenging with US. Findings described with these malformations are related to the presence and patency of fistulous connections between the colon and the genitourinary system. These signs include detection of a fluid-filled dilated rectum with or without calcified concretions of meconium, also known as enteroliths (Fig. 9), and presence of those calcifications within the genitourinary system.20,21 However, not all forms have a fistula and, if present, it might not be patent or large enough to be detectable with this technique. More recently, US analysis of the fetal anus has been reported. However, this evaluation can be impaired or
impossible when the amniotic fluid is severely decreased or absent or when the fetal anatomy is distorted.22,23 These challenging technical factors do not affect the bowel assessment with fetal MRI. In fact, during the second and third trimesters, this modality allows analysis of the size, signal, and length of the rectum, as well as the configuration and signal in the genitourinary system, and provides a more precise evaluation of anorectal malformations. Bowel findings described in the presence of long common channel cloacas and cloacal dysgenesis include a short and dilated rectum with normal or increased fluid signal characteristics, depending on the presence and degree of patency of the rectourinary fistula (Figs. 911).12,20 The absence of perineal openings in cloacal dysgenesis is only diagnosed with direct visual inspection. Another entity associated with fetal megacystis that has specific bowel findings is megacystis-microcolon-intestinal
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Figure 6 A 17-week female fetus with bladder outlet obstruction and patent urachus in the setting of cloacal malformation. (A) Sagittal US and (B) sagittal fetal MRI balanced steady-state free-precession images. Note the enlarged and elongated bladder (asterisk) that extends toward the abdominal cord insertion (dashed arrow), as expected with a patent urachus. There was no spinal dysraphic defect, but on fetal MRI the conus was noted to be low at the lumbosacral junction (arrow), concerning for tethered cord.
hypoperistalsis syndrome. In this condition, the rectum and colon are barely detectable by the time they are expected to be well distended with meconium, correlating with a microrectum and a microcolon.5
Internal and External Genitalia In normal circumstances, the fetal female internal genitalia cannot be visualized. However, in the presence of vaginal obstruction, urogenital sinus, or cloacal malformation, fluid distention of the genital tract, generically known as hydrocolpos, can develop. US can detect a hydrocolpos as a cystic mass posterior to the bladder and fetal MRI allows further characterization. Fetal megacystis and hydrocolpos can be seen in long common channel cloacas with a high and dilated rectum (Fig. 10).24 A normal to small size bladder
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and hydrocolpos can also be seen with vaginal obstruction and urogenital sinus, a malformation defined by the presence of a single exit for the bladder and genital tract. In both conditions the rectum is anterior to the sacrum, normal in length and not dilated (Fig. 12).25 Isolated hydrocolpos due to vaginal obstruction is not expected to be associated with obstructive uropathy.26 However, urogenital sinuses with stenotic outflow present a continuous backflow of urine from the bladder into the genital tract leading to progressive hydrocolpos that can compress the trigone and result in hydroureteronephrosis.27 Some fetuses already have a known karyotype at the time of the imaging referral, but not all. Evaluation of the external genitalia helps provide a gender-specific differential diagnosis. In addition, the detection of ambiguous or incompletely formed external genitalia should raise awareness of the
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Figure 7 Megacystis containing meconium in cloacal malformation. (A) Axial US image of the bladder in a 26-week female fetus. There is large amount of layering echogenic content (curved arrow). (B) Fetal MRI T2-weighted axial abdomen image of the same fetus at 29 weeks. The bladder remains massively enlarged and thick walled with layering lower T2 signal content (curved arrow). Note also abnormal fluid signal in the left colon (arrow). (C) Neonatal US axial image of the bladder with corresponding findings.
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Figure 8 Normal sagittal fetal MRI anatomy of the rectum after 20 weeks' gestational age. (A) Sagittal T1-weighted image shows bright meconium-filled rectum (arrow) in a 26-week female fetus. (B) Sagittal T2-weighted image of the rectum (arrow) at 34 weeks. Note in both images that the rectum extends caudal to the bladder (asterisk) and increases homogeneously in size with gestational age.
potential for an underlying more complex genitourinary malformation (Fig. 12).11,28
Kidneys The depiction of the kidneys with US is satisfactory under standard conditions. However, it can be more challenging in
the presence of oligohydramnios. Higher frequency transducers can help improve renal visualization, but they can be used only in specific conditions such as in patients without an anterior placenta or a large body habitus. Cystic renal dysplasia can develop in utero because of severe obstruction. High-grade obstruction could lead to spontaneous decompression of the bladder though rupture of the
Table 1 Classification of Anorectal Malformations by Gender Females
Rectoperineal fistula Rectovestibular fistula Anal stenosis Rectal atresia Imperforate anus without fistula Cloacal malformations with short common channel (measuring o3 cm from the perineum to first bifurcation) Cloacal malformations with long common channel (measuring 43 cm from the perineum to first bifurcation)
Males
Rectoperineal fistula Anal stenosis Rectal atresia Imperforate anus without fistula Rectourethral (bulbar) fistula Rectourethral (prostatic) fistula Rectovesical fistula
Complex defects (both genders)
Cloacal dysgenesis (also known as urorectal septum malformation) Cloacal exstrophy Covered cloacal exstrophy
Adapted from Levitt and Peña.18
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Figure 9 A 20-week female fetus with cloacal malformation. (A) Sagittal US image shows megacystis (asterisk) and adjacent dilated distal colon (arrow). The detection of a few subtle echogenic foci within the distal bowel (dashed arrow) was concerning for enteroliths. Fetal MRI coronal T2-weighted (B) and T1-weighted (C) images. The dilated bowel seen on US correlates with a dilated blind-ending distal colon, as expected for an anorectal malformation (arrow). In this case, the signal content in that dilated bowel had no obvious increased fluid content. The combined use of both techniques helped provide a confident diagnosis of cloacal malformation.
