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Evaluation of the Compromised Neonatal Foal
S E C T I O N
XVI The Foal
Evaluation of the Compromised Neonatal Foal
C H A P T E R
172
KEVIN T. CORLEY JONNA M. JOKISALO
MAJOR BODY SYSTEM ASSESSMENT (TRIAGE)
When a compromised neonatal foal is first presented, it is important to establish whether immediate emergency treatment is required. Three treatments that may need to be instituted in advance of obtaining a full history and clinical examination are fluid therapy, oxygen therapy, and cardiopulmonary resuscitation. The emergent need for one of these interventions is determined on the basis of quick assessment of the overall appearance of the foal, the heart rate and rhythm, the breathing rate and pattern, and mucous membrane color. Foals requiring immediate oxygen are likely to have low or high respiratory rates, may have respiratory distress, and may have cyanotic mucous membranes. Placing a normal neonatal foal in lateral recumbency reduces the arterial oxygen tension by approximately 10 mm Hg (1.3 kPa), which can make a substantial difference in a compromised foal. Therefore oxygen supplementation should be considered in foals in which restraint in lateral recumbency is needed, as during intravenous catheterization. Assessing whether a neonatal foal needs emergency fluids is not always straightforward. The clinical signs of hypovolemia (high heart rate, cold extremities, tachypnea, reduced jugular fill, poor pulse pressure) are not invariably present in hypovolemic foals. Foals that have not nursed for 3 hours or more are likely to be dehydrated, and foals that have not nursed for more than 6 hours are almost certain to be hypovolemic and require emergency fluids. The decision of whether to give immediate fluids is based on clinical signs, history, and a high index of suspicion. There are extremely few conditions in which giving 2 L of balanced electrolyte solution to a neonatal foal will cause harm, and in many cases it can significantly improve the foal’s chances of survival. Two (uncommon) conditions in which only small volumes of fluids should be given are uncontrolled hemorrhage and anuric renal failure.
HISTORY
Key questions to ask in the history of a neonatal foal are: What signs is the foal showing? For how long has the foal had these signs? When was the foal born? Were any problems seen at the time of delivery? How was the foal’s behavior
immediately after birth? How long was gestation? Has the mare had previous foals, and did they have problems? What is the parentage of the foal? For sick foals in the first couple of days of life, one of essentially two histories is likely. Either the foal looked well immediately after birth and then began to grow weak, or it was weak from birth. Both of these histories are common in foals with perinatal asphyxia syndrome and sepsis. Perhaps surprisingly, foals with conditions related to presumed birth trauma, such as ruptured bladder and diaphragmatic hernia, often appear normal after birth and only start showing clinical signs at 3 to 5 days. Likewise, foals with neonatal isoerythrolysis usually also are apparently initially normal and begin to show clinical signs at 1 to 7 days, most often 3 to 5 days after birth. The events at birth can profoundly affect the foal, with clinical signs usually becoming apparent within the first week of life but sometimes later. Prolonged birth (stage 2 lasting more than 40 minutes), premature placental separation, dystocia, and delivery by cesarean section are all risk factors for development of perinatal asphyxia syndrome, although the condition can also occur in foals in which no predisposing factors were recognized. Foals that require more than 60 minutes to stand or that do not nurse within the first 120 minutes after birth have significantly higher mortality and morbidity rates, compared with foals that stand and nurse inside these timepoints. The length of gestation is an important factor in the history and may direct early diagnostic testing to enable the owner to make an informed decision regarding whether to pursue treatment. However, gestation length alone is not sufficient grounds for determining prognosis because foals can be dysmature when born at term and or can be born early but have adequately mature organs and bone ossification to warrant giving a reasonable prognosis. The normal gestation length of mares ranges from 315 to 365 days, with a mean of 341 days. One critical factor in determining the prognosis for a foal that is born early is the state of the placenta and placental fluid. Perhaps counterintuitively, foals that are born from a “dirty” uterine environment are more likely to survive than those in which the placenta and uterine fluid appeared normal. This is because of precocious in utero maturation, in which increased corticosteroid
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concentrations in the mare induce hastened maturation of critical body systems in the fetus. The reproductive history of the mare can help in creation of a list of the likely differential diagnoses. If the mare has had previous foals, the length of prior gestations can help to determine whether the current pregnancy progressed normally. A small percentage of mares seems to repeatedly have foals with problems, although the problem may not be the same in every foal. Neonatal isoerythrolysis requires priming of the mare’s immune system by mismatching red blood cells, and is therefore rare (but not unheard of) in primiparous mares. Knowledge of a foal’s parentage provides an immediate indication of the foal’s potential future value and therefore the likely budget and expectations for treatment. The breeding can also give information as to the expected size of the foal (e.g., some stallions sire smaller foals than others) and any likely inherited flaws or characteristics.
CLINICAL EXAMINATION
Clinical examination is typically divided into three phases. The first, the major body system assessment, was discussed previously. The second phase consists of a general overview of the foal and is often done subconsciously by experienced veterinarians from the moment they first see the foal. The third phase is a detailed head-to-toe examination of the foal. Attention to detail here is vital. There is little benefit in heroically saving a foal with severe sepsis if it will have permanently compromised vision because of undetected entropion.
General Overview Much that determines both disease management and prognosis can be learned from the veterinarian’s general overview of the foal. The most important of these is the ability of the foal to stand and nurse unassisted. A foal that is not nursing will need both hydration and nutritional support. A foal that is unable to stand will require considerable nursing resources. It is also important to observe issues that will affect the future value of the foal; for the owner, these may be instrumental in deciding whether to invest in treatment. In many instances, the owner may not have seen the foal and is relying on the veterinarian to find any defects before embarking on treatment. Even if the owners are present, they may be so concerned about the primary disease that they miss other issues that will become a problem in the future. Issues that can be observed during the general overview include limb conformation, scoliosis or other vertebral column defects, and parrot mouth. More obvious defects, such as hydrocephalus or congenital skin lesions, are likely to be part of the presenting complaint.
Cardiovascular Assessment Although in general it is best to conduct a head-to-tail examination, the cardiovascular system is of such priority that it is usually assessed first. A brief examination of this system (heart rate and mucous membrane color) is included in the initial major body system assessment. At that point, it is usually decided what other assessments of the cardiovascular system are necessary. At a minimum, in addition to being checked for heart rate and mucous membrane color, the foal should be examined for jugular vein filling, cold extremities, skin tenting, and pulse pressure. Pulse pressure is simply the difference between systolic and diastolic arterial pressure and does not give information about the mean pressure (e.g., if the diastolic pressure is low, the pulse pressure can feel
normal despite a low mean arterial pressure). In a recumbent foal presented to a hospital, indirect mean arterial pressure should also be measured.
EXAMINATION FROM HEAD TO TAIL
Many similarities exist in the head-to-tail physical examination between the foal and adult horse. However, there are also many differences, and some parts of the animal that would normally only receive a cursory examination in an adult horse unless there is a specific indication warrant detailed evaluation in the foal. One example of this is the aural pinnae.
Head and Neck Start with the mouth and oral mucous membranes. The mucous membranes should be pink and moist, with capillary refill time less than 2 seconds. The mucous membranes can indicate changes in hydration, alterations in microcirculation, and the coagulation system. Dry or tacky mucous membranes are associated with inadequate hydration. Purple mucous membranes indicate poor oxygenation, which can be caused by a lung problem or heart defect that causes rightto-left shunting of blood, whereas red, injected mucous membranes can be seen with peripheral vasodilation. Yellow or icteric mucous membranes are most commonly a sign of neonatal isoerythrolysis. However, sepsis, neonatal hyperbilirubinemia, impaired hepatic function, equine herpesvirus type 1 infection, iron toxicosis, and meconium impaction can all increase circulating bilirubin concentrations and therefore also manifest as jaundice. Petechiae can also form in the oral mucous membranes and can be a result of sepsis or of the ulcerative dermatitis, thrombocytopenia, and neutropenia syndrome. The tongue should be examined for signs of infection, such as candidiasis. Cleft palate can usually only be detected on physical examination if the foal is comatose or anesthetized, or if it involves the hard palate (a rare occurrence). A cleft soft palate is usually detected endoscopically. Brachygnathia (parrot mouth), wry nose, congenital oral tumors such as hemangiosarcoma, and other abnormalities can also be detected at this time. The nares should be checked for airflow during exhalation because unilateral choanal atresia may not be clinically obvious in neonatal foals. Confirmation of choanal atresia is made by endoscopy. As in adult horses, the nares should also be checked for discharge. There are two primary differential diagnoses for milk coming out of the nares during nursing: milk regurgitation and physical interference with swallowing, such as that caused by a cleft palate or subepiglottic cyst. Milk regurgitation appears to be an organ-specific form of perinatal asphyxia syndrome in which there is a temporary swallowing defect. It is treated by preventing nursing while feeding the foal by nasogastric tube. In most cases, swallowing ability is restored in 48 to 96 hours. If left untreated or caught late, these foals will develop aspiration pneumonia. Milk regurgitation must be differentiated by endoscopy from physical problems affecting swallowing. Milk or saliva escaping from the nose and mouth can occur both with esophageal obstruction and with esophageal ulceration. Serous nasal discharge can develop with neonatal viral diseases, such as equine influenza (especially in an unvaccinated population), and mucopurulent discharge is often a sign of bacterial disease. The pupillary light reflex is universally present in newborn foals but can appear slightly sluggish, compared with the reflex in adult horses. The menace response only develops in
CHAPTER
172 Evaluation of the Compromised Neonatal Foal
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unlikely to be viable. Enlargement of the thyroid gland may be evident in some foals with congenital hypothyroidism. Congenital fluid-filled swellings (e.g., salivary mucoceles) have been reported in the neck area of foals as a consequence of pharyngeal pouch malformation.
Seizures
Figure 172-1 Photograph of a premature foal, showing the domed head, floppy ears, and fine haircoat typical of premature foals. (Image courtesy Kevin Corley and Jane Axon, 2004.)
Seizure activity is primarily seen in the head in foals. There are many causes of seizures in the neonate, including perinatal asphyxia syndrome, septicemia, meningitis, electrolyte disorders, hypoglycemia, hepatoencephalopathy, cranial or vertebral trauma, and congenital malformations. Seizures in foals are usually characterized by nystagmus, with or without swallowing activity. Occasionally rapid eye movement sleep is mistaken for a seizure. In rapid eye movement sleep, the animal can be roused, whereas seizure activity is not changed by actions such as poking a finger into the foal’s ear. Opisthotonus can sometimes be seen in conjunction with seizures in foals, particularly with seizures associated with hyponatremia, kernicterus, and lavender foal syndrome. Abnormal jaw movements (especially in kernicterus) and tongue movements (especially in hyponatremia) can also occur. Limb signs such as spasticity or paddling are frequent but not universally present in foals with seizures.