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Figure 10 A 29-week female fetus with a long common channel cloaca. Fetal MRI sagittal T2-weighted midline (A) and right parasagittal (B) images show megacystis (asterisk) anterior to a very large hydrocolpos (curved arrow) with layering lower T2 signal content (arrowhead) and ascites (a). The most distal bowel (arrow) is dilated, located very high in the abdomen, and contains abnormally increased T2 signal, which supports increased fluid content and cloacal malformation.
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Figure 11 A 21-week male fetus with cloacal dysgenesis. (A) Fetal MRI sagittal T2-weighted image shows megacystis (asterisk) and multicystic dysplastic appearance of the left kidney (LK). In the setting of severe oligohydramnios, the chest (curved arrow) is small and the lower limb (L) abuts the anterior abdominal wall. (B) Right parasagittal oblique T1-weighted image shows an abnormally high and dilated rectum (arrow) with normal signal. The right kidney (RK) had a multicystic dysplastic appearance as well. Urethral atresia and blind-ending colon connected to the obstructed bladder in conjunction with a smooth perineum and abnormal genitalia were found at necropsy.
urinary tract with subsequent development of perinephric urinomas and urinary ascites.29 In female fetuses, ascites can also be seen in the context of a cloacal malformation (Fig. 10) and urogenital sinus (Fig. 12). Urine and meconium in the former or urine in the latter could reach the peritoneal space through the genital tract, leading to hydrocolpos and intraperitoneal fluid.24 In the context of any obstructive uropathy, the kidneys may become echogenic and eventually develop
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cortical cysts.30 In early stages, these cystic changes are detected only (or better) with fetal MRI (Fig. 13).
Spine Careful evaluation of the spine should be performed with both techniques, although fetal MRI allows a more precise assessment. Spinal dysraphic defects, segmentation anomalies, and
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Figure 12 Fetal MRI on a 27-week female fetus with urogenital sinus. (A) Sagittal T2-weighted image. There is a partially septated hydrometrocolpos (curved arrow) posterior and superior to a normal size bladder (asterisk), ascites (a) and ambiguous genitalia (arrowhead). (B) Sagittal T1-weighted image shows a normal rectum (arrow) posterior to the hydrometrocolpos (curved arrow).
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Figure 13 US and fetal MRI images in a 20-week male fetus with lower urinary tract obstruction and cystic renal dysplasia. (A) Axial US image through the upper abdomen shows an enlarged bladder (curved arrow) and absent amniotic fluid. There is associated bilateral urinary tract dilatation (asterisk) and small cortical cysts in the left kidney (arrow) consistent with cystic renal dysplasia. Corresponding fetal MRI axial (B) and coronal (C) T2-weighted images depict bilateral cystic renal dysplasia. There are multiple abnormal high-T2-signal renal foci consistent with small bilateral renal cysts.
tethered cord (Fig. 6) can be seen in the context of an underlying anorectal malformation and could also point toward a syndromic association, such as VACTERL.12,31
Differential Diagnosis of Fetal Megacystis Megacystis can be seen because of lower urinary tract obstructions. However, in other cases the underlying problem is not a congenital malformation but bladder dysfunction (Table 2).2,32 The clues collected during the imaging analysis help determine whether the megacystis is due to obstruction or dysfunction and which specific diagnosis or main differential considerations should be entertained.
Obstructive Megacystis When a lower urinary tract obstruction is severe enough, oligohydramnios is expected to develop, unless it is balanced by the effect of a combined upper gastrointestinal obstruction such as esophageal atresia in VACTERL association (Fig. 1).12 Obstructive megacystis in male fetuses is frequently owing to posterior urethral valves.33 Alternative diagnoses include urethral stenosis, and with a much earlier presentation, urethral atresia (Figs. 4 and 5).11,34 In females, obstructive megacystis can be seen with urethral stenosis or atresia and with cloacal malformations (Figs. 6, 7, 9, and 10). In both male and female fetuses, bladder outlet obstruction can be seen with cloacal dysgenesis (Fig. 11), ectopic or prolapsed ureteroceles, and pelvic masses such as type III-IV sacrococcygeal teratomas (Fig. 14).11,12,19,35 In all these scenarios, fetal MRI analysis of
Table 2 Differential Considerations for Fetal Megacystis2 Megacystis with anatomical obstruction: Urethral obstruction Stenosis or atresia (♂, ♀) Posterior urethral valves (♂) Anorectal malformations (♂, ♀) Extrinsic or intrinsic “masses” Sacrococcygeal teratoma (♂, ♀) Prolapsed ureterocele (♂, ♀) Hydrocolpos: cloaca (♀) Megacystis without anatomical obstruction: Prune-belly syndrome* (4♂) Megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS) 4polyhydramnios after 30 weeks, (4♀) Megacystis-megaureter association (4♂) The gender distribution is described in parentheses. ♀, female; ♂, male; 4, more frequent. *Prune-belly syndrome is also known as triad syndrome and Eagle-Barret syndrome.