Thorax
Figure 172-2 Petechiae in the ear of a septic foal. (Image courtesy Veterinary Advances Ltd., 2013.)
the first week of life. In one study of 26 foals, 1 foal had a menace response on day 1 of life, most foals had a menace response by day 7, and all foals had a menace response by the ninth day. Entropion is relatively common in neonatal foals, especially compromised foals, and should be corrected if observed. Sepsis can manifest in the eyes as hypopyon (pus in the anterior chamber) or hyphema (blood in the anterior chamber of the eye). The eyes should also be checked for size because congenital microophthalmia can occur. The conjunctival mucous membranes can develop color changes similar to those in the mouth, but the changes are sometimes more obvious in the conjunctiva. The sclera is often the first place where subtle jaundice is appreciated. A domed head and floppy ears are signs of prematurity in the foal (Figure 172-1); premature foals can also have fine, silky hair coats. The inside of the ears (aural pinnae) are the classically described place to detect petechiation and ecchymosis in the foal (Figure 172-2). However, petechiae can be fairly subtle in this location, and other mucous membranes should also be examined for these lesions. Wry neck can be a cause of dystocia and may result in the foal having to be delivered by cesarean section. It is important to detect defects of this sort immediately after delivery to avoid expensive resuscitation efforts on an animal that is
The ribs, respiratory system, and cardiovascular system are all considered during assessment of the thorax. Thoracic trauma is very common in foals at birth, and was reported in 20% of foals in one study. In that study, rib fractures were detected in 3.5% of foals. Although rib fractures are often suspected on palpation, they should be confirmed with medical imaging. In one study, ultrasonography was found to be more accurate than radiography in detecting rib fractures. Ultrasonography is also useful at detecting associated problems such as hemothorax, which could be missed on radiography. Observation of respiratory patterns in foals is particularly useful. Increased rate, increased effort, paradoxical breathing, and flail chest all are readily apparent and may necessitate immediate intervention, such as intranasal oxygen supplementation. Auscultation is sometimes very helpful in the foal and in principle is similar to auscultation in the adult horse. However, respiratory sounds are more evident and more harsh in the normal foal than in normal adults, and unexpanded or collapsed lungs where little or no air movement can be auscultated are much more common in neonates than in adult horses. During clinical examination of the thorax, the heart is auscultated. Neonatal foals often have a soft systolic murmur audible on the left side for the first several days after birth. Auscultation is extremely important for detecting congenital abnormalities, such as ventricular septal defects, that may affect the future athletic ability of the horse. Echocardiography is used to confirm congenital cardiac defects. Persistent fetal circulation is seen in a small percentage of foals presenting to the author’s hospital. The pulmonary circulation is vasoconstricted in utero, and much of the blood bypasses the lungs through the foramen ovale and ductus arteriosus. When this fetal circulation persists after birth, the right ventricle must pump against a high pressure and therefore dilates. The foal may also show respiratory distress. Treatment with sildenafil (0.5 to 2.5 mg/kg, PO, up to every 4 hours) may reduce the pulmonary vasoconstriction and allow the circulation to change to the normal postnatal configuration. The author often begins with a dosage of
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0.75 mg/kg; the higher dosages in this range can cause systemic hypotension. During examination of the thorax, any brisket edema or any other swellings or abnormalities should also be noted.
Abdomen and Hindquarters Gastrointestinal sounds tend to be softer in the foal than in the adult horse, and no cecal “flushing” sounds are auscultated. The shape and size of the abdomen should be assessed. Increases in size can occur with gas in colic and with fluid in ruptured bladder and peritonitis. The external umbilicus should be thoroughly examined in the foal. This structure should be small and dry. Wetness may indicate a patent urachus, although some compromised male foals do not extrude the penis from the sheath during micturition and urinate on the umbilical stump. Swelling may indicate a hernia, infection such as an umbilical abscess, or, rarely, edema. Observation of a normal-appearing external umbilicus does not rule out infected urachus, umbilical arteries, or umbilical vein, and these structures should be investigated by ultrasound if a site of infection is suspected from clinical or laboratory examination. The rectal temperature of the foal should be taken, and the anus and sexual organs examined. Inguinal hernias manifest as large, almost always unilateral, swellings in the inguinal area. Icterus can sometimes be appreciated in the vulval mucous membranes. It is important not to miss rare congenital abnormalities such as atresia ani, hypospadias, and male pseudohermaphroditism. In a foal that is younger than 2 days and being evaluated for colic, it is important to perform a one-fingered examination per rectum because meconium impactions can often be appreciated in this manner. If the finger comes back completely clean with no fecal staining, atresia recti or atresia coli should be suspected and can be confirmed with a barium enema radiographic study.
Limbs Each limb should be carefully palpated, with focus concentrated on the joints. Synovial swellings are usually indicative of joint sepsis but can develop as a sterile effusion secondary to systemic disease. Synovial effusions are differentiated from swellings in the region of the joint by feeling for distension in one joint pouch while pressing on another pouch in the same joint. As far as possible, limb conformation should be assessed and findings communicated to the owner during the initial workup of the foal. The hooves should also be assessed. The presence of eponychium (also called “foal slippers”) on the feet (Figure 172-3) is indicative that the foal has not yet stood. Hyperemia of the coronary band area can be seen in foals with circulatory disturbances such as those caused by sepsis.
LABORATORY INVESTIGATIONS
Plasma lactate concentration is a rapidly performed test that adds to the information from the major body system assessment and cardiovascular examination. A high lactate concentration most commonly indicates inadequate organ perfusion or sepsis and therefore (at initial assessment) the requirement for fluid therapy and often hospital care. Lactate concentrations at admission and changes in response to treatments are also fairly tightly associated with survival in hospitalized foals. The lactate concentration is therefore very useful during initial assessment to aid prognostication. Normal lactate concentrations in foals are higher immediately after birth (0.4 to 4.4 mmol/L) than in 3-day-old foals and adult horses (0.2 to 0.7 mmol/L).