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megacystis-microcolon-intestinal hypoperistalsis syndrome (Fig. 16), and megacystis-megaureter association (Fig. 17). In most of these scenarios the amniotic fluid volume is normal. However, polyhydramnios is frequently encountered during the third trimester in megacystis-microcolon-intestinal hypoperistalsis syndrome.2,7,36 The underlying etiology of megacystis is massive vesicoureteral reflux in megacystis-megaureter association, and bladder and bowel dysfunction in megacystis-microcolon-intestinal hypoperistalsis syndrome.7,37 Voiding dysfunction and vesicoureteral reflux can also be seen with prune-belly syndrome, although it is suspected to have a connection with transient lower urinary tract obstructions in utero. The abdominal wall is lax and protuberant due to deficient development of the abdominal musculature and in male fetuses the testes are frequently undescended.38 Fetal MRI helps to define the appearance of the colon and rectum, expected to be normal in prune-belly syndrome and in megacystis-megaureter association. Alternatively, it can display the very characteristic findings of microcolon and microrectum in megacystis-microcolon-intestinal hypoperistalsis syndrome (Table 4).5,16
Figure 14 A 25-week fetus with type-III sacrococcygeal teratoma (arrows). Fetal MRI sagittal T2-weighted image. The bladder (asterisk) is enlarged and superiorly displaced by the mass. There is severe oligohydramnios supporting bladder outlet obstruction.
Conclusions There are several diagnostic considerations in the setting of fetal megacystis. Estimation of amniotic fluid volume guides the differential diagnostic considerations that include obstructive vs functional etiologies. A subsequent detailed anatomical analysis of the fetal genitourinary system and adjacent bowel should be obtained. This systematic approach and combined use of US and MRI can provide a more confident and specific fetal diagnosis.
the rectum and adjacent genitourinary tract allows improved differentiation between the disorders (Table 3).
Functional Megacystis Megacystis may occur in the absence of an obstructive malformation, such as in prune-belly syndrome (Fig. 15), Table 3 Fetal MRI Findings in Lower Urinary Tract Obstructions Lower Urinary Tract Short/Dilated Rectum Obstruction
Abnormal Signal in Bladder/ Rectum
Megacystis
Urethral stenosis or No atresia (♀, ♂) Posterior urethral valves No (♂) Anorectal Yes malformations (♂, ♀)
No
Yes
No
Yes
Possible: Meconium in bladder ↑T1, ↓T2 Fluid in the rectum ↓T1, ↑T2
Possible
No
Possible
No
Possible
Possible: Meconium in bladder or hydrocolpos (↑T1, ↓T2 layering content), Fluid in the rectum (↓T1, ↑T2 signal) No
Possible
Extrinsic or intrinsic “masses” Sacrococcygeal No (due to mass effect, anteriorly teratoma (♀, ♂) displaced or compressed) Prolapsed ureterocele No (♀, ♂) Hydrocolpos (♀): cloaca Yes (long common channel cloaca)
Hydrocolpos (♀): urogenital sinus
No (rectum is seen posterior to the hydrocolpos)
Gender distribution is described in parentheses. ♀, female; ♂, male.
Hydroureteronephrosis but no megacystis
Imaging evaluation of fetal megacystis
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Figure 15 Prenatal findings and postnatal correlation in prune-belly syndrome. (A-C) Fetal MRI balanced steady-state freeprecession axial images through the abdomen and pelvis in a 31-week male fetus with normal amniotic fluid volume. The abdominal wall is protuberant (arrowheads) and the bladder is enlarged (asterisk). Both upper ureters were dilated (not shown) with more prominent distal hydroureter noted on the right (arrow). The scrotum is empty (curved arrow). (D) Abdomen x-ray image shows the characteristic bulging flanks (especially on the right) and a distended bladder (asterisk) displacing the adjacent bowel. (E) Lateral voiding cystourethrogram shows a large bladder capacity and failure to spontaneously void. Voiding dysfunction requires intermittent catheterization in this patient.
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Figure 16 Megacystis-microcolon-intestinal hypoperistalsis syndrome. (A) Fetal MRI axial T2-weighted image of the upper abdomen in a 19-week female fetus. There is an enlarged bladder (b) and bilateral urinary tract dilatation (dashed arrows). The amniotic fluid (asterisk) volume is normal. (B) Lateral cystourethrogram and (C) frontal contrast enema on the third day of life (the baby failed to pass meconium). There is an atonic large-capacity bladder (b) with absent spontaneous voiding. Contrast enema after bladder drainage shows microcolon with centralized position in the abdomen consistent with malrotation. Note also minimal aeration of the proximal bowel in the right upper quadrant (arrow). Midgut malrotation and severe gastroesophageal reflux were confirmed on subsequent upper gastrointestinal series.
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Figure 17 A 29-week male fetus with megacystis-megaureter association and postnatal correlation. (A) Fetal MRI coronal T2-weighted and (B) sagittal T1-weighted images. There is bilateral urinary tract dilatation (dashed arrows), an enlarged bladder (b), normal rectum (arrow), and adequate amount of amniotic fluid (asterisk). (C and D) Frontal and lateral voiding cystourethrogram in the first day of life. There is a large-capacity bladder (b). Vesicoureteral reflux (dashed arrows) started as soon as the contrast entered the bladder and was severe. The urethra is normal.
Table 4 Fetal MRI Findings in the Setting of Impaired Bladder Emptying Without an Obstructive Anatomical Etiology Functional Megacystis
Short/Dilated Rectum
Abnormal Signal in Bladder/Rectum
Megacystis Microcolon
Prune-belly syndrome (4♂) Megacystis-microcolon-intestinal hypoperistalsis syndrome (4♀) Megacystis-megaureter association (4♂)
No No, in fact, microrectum! No
No No
Yes Yes
No Yes
No
Yes
No
Gender distribution is in parentheses. ♀, female; ♂, male; 4, more frequent.