Figure 172-3 Eponychium on neonatal foal hooves, indicating that the foal has not yet stood. (Image courtesy Veterinary Advances Ltd., 2013.)
In foals as in adult horses, a high serum creatinine concentration can result from inadequate perfusion of the kidneys, nephropathy, or postrenal problems such as ruptured bladder. In foals in the first 36 hours of life, however, a high creatinine concentration most commonly reflects compromised placental function in utero. High creatinine concentration caused by placental insufficiency should decrease by 50% within the first 24 hours, and to normal concentration within the first 3 days of life. Urine specific gravity can be used as an indicator of hydration in foals without renal disease. The specific gravity should be less than 1.012, with higher values indicating hypovolemia. Serum amyloid A (SAA) is an extremely useful laboratory test for supporting a clinical suspicion of infection in the neonatal foal. Normal foals have an SAA concentration of less than 30 mg/L. An SAA concentration over 100 mg/L is highly suspicious for acute infection in the foal. This test is particularly useful as a support for suspected sepsis in difficult to palpate joints such as the acetabulofemoral joint and to differentiate sepsis from perinatal asphyxia syndrome. This test only indicates acute infection, however, and foals with well walled-off chronic infection such as umbilical abscesses (or Rhodococcus equi) can have normal SAA concentrations. All laboratory tests in the foal should be compared with foal-specific reference ranges. The reference ranges for γ-glutamyltransferase and aspartate aminotransferase activities and for bile acids and thyroid hormone concentrations are particularly different from those in adult horses. Arterial and venous blood gas partial pressures and acidbase analysis can be very useful in assessment of animals with respiratory and cardiovascular compromise and are commonly performed in the hospital setting.
MEDICAL IMAGING
Ultrasonography is extremely useful in the initial stages of assessing a neonatal foal. The areas examined and extent of the examination depend on the initial history and clinical signs. Transabdominal ultrasound examination is virtually essential in foals with colic and is often extremely useful in foals with diarrhea. Transabdominal ultrasonography is best performed with the animal standing or held in a standing position. This is because potential lesions such as intussusceptions and abscesses are generally heavier than the surrounding intestines, and they can often be found in the
CHAPTER
Figure 172-4 Color Doppler image of patent ductus arteriosus in a foal with persistent fetal circulation (see Color Plate 172-1). (Image courtesy Kevin Corley, 2013.)
172 Evaluation of the Compromised Neonatal Foal
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Figure 172-6 Photograph of the same foal as in Figure 172-5. The abscess cavity has been surgically opened for lavage and drainage. (Image courtesy Veterinary Advances Ltd., 2011.)
foal is stable enough to undergo the procedure. Lack of ossification of these bones is common in severely premature foals and greatly decreases the prognosis for an athletic outcome. In the foal with suspected or confirmed septic joints, radiographs of the affected joint enable assessment of bony involvement, which influences both the prognosis and the duration of treatment. One major difference between foals and adult horses in radiography is the ability to easily obtain diagnostic images of the abdomen. This allows for obtaining both plain radiographs and contrast studies. Plain abdominal radiographs are most commonly used in assessment of colic and may reveal lesions such as high meconium impaction that in some instances can be difficult to detect with ultrasound. Contrast studies are useful in a limited number of cases and can highlight incongruencies in the urinary tract or atresia of the intestine, for example.
CONCLUSION Figure 172-5 Ultrasound image of a fluid-filled cavity separate from the joint in a foal with swelling in the hock area. (Image courtesy Kevin Corley, 2011.)
ventral midline area in a standing animal. Always performing the examination with the animal in the same position aids in identification of all the organs and any abnormalities. Thoracic ultrasound can also be useful in the diagnostic evaluation of the neonatal foal. Echocardiography can reveal congenital lesions or persistent fetal circulation (Figure 172-4) and can provide an indicator of the foal’s fluid status. Ultrasound of the pleural surface of the lungs can give some indication of the extent of disease processes within the lungs, and fluid or air can be identified in the pleural space. As mentioned, ultrasonography is reported to be the best method for detecting fractured ribs in the foal. Other areas are perhaps less commonly imaged with ultrasound, but useful information can be obtained in some cases. For example, ultrasound of the joints can confirm effusions, reveal fibrin in the joint fluid, and reveal perisynovial abscesses (Figures 172-5 and 172-6). Radiography can also be extremely useful in assessment of the foal. In the premature foal, radiographic imaging of the small tarsal and carpal bones is a priority after the
Evaluation of the compromised foal requires the veterinarian to adopt an ordered, logical approach. However, it is important to address the immediate treatment needs of the foal before embarking on a more extensive diagnostic workup. It is also important to remember that disease progression can be extremely rapid in the foal, and compromised foals represent highly dynamic cases. Therefore a foal will need to be evaluated serially and treatments adjusted accordingly throughout the period of treatment.
Suggested Readings Borchers A, Wilkins PA, Marsh PM, et al. Association of admission L-lactate concentration in hospitalised equine neonates with presenting complaint, periparturient events, clinical diagnosis and outcome: a prospective multicentre study. Equine Vet J Suppl 2012;Feb:57-63. Jean D, Picandet V, Macieria S, et al. Detection of rib trauma in newborn foals in an equine critical care unit: a comparison of ultrasonography, radiography and physical examination. Equine Vet J 2007;39:158-163. Stoneham SJ, Palmer L, Cash R, et al. Measurement of serum amyloid A in the neonatal foal using a latex agglutination immunoturbidimetric assay: determination of the normal range, variation with age and response to disease. Equine Vet J 2001;33:599-603.