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high-risk population. Ultrasound Obstet Gynecol 39:521-527, 2012 Shono T, Taguchi T, Suita S, et al: Prenatal ultrasonographic and magnetic resonance imaging findings of congenital cloacal anomalies associated with meconium peritonitis. J Pediatr Surg 42:681-684, 2007 Capito C, Belarbi N, Paye Jaouen A, et al: Prenatal pelvic MRI: Additional clues for assessment of urogenital obstructive anomalies. J Pediatr Urol 10:162-166, 2014 Picone O, Laperelle J, Sonigo P, et al: Fetal magnetic resonance imaging in the antenatal diagnosis and management of hydrocolpos. Ultrasound Obstet Gynecol 30:105-109, 2007 Pauleta J, Melo MA, Borges G, et al: Prenatal diagnosis of persistent urogenital sinus with duplicated hydrometrocolpos and ascites—A case report. Fetal Diagn Ther 28:229-232, 2010 Calvo-Garcia MA, Kline-Fath BM, Rubio EI, et al: Fetal MRI of cloacal exstrophy. Pediatr Radiol 43:593-604, 2013 Aslan AR, Kogan BA: The effect of bladder outlet obstruction on the developing kidney. BJU Int 92(suppl 1):38-41, 2003 Carr MC, Kim SS: Prenatal management of urogenital disorders. Urol clin North Am 37:149-158, 2010 Alamo L, Meyrat BJ, Meuwly JY, et al: Anorectal malformations: Finding the pathway out of the Labyrinth. Radiographics 33:491-512, 2013 Lissauer D, Morris RK, Kilby MD: Fetal lower urinary tract obstruction. Semin Fetal Neonat Med 12:464-470, 2007 Quintero RA, Johnson MP, Romero R, et al: In-utero percutaneous cystoscopy in the management of fetal lower obstructive uropathy. Lancet 346:537-540, 1995 Robyr R, Benachi A, Daikha-Dahmane F, et al: Correlation between ultrasound and anatomical findings in fetuses with lower urinary tract obstruction in the first half of pregnancy. Ultrasound Obstet Gynecol 25:478-482, 2005 Hedrick HL, Flake AW, Crombleholme TM, et al: Sacrococcygeal teratoma: Prenatal assessment, fetal intervention, and outcome. J Pediatr Surg 39:430-438, 2004. [discussion 430-438] Pinette MG, Blackstone J, Wax JR, et al: Enlarged fetal bladder: Differential diagnosis and outcomes. J Clin Ultrasound 31:328-334, 2003 Mandell J, Lebowitz RL, Peters CA, et al: Prenatal diagnosis of the megacystis-megaureter association. J Urol 148:1487-1489, 1992 Herman TE, Siegel MJ: Special imaging casebook. Prune-belly syndrome with urachal diverticular calcification, posterior urethral valves, and patent utricle. J Perinatol 19:610-612, 1999
Introduction
M
egacystis is the term used to describe an enlarged bladder at any gestational age. During the second and third trimesters, this diagnosis is based on an overall subjective assessment and visualization of a persistently enlarged bladder. However, in the first trimester, specific size criteria have been defined.1,2 The fetal bladder is first seen at approximately 10-12 weeks and its diameter should be no more than 68 mm. If the bladder is not seen by 15 weeks or if it is persistently enlarged (especially beyond 15 mm), concern for an underlying genitourinary malformation should be raised and further detailed evaluation during the second trimester is warranted.3 This assessment typically includes a detailed ultrasound (US) of the kidneys, bladder, external genitalia, and amniotic fluid volume. The US may be complemented with fetal magnetic resonance imaging (MRI), which improves the evaluation of the kidneys with better detection of renal ectopia and cystic dysplasia. It also helps detect underlying anorectal malformations and allows better prediction of lung hypoplasia through calculation of fetal lung volumes in the third trimester, if needed.4,5
Department of Radiology, Cincinnati Children's Hospital Medical Center, Cincinnati, OH. The objective of this article is to describe the imaging evaluation of fetal megacystis and discuss how a combined approach with US and fetal MRI can be beneficial. Address reprint requests to Maria A. Calvo-Garcia, MD, Department of Radiology, MLC 5031, Cincinnati Children's Hospital Medical Center, 3333 Burnet Ave, Cincinnati, OH. E-mail: [email protected]
http://dx.doi.org/10.1053/j.sult.2015.05.017 0887-2171/& 2015 Elsevier Inc. All rights reserved.
Imaging Checklist for Fetal Megacystis A systematic and organized approach to any imaging analysis enables collection of all the essential imaging clues and identification of the most likely diagnosis or at least the main differential considerations. The most important elements to be assessed in the presence of fetal megacystis are discussed.
Amniotic Fluid Volume US can quantify the amniotic fluid volume. A frequently used method is the evaluation of the single deepest vertical pocket free of fetal parts and umbilical cord. In normal conditions it measures between 2 and 8 cm.6 There is no standardized method for quantification of amniotic fluid volume with fetal MRI, but it allows a subjective overall assessment. After the first trimester, if there is persistent renal dysfunction or lower urinary tract obstruction, the amniotic fluid volume is expected to decrease. Preserved amniotic fluid volume can be seen in less severe forms of lower urinary tract obstruction, in functional megacystis (where no anatomical obstruction is present), and in cases of obstructive megacystis with a combined proximal gastrointestinal obstruction (Fig. 1).2 Alternatively, development of polyhydramnios in the third trimester has been described with megacystismicrocolon-intestinal hypoperistalsis syndrome, due to associated bowel dysmotility.7,8
Bladder US evaluation of the bladder in the setting of massive megacystis, tortuous hydroureters, or extrinsic masses can be 1
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Figure 1 A 29-week female fetus with esophageal atresia and bladder outlet obstruction in the setting of cloacal malformation and VACTERL association. Fetal MRI T2-weighted coronal oblique image shows fluid dilatation of the cervical esophagus (arrow), known as the pouch sign, which is consistent with esophageal atresia; this finding was noted only once during the entire examination and was undetected with US. The stomach was small but the amniotic fluid volume (asterisk) was normal due to an associated bladder outlet obstruction. The bladder is enlarged (b) and there is hydrocolpos (curved arrow). Radial ray deficiency (not shown) and congenital heart defect were also present.
challenging owing to anatomical distortion and limitations in differentiating adjacent bowel. Fetal MRI allows a better understanding of the fetal anatomy, the level of obstruction, and associated findings. The bladder is adjacent to the anterior abdominal wall and bordered on each side by the umbilical arteries (Fig. 2).2 If a pelvic or abdominopelvic cystic structure keeps this anatomical
A
relationship with the umbilical arteries and the abdominal wall, it truly represents the bladder. Otherwise, the cystic lesion could represent a different structure, such as hydrocolpos (Fig. 3), enteric duplication cyst, bowel dilatation, meconium pseudocyst, lymphatic malformation, ovarian cyst, anterior sacral meningocele, or a type IV sacrococcygeal teratoma.9 The bladder configuration and content can provide diagnostic clues. Long-standing obstructive bladders may eventually develop wall thickening and trabeculation. In the presence of a dilated bladder and posterior urethra (known as the keyhole sign), posterior urethral obstruction should be considered (Fig. 4). Fetal MRI can help define more subtle or intermittent posterior urethral dilatation, as the examination allows visualization of the same anatomical area over a longer period of time (Fig. 5). Elongation of the bladder dome toward the abdominal cord insertion supports a patent urachus, potentially seen as a decompression pathway in the setting of severe bladder outlet obstruction (Fig. 6).10 Enlarged bladders with lobulated contours or with large diverticula have been reported in cloacal malformations, and in some cases they may have associated layering debris, which represents meconium and supports an abnormal fistulous connection with the colon (Fig. 7).11-13
Evaluation of the Large Bowel for Potential Underlying Anorectal Malformations or Microcolon Assessment of the bowel provides important clues in the context of genitourinary anomalies. Compared with US, fetal MRI is advantageous because it is able to easily track the course of the entire colon and rectum once it is distended with meconium. In normal circumstances, meconium displays a dark T2 and bright T1-weighted signal, starts accumulating in the distal rectum at 20-21 weeks, and fills the entire colon after 25 weeks.14-16 Subsequently, the content and caliber of the large bowel increase homogeneously during the third trimester.17 On a sagittal view, the rectum should be seen
B
Figure 2 Normal fetal bladder with a 3-vessel cord. Axial color-Doppler US (A) and fetal MRI, balanced steady-state freeprecession (B) images. The umbilical arteries (arrows) run from the abdominal cord insertion (ACI) parallel to the lateral walls of the bladder (asterisk) to join the iliac arteries. (Color version of figure is available online.)
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Figure 3 A 18-week female fetus with cloacal malformation, 2-vessel cord, and absent amniotic fluid. (A) Coronal US image shows a hydrocolpos (arrow) simulating an enlarged bladder. The heart (H) has been labeled for anatomical reference. (B) Axial color-Doppler US image shows, in fact, a very small bladder (asterisk) anterior to a large hydrocolpos (arrow). The first and more anterior abdominal structure that the umbilical artery (curved arrow) outlines from the abdominal cord insertion (ACI) is the bladder. (Color version of figure is available online.)
immediately posterior and caudal to the bladder, and its length should be at least 10 mm from the bladder base to the perineum (Fig. 8).14 If the fetal rectal anatomy does not follow this pattern, it should immediately raise concern for an underlying bowel abnormality, such as an anorectal malformation or an impaired transfer of meconium to this region (as in a bowel obstruction).
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Anorectal malformations include a wide spectrum of anomalies in male and female fetuses affecting the anus, rectum, and genitourinary tract (Table 1).18 Two of these malformations, long common channel cloacas (in female fetus) and cloacal dysgenesis (in both genders), can present in utero with megacystis and lower urinary tract obstruction. In the case of long common channel cloacas, the urinary tract, vagina, and
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Figure 4 A 16-week male fetus with megacystis and severe oligohydramnios. (A) US coronal and (B) fetal MRI sagittal T2weighted images. There is dilatation of the bladder (asterisk) and posterior urethra (curved arrow), also called the keyhole sign. The main consideration at the time of the workup was posterior urethral valves, although urethral stenosis from other etiology or even atresia could not be excluded, given the severity of dilatation and early oligohydramnios. The family opted for no intervention and there was intrauterine fetal demise 3 weeks later.
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Figure 5 Posterior urethral valves with intermittent keyhole configuration of the fetal bladder. (A and B) Fetal MRI coronal T2-weighted images in a 28-week male fetus with megacystis. The posterior urethra (curved arrow) was barely detectable at the beginning of the examination (A) but eventually became dilated (B). (C) Sagittal T1-weighted image. Note normal meconium-filled rectum (arrow), posterior to the enlarged bladder (asterisk). (D) Lateral voiding cystourethrogram on the first day of life with a urethral catheter in place. The bladder (asterisk) is elongated and irregular in contour. The posterior urethra is dilated (curved arrow) and there is a linear filling defect consistent with urethral valves (dashed arrow) as well as incidental faint opacification of the prostatic utricle (arrowhead). Note also the presence of severe vesicoureteral reflux (black arrow).