XVI The Foal
Evaluation of the Compromised Neonatal Foal
C H A P T E R
172
KEVIN T. CORLEY JONNA M. JOKISALO
MAJOR BODY SYSTEM ASSESSMENT (TRIAGE)
When a compromised neonatal foal is first presented, it is important to establish whether immediate emergency treatment is required. Three treatments that may need to be instituted in advance of obtaining a full history and clinical examination are fluid therapy, oxygen therapy, and cardiopulmonary resuscitation. The emergent need for one of these interventions is determined on the basis of quick assessment of the overall appearance of the foal, the heart rate and rhythm, the breathing rate and pattern, and mucous membrane color. Foals requiring immediate oxygen are likely to have low or high respiratory rates, may have respiratory distress, and may have cyanotic mucous membranes. Placing a normal neonatal foal in lateral recumbency reduces the arterial oxygen tension by approximately 10 mm Hg (1.3 kPa), which can make a substantial difference in a compromised foal. Therefore oxygen supplementation should be considered in foals in which restraint in lateral recumbency is needed, as during intravenous catheterization. Assessing whether a neonatal foal needs emergency fluids is not always straightforward. The clinical signs of hypovolemia (high heart rate, cold extremities, tachypnea, reduced jugular fill, poor pulse pressure) are not invariably present in hypovolemic foals. Foals that have not nursed for 3 hours or more are likely to be dehydrated, and foals that have not nursed for more than 6 hours are almost certain to be hypovolemic and require emergency fluids. The decision of whether to give immediate fluids is based on clinical signs, history, and a high index of suspicion. There are extremely few conditions in which giving 2 L of balanced electrolyte solution to a neonatal foal will cause harm, and in many cases it can significantly improve the foal’s chances of survival. Two (uncommon) conditions in which only small volumes of fluids should be given are uncontrolled hemorrhage and anuric renal failure.
HISTORY
Key questions to ask in the history of a neonatal foal are: What signs is the foal showing? For how long has the foal had these signs? When was the foal born? Were any problems seen at the time of delivery? How was the foal’s behavior
immediately after birth? How long was gestation? Has the mare had previous foals, and did they have problems? What is the parentage of the foal? For sick foals in the first couple of days of life, one of essentially two histories is likely. Either the foal looked well immediately after birth and then began to grow weak, or it was weak from birth. Both of these histories are common in foals with perinatal asphyxia syndrome and sepsis. Perhaps surprisingly, foals with conditions related to presumed birth trauma, such as ruptured bladder and diaphragmatic hernia, often appear normal after birth and only start showing clinical signs at 3 to 5 days. Likewise, foals with neonatal isoerythrolysis usually also are apparently initially normal and begin to show clinical signs at 1 to 7 days, most often 3 to 5 days after birth. The events at birth can profoundly affect the foal, with clinical signs usually becoming apparent within the first week of life but sometimes later. Prolonged birth (stage 2 lasting more than 40 minutes), premature placental separation, dystocia, and delivery by cesarean section are all risk factors for development of perinatal asphyxia syndrome, although the condition can also occur in foals in which no predisposing factors were recognized. Foals that require more than 60 minutes to stand or that do not nurse within the first 120 minutes after birth have significantly higher mortality and morbidity rates, compared with foals that stand and nurse inside these timepoints. The length of gestation is an important factor in the history and may direct early diagnostic testing to enable the owner to make an informed decision regarding whether to pursue treatment. However, gestation length alone is not sufficient grounds for determining prognosis because foals can be dysmature when born at term and or can be born early but have adequately mature organs and bone ossification to warrant giving a reasonable prognosis. The normal gestation length of mares ranges from 315 to 365 days, with a mean of 341 days. One critical factor in determining the prognosis for a foal that is born early is the state of the placenta and placental fluid. Perhaps counterintuitively, foals that are born from a “dirty” uterine environment are more likely to survive than those in which the placenta and uterine fluid appeared normal. This is because of precocious in utero maturation, in which increased corticosteroid
717
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XVI The Foal
concentrations in the mare induce hastened maturation of critical body systems in the fetus. The reproductive history of the mare can help in creation of a list of the likely differential diagnoses. If the mare has had previous foals, the length of prior gestations can help to determine whether the current pregnancy progressed normally. A small percentage of mares seems to repeatedly have foals with problems, although the problem may not be the same in every foal. Neonatal isoerythrolysis requires priming of the mare’s immune system by mismatching red blood cells, and is therefore rare (but not unheard of) in primiparous mares. Knowledge of a foal’s parentage provides an immediate indication of the foal’s potential future value and therefore the likely budget and expectations for treatment. The breeding can also give information as to the expected size of the foal (e.g., some stallions sire smaller foals than others) and any likely inherited flaws or characteristics.
CLINICAL EXAMINATION
Clinical examination is typically divided into three phases. The first, the major body system assessment, was discussed previously. The second phase consists of a general overview of the foal and is often done subconsciously by experienced veterinarians from the moment they first see the foal. The third phase is a detailed head-to-toe examination of the foal. Attention to detail here is vital. There is little benefit in heroically saving a foal with severe sepsis if it will have permanently compromised vision because of undetected entropion.
General Overview Much that determines both disease management and prognosis can be learned from the veterinarian’s general overview of the foal. The most important of these is the ability of the foal to stand and nurse unassisted. A foal that is not nursing will need both hydration and nutritional support. A foal that is unable to stand will require considerable nursing resources. It is also important to observe issues that will affect the future value of the foal; for the owner, these may be instrumental in deciding whether to invest in treatment. In many instances, the owner may not have seen the foal and is relying on the veterinarian to find any defects before embarking on treatment. Even if the owners are present, they may be so concerned about the primary disease that they miss other issues that will become a problem in the future. Issues that can be observed during the general overview include limb conformation, scoliosis or other vertebral column defects, and parrot mouth. More obvious defects, such as hydrocephalus or congenital skin lesions, are likely to be part of the presenting complaint.