rectum converge in a common channel longer than 3 cm that opens to the perineum. In cloacal dysgenesis, the bladder, rectum, and vagina (in female fetus) are connected or end blindly, there is no perineal opening, and the external genitalia are incompletely formed or abnormal.11,12,19 The depiction of these anomalies remains challenging with US. Findings described with these malformations are related to the presence and patency of fistulous connections between the colon and the genitourinary system. These signs include detection of a fluid-filled dilated rectum with or without calcified concretions of meconium, also known as enteroliths (Fig. 9), and presence of those calcifications within the genitourinary system.20,21 However, not all forms have a fistula and, if present, it might not be patent or large enough to be detectable with this technique. More recently, US analysis of the fetal anus has been reported. However, this evaluation can be impaired or
impossible when the amniotic fluid is severely decreased or absent or when the fetal anatomy is distorted.22,23 These challenging technical factors do not affect the bowel assessment with fetal MRI. In fact, during the second and third trimesters, this modality allows analysis of the size, signal, and length of the rectum, as well as the configuration and signal in the genitourinary system, and provides a more precise evaluation of anorectal malformations. Bowel findings described in the presence of long common channel cloacas and cloacal dysgenesis include a short and dilated rectum with normal or increased fluid signal characteristics, depending on the presence and degree of patency of the rectourinary fistula (Figs. 911).12,20 The absence of perineal openings in cloacal dysgenesis is only diagnosed with direct visual inspection. Another entity associated with fetal megacystis that has specific bowel findings is megacystis-microcolon-intestinal
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Figure 6 A 17-week female fetus with bladder outlet obstruction and patent urachus in the setting of cloacal malformation. (A) Sagittal US and (B) sagittal fetal MRI balanced steady-state free-precession images. Note the enlarged and elongated bladder (asterisk) that extends toward the abdominal cord insertion (dashed arrow), as expected with a patent urachus. There was no spinal dysraphic defect, but on fetal MRI the conus was noted to be low at the lumbosacral junction (arrow), concerning for tethered cord.
hypoperistalsis syndrome. In this condition, the rectum and colon are barely detectable by the time they are expected to be well distended with meconium, correlating with a microrectum and a microcolon.5
Internal and External Genitalia In normal circumstances, the fetal female internal genitalia cannot be visualized. However, in the presence of vaginal obstruction, urogenital sinus, or cloacal malformation, fluid distention of the genital tract, generically known as hydrocolpos, can develop. US can detect a hydrocolpos as a cystic mass posterior to the bladder and fetal MRI allows further characterization. Fetal megacystis and hydrocolpos can be seen in long common channel cloacas with a high and dilated rectum (Fig. 10).24 A normal to small size bladder
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and hydrocolpos can also be seen with vaginal obstruction and urogenital sinus, a malformation defined by the presence of a single exit for the bladder and genital tract. In both conditions the rectum is anterior to the sacrum, normal in length and not dilated (Fig. 12).25 Isolated hydrocolpos due to vaginal obstruction is not expected to be associated with obstructive uropathy.26 However, urogenital sinuses with stenotic outflow present a continuous backflow of urine from the bladder into the genital tract leading to progressive hydrocolpos that can compress the trigone and result in hydroureteronephrosis.27 Some fetuses already have a known karyotype at the time of the imaging referral, but not all. Evaluation of the external genitalia helps provide a gender-specific differential diagnosis. In addition, the detection of ambiguous or incompletely formed external genitalia should raise awareness of the
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Figure 7 Megacystis containing meconium in cloacal malformation. (A) Axial US image of the bladder in a 26-week female fetus. There is large amount of layering echogenic content (curved arrow). (B) Fetal MRI T2-weighted axial abdomen image of the same fetus at 29 weeks. The bladder remains massively enlarged and thick walled with layering lower T2 signal content (curved arrow). Note also abnormal fluid signal in the left colon (arrow). (C) Neonatal US axial image of the bladder with corresponding findings.
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Figure 8 Normal sagittal fetal MRI anatomy of the rectum after 20 weeks' gestational age. (A) Sagittal T1-weighted image shows bright meconium-filled rectum (arrow) in a 26-week female fetus. (B) Sagittal T2-weighted image of the rectum (arrow) at 34 weeks. Note in both images that the rectum extends caudal to the bladder (asterisk) and increases homogeneously in size with gestational age.
potential for an underlying more complex genitourinary malformation (Fig. 12).11,28
Kidneys The depiction of the kidneys with US is satisfactory under standard conditions. However, it can be more challenging in
the presence of oligohydramnios. Higher frequency transducers can help improve renal visualization, but they can be used only in specific conditions such as in patients without an anterior placenta or a large body habitus. Cystic renal dysplasia can develop in utero because of severe obstruction. High-grade obstruction could lead to spontaneous decompression of the bladder though rupture of the
Table 1 Classification of Anorectal Malformations by Gender Females
Rectoperineal fistula Rectovestibular fistula Anal stenosis Rectal atresia Imperforate anus without fistula Cloacal malformations with short common channel (measuring o3 cm from the perineum to first bifurcation) Cloacal malformations with long common channel (measuring 43 cm from the perineum to first bifurcation)
Males
Rectoperineal fistula Anal stenosis Rectal atresia Imperforate anus without fistula Rectourethral (bulbar) fistula Rectourethral (prostatic) fistula Rectovesical fistula
Complex defects (both genders)
Cloacal dysgenesis (also known as urorectal septum malformation) Cloacal exstrophy Covered cloacal exstrophy
Adapted from Levitt and Peña.18
Imaging evaluation of fetal megacystis
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Figure 9 A 20-week female fetus with cloacal malformation. (A) Sagittal US image shows megacystis (asterisk) and adjacent dilated distal colon (arrow). The detection of a few subtle echogenic foci within the distal bowel (dashed arrow) was concerning for enteroliths. Fetal MRI coronal T2-weighted (B) and T1-weighted (C) images. The dilated bowel seen on US correlates with a dilated blind-ending distal colon, as expected for an anorectal malformation (arrow). In this case, the signal content in that dilated bowel had no obvious increased fluid content. The combined use of both techniques helped provide a confident diagnosis of cloacal malformation.
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Figure 10 A 29-week female fetus with a long common channel cloaca. Fetal MRI sagittal T2-weighted midline (A) and right parasagittal (B) images show megacystis (asterisk) anterior to a very large hydrocolpos (curved arrow) with layering lower T2 signal content (arrowhead) and ascites (a). The most distal bowel (arrow) is dilated, located very high in the abdomen, and contains abnormally increased T2 signal, which supports increased fluid content and cloacal malformation.
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Figure 11 A 21-week male fetus with cloacal dysgenesis. (A) Fetal MRI sagittal T2-weighted image shows megacystis (asterisk) and multicystic dysplastic appearance of the left kidney (LK). In the setting of severe oligohydramnios, the chest (curved arrow) is small and the lower limb (L) abuts the anterior abdominal wall. (B) Right parasagittal oblique T1-weighted image shows an abnormally high and dilated rectum (arrow) with normal signal. The right kidney (RK) had a multicystic dysplastic appearance as well. Urethral atresia and blind-ending colon connected to the obstructed bladder in conjunction with a smooth perineum and abnormal genitalia were found at necropsy.
urinary tract with subsequent development of perinephric urinomas and urinary ascites.29 In female fetuses, ascites can also be seen in the context of a cloacal malformation (Fig. 10) and urogenital sinus (Fig. 12). Urine and meconium in the former or urine in the latter could reach the peritoneal space through the genital tract, leading to hydrocolpos and intraperitoneal fluid.24 In the context of any obstructive uropathy, the kidneys may become echogenic and eventually develop
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cortical cysts.30 In early stages, these cystic changes are detected only (or better) with fetal MRI (Fig. 13).
Spine Careful evaluation of the spine should be performed with both techniques, although fetal MRI allows a more precise assessment. Spinal dysraphic defects, segmentation anomalies, and
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Figure 12 Fetal MRI on a 27-week female fetus with urogenital sinus. (A) Sagittal T2-weighted image. There is a partially septated hydrometrocolpos (curved arrow) posterior and superior to a normal size bladder (asterisk), ascites (a) and ambiguous genitalia (arrowhead). (B) Sagittal T1-weighted image shows a normal rectum (arrow) posterior to the hydrometrocolpos (curved arrow).
Imaging evaluation of fetal megacystis
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Figure 13 US and fetal MRI images in a 20-week male fetus with lower urinary tract obstruction and cystic renal dysplasia. (A) Axial US image through the upper abdomen shows an enlarged bladder (curved arrow) and absent amniotic fluid. There is associated bilateral urinary tract dilatation (asterisk) and small cortical cysts in the left kidney (arrow) consistent with cystic renal dysplasia. Corresponding fetal MRI axial (B) and coronal (C) T2-weighted images depict bilateral cystic renal dysplasia. There are multiple abnormal high-T2-signal renal foci consistent with small bilateral renal cysts.
tethered cord (Fig. 6) can be seen in the context of an underlying anorectal malformation and could also point toward a syndromic association, such as VACTERL.12,31
Differential Diagnosis of Fetal Megacystis Megacystis can be seen because of lower urinary tract obstructions. However, in other cases the underlying problem is not a congenital malformation but bladder dysfunction (Table 2).2,32 The clues collected during the imaging analysis help determine whether the megacystis is due to obstruction or dysfunction and which specific diagnosis or main differential considerations should be entertained.
Obstructive Megacystis When a lower urinary tract obstruction is severe enough, oligohydramnios is expected to develop, unless it is balanced by the effect of a combined upper gastrointestinal obstruction such as esophageal atresia in VACTERL association (Fig. 1).12 Obstructive megacystis in male fetuses is frequently owing to posterior urethral valves.33 Alternative diagnoses include urethral stenosis, and with a much earlier presentation, urethral atresia (Figs. 4 and 5).11,34 In females, obstructive megacystis can be seen with urethral stenosis or atresia and with cloacal malformations (Figs. 6, 7, 9, and 10). In both male and female fetuses, bladder outlet obstruction can be seen with cloacal dysgenesis (Fig. 11), ectopic or prolapsed ureteroceles, and pelvic masses such as type III-IV sacrococcygeal teratomas (Fig. 14).11,12,19,35 In all these scenarios, fetal MRI analysis of
Table 2 Differential Considerations for Fetal Megacystis2 Megacystis with anatomical obstruction: Urethral obstruction Stenosis or atresia (♂, ♀) Posterior urethral valves (♂) Anorectal malformations (♂, ♀) Extrinsic or intrinsic “masses” Sacrococcygeal teratoma (♂, ♀) Prolapsed ureterocele (♂, ♀) Hydrocolpos: cloaca (♀) Megacystis without anatomical obstruction: Prune-belly syndrome* (4♂) Megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS) 4polyhydramnios after 30 weeks, (4♀) Megacystis-megaureter association (4♂) The gender distribution is described in parentheses. ♀, female; ♂, male; 4, more frequent. *Prune-belly syndrome is also known as triad syndrome and Eagle-Barret syndrome.
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megacystis-microcolon-intestinal hypoperistalsis syndrome (Fig. 16), and megacystis-megaureter association (Fig. 17). In most of these scenarios the amniotic fluid volume is normal. However, polyhydramnios is frequently encountered during the third trimester in megacystis-microcolon-intestinal hypoperistalsis syndrome.2,7,36 The underlying etiology of megacystis is massive vesicoureteral reflux in megacystis-megaureter association, and bladder and bowel dysfunction in megacystis-microcolon-intestinal hypoperistalsis syndrome.7,37 Voiding dysfunction and vesicoureteral reflux can also be seen with prune-belly syndrome, although it is suspected to have a connection with transient lower urinary tract obstructions in utero. The abdominal wall is lax and protuberant due to deficient development of the abdominal musculature and in male fetuses the testes are frequently undescended.38 Fetal MRI helps to define the appearance of the colon and rectum, expected to be normal in prune-belly syndrome and in megacystis-megaureter association. Alternatively, it can display the very characteristic findings of microcolon and microrectum in megacystis-microcolon-intestinal hypoperistalsis syndrome (Table 4).5,16
Figure 14 A 25-week fetus with type-III sacrococcygeal teratoma (arrows). Fetal MRI sagittal T2-weighted image. The bladder (asterisk) is enlarged and superiorly displaced by the mass. There is severe oligohydramnios supporting bladder outlet obstruction.