Cardiovascular Assessment Although in general it is best to conduct a head-to-tail examination, the cardiovascular system is of such priority that it is usually assessed first. A brief examination of this system (heart rate and mucous membrane color) is included in the initial major body system assessment. At that point, it is usually decided what other assessments of the cardiovascular system are necessary. At a minimum, in addition to being checked for heart rate and mucous membrane color, the foal should be examined for jugular vein filling, cold extremities, skin tenting, and pulse pressure. Pulse pressure is simply the difference between systolic and diastolic arterial pressure and does not give information about the mean pressure (e.g., if the diastolic pressure is low, the pulse pressure can feel
normal despite a low mean arterial pressure). In a recumbent foal presented to a hospital, indirect mean arterial pressure should also be measured.
EXAMINATION FROM HEAD TO TAIL
Many similarities exist in the head-to-tail physical examination between the foal and adult horse. However, there are also many differences, and some parts of the animal that would normally only receive a cursory examination in an adult horse unless there is a specific indication warrant detailed evaluation in the foal. One example of this is the aural pinnae.
Head and Neck Start with the mouth and oral mucous membranes. The mucous membranes should be pink and moist, with capillary refill time less than 2 seconds. The mucous membranes can indicate changes in hydration, alterations in microcirculation, and the coagulation system. Dry or tacky mucous membranes are associated with inadequate hydration. Purple mucous membranes indicate poor oxygenation, which can be caused by a lung problem or heart defect that causes rightto-left shunting of blood, whereas red, injected mucous membranes can be seen with peripheral vasodilation. Yellow or icteric mucous membranes are most commonly a sign of neonatal isoerythrolysis. However, sepsis, neonatal hyperbilirubinemia, impaired hepatic function, equine herpesvirus type 1 infection, iron toxicosis, and meconium impaction can all increase circulating bilirubin concentrations and therefore also manifest as jaundice. Petechiae can also form in the oral mucous membranes and can be a result of sepsis or of the ulcerative dermatitis, thrombocytopenia, and neutropenia syndrome. The tongue should be examined for signs of infection, such as candidiasis. Cleft palate can usually only be detected on physical examination if the foal is comatose or anesthetized, or if it involves the hard palate (a rare occurrence). A cleft soft palate is usually detected endoscopically. Brachygnathia (parrot mouth), wry nose, congenital oral tumors such as hemangiosarcoma, and other abnormalities can also be detected at this time. The nares should be checked for airflow during exhalation because unilateral choanal atresia may not be clinically obvious in neonatal foals. Confirmation of choanal atresia is made by endoscopy. As in adult horses, the nares should also be checked for discharge. There are two primary differential diagnoses for milk coming out of the nares during nursing: milk regurgitation and physical interference with swallowing, such as that caused by a cleft palate or subepiglottic cyst. Milk regurgitation appears to be an organ-specific form of perinatal asphyxia syndrome in which there is a temporary swallowing defect. It is treated by preventing nursing while feeding the foal by nasogastric tube. In most cases, swallowing ability is restored in 48 to 96 hours. If left untreated or caught late, these foals will develop aspiration pneumonia. Milk regurgitation must be differentiated by endoscopy from physical problems affecting swallowing. Milk or saliva escaping from the nose and mouth can occur both with esophageal obstruction and with esophageal ulceration. Serous nasal discharge can develop with neonatal viral diseases, such as equine influenza (especially in an unvaccinated population), and mucopurulent discharge is often a sign of bacterial disease. The pupillary light reflex is universally present in newborn foals but can appear slightly sluggish, compared with the reflex in adult horses. The menace response only develops in
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172 Evaluation of the Compromised Neonatal Foal
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unlikely to be viable. Enlargement of the thyroid gland may be evident in some foals with congenital hypothyroidism. Congenital fluid-filled swellings (e.g., salivary mucoceles) have been reported in the neck area of foals as a consequence of pharyngeal pouch malformation.
Seizures
Figure 172-1 Photograph of a premature foal, showing the domed head, floppy ears, and fine haircoat typical of premature foals. (Image courtesy Kevin Corley and Jane Axon, 2004.)
Seizure activity is primarily seen in the head in foals. There are many causes of seizures in the neonate, including perinatal asphyxia syndrome, septicemia, meningitis, electrolyte disorders, hypoglycemia, hepatoencephalopathy, cranial or vertebral trauma, and congenital malformations. Seizures in foals are usually characterized by nystagmus, with or without swallowing activity. Occasionally rapid eye movement sleep is mistaken for a seizure. In rapid eye movement sleep, the animal can be roused, whereas seizure activity is not changed by actions such as poking a finger into the foal’s ear. Opisthotonus can sometimes be seen in conjunction with seizures in foals, particularly with seizures associated with hyponatremia, kernicterus, and lavender foal syndrome. Abnormal jaw movements (especially in kernicterus) and tongue movements (especially in hyponatremia) can also occur. Limb signs such as spasticity or paddling are frequent but not universally present in foals with seizures.