Conclusions There are several diagnostic considerations in the setting of fetal megacystis. Estimation of amniotic fluid volume guides the differential diagnostic considerations that include obstructive vs functional etiologies. A subsequent detailed anatomical analysis of the fetal genitourinary system and adjacent bowel should be obtained. This systematic approach and combined use of US and MRI can provide a more confident and specific fetal diagnosis.
the rectum and adjacent genitourinary tract allows improved differentiation between the disorders (Table 3).
Functional Megacystis Megacystis may occur in the absence of an obstructive malformation, such as in prune-belly syndrome (Fig. 15), Table 3 Fetal MRI Findings in Lower Urinary Tract Obstructions Lower Urinary Tract Short/Dilated Rectum Obstruction
Abnormal Signal in Bladder/ Rectum
Megacystis
Urethral stenosis or No atresia (♀, ♂) Posterior urethral valves No (♂) Anorectal Yes malformations (♂, ♀)
No
Yes
No
Yes
Possible: Meconium in bladder ↑T1, ↓T2 Fluid in the rectum ↓T1, ↑T2
Possible
No
Possible
No
Possible
Possible: Meconium in bladder or hydrocolpos (↑T1, ↓T2 layering content), Fluid in the rectum (↓T1, ↑T2 signal) No
Possible
Extrinsic or intrinsic “masses” Sacrococcygeal No (due to mass effect, anteriorly teratoma (♀, ♂) displaced or compressed) Prolapsed ureterocele No (♀, ♂) Hydrocolpos (♀): cloaca Yes (long common channel cloaca)
Hydrocolpos (♀): urogenital sinus
No (rectum is seen posterior to the hydrocolpos)
Gender distribution is described in parentheses. ♀, female; ♂, male.
Hydroureteronephrosis but no megacystis
Imaging evaluation of fetal megacystis
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Figure 15 Prenatal findings and postnatal correlation in prune-belly syndrome. (A-C) Fetal MRI balanced steady-state freeprecession axial images through the abdomen and pelvis in a 31-week male fetus with normal amniotic fluid volume. The abdominal wall is protuberant (arrowheads) and the bladder is enlarged (asterisk). Both upper ureters were dilated (not shown) with more prominent distal hydroureter noted on the right (arrow). The scrotum is empty (curved arrow). (D) Abdomen x-ray image shows the characteristic bulging flanks (especially on the right) and a distended bladder (asterisk) displacing the adjacent bowel. (E) Lateral voiding cystourethrogram shows a large bladder capacity and failure to spontaneously void. Voiding dysfunction requires intermittent catheterization in this patient.
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Figure 16 Megacystis-microcolon-intestinal hypoperistalsis syndrome. (A) Fetal MRI axial T2-weighted image of the upper abdomen in a 19-week female fetus. There is an enlarged bladder (b) and bilateral urinary tract dilatation (dashed arrows). The amniotic fluid (asterisk) volume is normal. (B) Lateral cystourethrogram and (C) frontal contrast enema on the third day of life (the baby failed to pass meconium). There is an atonic large-capacity bladder (b) with absent spontaneous voiding. Contrast enema after bladder drainage shows microcolon with centralized position in the abdomen consistent with malrotation. Note also minimal aeration of the proximal bowel in the right upper quadrant (arrow). Midgut malrotation and severe gastroesophageal reflux were confirmed on subsequent upper gastrointestinal series.
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Figure 17 A 29-week male fetus with megacystis-megaureter association and postnatal correlation. (A) Fetal MRI coronal T2-weighted and (B) sagittal T1-weighted images. There is bilateral urinary tract dilatation (dashed arrows), an enlarged bladder (b), normal rectum (arrow), and adequate amount of amniotic fluid (asterisk). (C and D) Frontal and lateral voiding cystourethrogram in the first day of life. There is a large-capacity bladder (b). Vesicoureteral reflux (dashed arrows) started as soon as the contrast entered the bladder and was severe. The urethra is normal.
Table 4 Fetal MRI Findings in the Setting of Impaired Bladder Emptying Without an Obstructive Anatomical Etiology Functional Megacystis
Short/Dilated Rectum
Abnormal Signal in Bladder/Rectum
Megacystis Microcolon
Prune-belly syndrome (4♂) Megacystis-microcolon-intestinal hypoperistalsis syndrome (4♀) Megacystis-megaureter association (4♂)
No No, in fact, microrectum! No
No No
Yes Yes
No Yes
No
Yes
No
Gender distribution is in parentheses. ♀, female; ♂, male; 4, more frequent.
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5. Chauvin NA, Epelman M, Victoria T, et al: Complex genitourinary abnormalities on fetal MRI: Imaging findings and approach to diagnosis. Am J Roentgenol 199:W222-W231, 2012 6. Magann EF, Chauhan SP, Doherty DA, et al: The evidence for abandoning the amniotic fluid index in favor of the single deepest pocket. Am J Perinatol 24:549-555, 2007 7. Puri P, Shinkai M: Megacystis microcolon intestinal hypoperistalsis syndrome. Semin Pediatr Surg 14:58-63, 2005 8. Berdon WE, Baker DH, Blanc WA, et al: Megacystis-microcolon-intestinal hypoperistalsis syndrome: A new cause of intestinal obstruction in the
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