Thorax
Figure 172-2 Petechiae in the ear of a septic foal. (Image courtesy Veterinary Advances Ltd., 2013.)
the first week of life. In one study of 26 foals, 1 foal had a menace response on day 1 of life, most foals had a menace response by day 7, and all foals had a menace response by the ninth day. Entropion is relatively common in neonatal foals, especially compromised foals, and should be corrected if observed. Sepsis can manifest in the eyes as hypopyon (pus in the anterior chamber) or hyphema (blood in the anterior chamber of the eye). The eyes should also be checked for size because congenital microophthalmia can occur. The conjunctival mucous membranes can develop color changes similar to those in the mouth, but the changes are sometimes more obvious in the conjunctiva. The sclera is often the first place where subtle jaundice is appreciated. A domed head and floppy ears are signs of prematurity in the foal (Figure 172-1); premature foals can also have fine, silky hair coats. The inside of the ears (aural pinnae) are the classically described place to detect petechiation and ecchymosis in the foal (Figure 172-2). However, petechiae can be fairly subtle in this location, and other mucous membranes should also be examined for these lesions. Wry neck can be a cause of dystocia and may result in the foal having to be delivered by cesarean section. It is important to detect defects of this sort immediately after delivery to avoid expensive resuscitation efforts on an animal that is
The ribs, respiratory system, and cardiovascular system are all considered during assessment of the thorax. Thoracic trauma is very common in foals at birth, and was reported in 20% of foals in one study. In that study, rib fractures were detected in 3.5% of foals. Although rib fractures are often suspected on palpation, they should be confirmed with medical imaging. In one study, ultrasonography was found to be more accurate than radiography in detecting rib fractures. Ultrasonography is also useful at detecting associated problems such as hemothorax, which could be missed on radiography. Observation of respiratory patterns in foals is particularly useful. Increased rate, increased effort, paradoxical breathing, and flail chest all are readily apparent and may necessitate immediate intervention, such as intranasal oxygen supplementation. Auscultation is sometimes very helpful in the foal and in principle is similar to auscultation in the adult horse. However, respiratory sounds are more evident and more harsh in the normal foal than in normal adults, and unexpanded or collapsed lungs where little or no air movement can be auscultated are much more common in neonates than in adult horses. During clinical examination of the thorax, the heart is auscultated. Neonatal foals often have a soft systolic murmur audible on the left side for the first several days after birth. Auscultation is extremely important for detecting congenital abnormalities, such as ventricular septal defects, that may affect the future athletic ability of the horse. Echocardiography is used to confirm congenital cardiac defects. Persistent fetal circulation is seen in a small percentage of foals presenting to the author’s hospital. The pulmonary circulation is vasoconstricted in utero, and much of the blood bypasses the lungs through the foramen ovale and ductus arteriosus. When this fetal circulation persists after birth, the right ventricle must pump against a high pressure and therefore dilates. The foal may also show respiratory distress. Treatment with sildenafil (0.5 to 2.5 mg/kg, PO, up to every 4 hours) may reduce the pulmonary vasoconstriction and allow the circulation to change to the normal postnatal configuration. The author often begins with a dosage of
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XVI The Foal
0.75 mg/kg; the higher dosages in this range can cause systemic hypotension. During examination of the thorax, any brisket edema or any other swellings or abnormalities should also be noted.
Abdomen and Hindquarters Gastrointestinal sounds tend to be softer in the foal than in the adult horse, and no cecal “flushing” sounds are auscultated. The shape and size of the abdomen should be assessed. Increases in size can occur with gas in colic and with fluid in ruptured bladder and peritonitis. The external umbilicus should be thoroughly examined in the foal. This structure should be small and dry. Wetness may indicate a patent urachus, although some compromised male foals do not extrude the penis from the sheath during micturition and urinate on the umbilical stump. Swelling may indicate a hernia, infection such as an umbilical abscess, or, rarely, edema. Observation of a normal-appearing external umbilicus does not rule out infected urachus, umbilical arteries, or umbilical vein, and these structures should be investigated by ultrasound if a site of infection is suspected from clinical or laboratory examination. The rectal temperature of the foal should be taken, and the anus and sexual organs examined. Inguinal hernias manifest as large, almost always unilateral, swellings in the inguinal area. Icterus can sometimes be appreciated in the vulval mucous membranes. It is important not to miss rare congenital abnormalities such as atresia ani, hypospadias, and male pseudohermaphroditism. In a foal that is younger than 2 days and being evaluated for colic, it is important to perform a one-fingered examination per rectum because meconium impactions can often be appreciated in this manner. If the finger comes back completely clean with no fecal staining, atresia recti or atresia coli should be suspected and can be confirmed with a barium enema radiographic study.
Limbs Each limb should be carefully palpated, with focus concentrated on the joints. Synovial swellings are usually indicative of joint sepsis but can develop as a sterile effusion secondary to systemic disease. Synovial effusions are differentiated from swellings in the region of the joint by feeling for distension in one joint pouch while pressing on another pouch in the same joint. As far as possible, limb conformation should be assessed and findings communicated to the owner during the initial workup of the foal. The hooves should also be assessed. The presence of eponychium (also called “foal slippers”) on the feet (Figure 172-3) is indicative that the foal has not yet stood. Hyperemia of the coronary band area can be seen in foals with circulatory disturbances such as those caused by sepsis.
LABORATORY INVESTIGATIONS
Plasma lactate concentration is a rapidly performed test that adds to the information from the major body system assessment and cardiovascular examination. A high lactate concentration most commonly indicates inadequate organ perfusion or sepsis and therefore (at initial assessment) the requirement for fluid therapy and often hospital care. Lactate concentrations at admission and changes in response to treatments are also fairly tightly associated with survival in hospitalized foals. The lactate concentration is therefore very useful during initial assessment to aid prognostication. Normal lactate concentrations in foals are higher immediately after birth (0.4 to 4.4 mmol/L) than in 3-day-old foals and adult horses (0.2 to 0.7 mmol/L).
Figure 172-3 Eponychium on neonatal foal hooves, indicating that the foal has not yet stood. (Image courtesy Veterinary Advances Ltd., 2013.)
In foals as in adult horses, a high serum creatinine concentration can result from inadequate perfusion of the kidneys, nephropathy, or postrenal problems such as ruptured bladder. In foals in the first 36 hours of life, however, a high creatinine concentration most commonly reflects compromised placental function in utero. High creatinine concentration caused by placental insufficiency should decrease by 50% within the first 24 hours, and to normal concentration within the first 3 days of life. Urine specific gravity can be used as an indicator of hydration in foals without renal disease. The specific gravity should be less than 1.012, with higher values indicating hypovolemia. Serum amyloid A (SAA) is an extremely useful laboratory test for supporting a clinical suspicion of infection in the neonatal foal. Normal foals have an SAA concentration of less than 30 mg/L. An SAA concentration over 100 mg/L is highly suspicious for acute infection in the foal. This test is particularly useful as a support for suspected sepsis in difficult to palpate joints such as the acetabulofemoral joint and to differentiate sepsis from perinatal asphyxia syndrome. This test only indicates acute infection, however, and foals with well walled-off chronic infection such as umbilical abscesses (or Rhodococcus equi) can have normal SAA concentrations. All laboratory tests in the foal should be compared with foal-specific reference ranges. The reference ranges for γ-glutamyltransferase and aspartate aminotransferase activities and for bile acids and thyroid hormone concentrations are particularly different from those in adult horses. Arterial and venous blood gas partial pressures and acidbase analysis can be very useful in assessment of animals with respiratory and cardiovascular compromise and are commonly performed in the hospital setting.
MEDICAL IMAGING
Ultrasonography is extremely useful in the initial stages of assessing a neonatal foal. The areas examined and extent of the examination depend on the initial history and clinical signs. Transabdominal ultrasound examination is virtually essential in foals with colic and is often extremely useful in foals with diarrhea. Transabdominal ultrasonography is best performed with the animal standing or held in a standing position. This is because potential lesions such as intussusceptions and abscesses are generally heavier than the surrounding intestines, and they can often be found in the
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Figure 172-4 Color Doppler image of patent ductus arteriosus in a foal with persistent fetal circulation (see Color Plate 172-1). (Image courtesy Kevin Corley, 2013.)
172 Evaluation of the Compromised Neonatal Foal
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Figure 172-6 Photograph of the same foal as in Figure 172-5. The abscess cavity has been surgically opened for lavage and drainage. (Image courtesy Veterinary Advances Ltd., 2011.)
foal is stable enough to undergo the procedure. Lack of ossification of these bones is common in severely premature foals and greatly decreases the prognosis for an athletic outcome. In the foal with suspected or confirmed septic joints, radiographs of the affected joint enable assessment of bony involvement, which influences both the prognosis and the duration of treatment. One major difference between foals and adult horses in radiography is the ability to easily obtain diagnostic images of the abdomen. This allows for obtaining both plain radiographs and contrast studies. Plain abdominal radiographs are most commonly used in assessment of colic and may reveal lesions such as high meconium impaction that in some instances can be difficult to detect with ultrasound. Contrast studies are useful in a limited number of cases and can highlight incongruencies in the urinary tract or atresia of the intestine, for example.
CONCLUSION Figure 172-5 Ultrasound image of a fluid-filled cavity separate from the joint in a foal with swelling in the hock area. (Image courtesy Kevin Corley, 2011.)
ventral midline area in a standing animal. Always performing the examination with the animal in the same position aids in identification of all the organs and any abnormalities. Thoracic ultrasound can also be useful in the diagnostic evaluation of the neonatal foal. Echocardiography can reveal congenital lesions or persistent fetal circulation (Figure 172-4) and can provide an indicator of the foal’s fluid status. Ultrasound of the pleural surface of the lungs can give some indication of the extent of disease processes within the lungs, and fluid or air can be identified in the pleural space. As mentioned, ultrasonography is reported to be the best method for detecting fractured ribs in the foal. Other areas are perhaps less commonly imaged with ultrasound, but useful information can be obtained in some cases. For example, ultrasound of the joints can confirm effusions, reveal fibrin in the joint fluid, and reveal perisynovial abscesses (Figures 172-5 and 172-6). Radiography can also be extremely useful in assessment of the foal. In the premature foal, radiographic imaging of the small tarsal and carpal bones is a priority after the
Evaluation of the compromised foal requires the veterinarian to adopt an ordered, logical approach. However, it is important to address the immediate treatment needs of the foal before embarking on a more extensive diagnostic workup. It is also important to remember that disease progression can be extremely rapid in the foal, and compromised foals represent highly dynamic cases. Therefore a foal will need to be evaluated serially and treatments adjusted accordingly throughout the period of treatment.
Suggested Readings Borchers A, Wilkins PA, Marsh PM, et al. Association of admission L-lactate concentration in hospitalised equine neonates with presenting complaint, periparturient events, clinical diagnosis and outcome: a prospective multicentre study. Equine Vet J Suppl 2012;Feb:57-63. Jean D, Picandet V, Macieria S, et al. Detection of rib trauma in newborn foals in an equine critical care unit: a comparison of ultrasonography, radiography and physical examination. Equine Vet J 2007;39:158-163. Stoneham SJ, Palmer L, Cash R, et al. Measurement of serum amyloid A in the neonatal foal using a latex agglutination immunoturbidimetric assay: determination of the normal range, variation with age and response to disease. Equine Vet J 2001;33:599-603.