Ultrasonography of the neonate

Ultrasonography has become a routine and important part of medical examination of the foal. The small size of the foal, lack of muscle mass and body fat facilitate the exam. A variety of ultrasound machines and transducers are available. For the detection of most disorders in neonates, extremely expensive equipment is not required, and a linear rectal probe of variable frequency (5-13MHz) is sufficient for most exams. Although we often separate disorders of the thorax and abdomen into distinct categories, both body cavities are usually evaluated during the same ultrasound examination. The ultrasound examination is performed in a cranial-to-caudal manner—passing the ultrasound probe along each intercostal space, scanning from dorsal to ventral, and then sweeping the caudal and ventral abdomen behind and beneath the ribs.

Thorax

Clipping of the thorax is generally not required for routine examinations but may be needed where additional detail and a clearer image is needed.

Disorders of the thorax in the neonatal foal include rib fractures, pneumonia, and effusions (septic, hemorrhagic, or other). Rib fractures are common in hospitalized neonatal foals, and ultrasound is considered a more accurate method of detection compared with radiography and physical examination. Fractures are most often located with 3cm of the costochondral junction and more commonly involve the first few ribs behind the elbow. Fractures may be non-displaced to severely displaced and are categorised by the number of mm separating the fracture fragments. Fractures may displace over time as the foal becomes more mobile with the distal segment usually moving medially. There may be fluid or hemorrhage in the soft tissues surrounding the fracture ends. Injury to the underlying pulmonary parenchyma can vary from mild bruising with a few echogenic “comet tails” to progressively more involvement with parenchymal consolidation and occasional hemothorax or pneumothorax. Serial evaluation of the degree of displacement is recommended to determine if there is an indication to consider surgical stabilization versus conservative management of restricted mobility in the stall. Ultrasound can also be useful in monitoring the fracture healing process— determining when there is sufficient callus formation and fracture stability to allow more exercise. With fracture of more caudal ribs, there may be injury to the diaphragm and possible diaphragmatic hernia with intestinal structures within the pleural space. In cases where the intestine lies between the lung lobes and cannot be directly visualized there is often and increase in free pleural fluid. This combined with an abnormal respiratory rate of pattern would indicate the additional use of radiography.

Ultrasound has become a routine tool in the evaluation of pneumonia in the foal. Pneumonia can be a primary anatomic site of sepsis in the neonate. Patterns of changes on the ultrasound image can be helpful in predicting the type of lung injury present. Scattered echogenic “comet tails” may be present in the early stages of a variety of bacterial pneumonias, with ventral consolidation being evident with further progression or more serious pneumonia. Broad-based or diffuse echogenic shadowing is more consistent with interstitial lung disease (pulmonary edema or interstitial pneumonia) and suggests a more serious disease process. Serial ultrasonographic examination of the lung is useful in evaluating the progression of disease and can be a component for evaluating response to medical therapy.

Abdominal ultrasound

Ultrasound examination of the abdomen of foals is often used in the evaluation of foals with signs of colic and can be useful in differentiating causes of abdominal distention in foals with and without colic signs. Ultrasound is useful in extremely useful in the evaluation of disorders of the umbilical structures and abnormalities of the abdominal wall surrounding the umbilicus and the inguinal area (e.g., traumatic injury acquired during delivery and congenital defects).

The approach to the ultrasound examination of the foal with signs of acute abdominal pain is similar to that used for the adult equine patient. There are some special circumstances and lesions that may be unique to the younger foal that must be evaluated. The use of a high-frequency (5 to 7.5 mHz or higher) probe— whether linear or microconvex—is sufficient for imaging much of the abdominal cavity in the young foal with good resolution of structures. This can be performed with the foal standing or in a recumbent position. The ultrasound examination should proceed as with the adult patient by evaluating the ventral thorax and abdomen by passing the ultrasound probe dorsal to ventral along each intercostal space beginning just caudal to the triceps muscle on each side and progressing in a cranial to caudal fashion to the thigh. The exam is completed by then sweeping the ventral aspect of the abdomen to evaluate the umbilical structures and the urinary bladder.

Gastric distension can be evaluated in the foal in a manner similar to that seen in the adult. Causes of gastric distension may include ileus with or without enteritis or small intestinal strangulation obstruction. Small intestinal obstructive disorders such as volvulus or entrapment in scrotal hernias will appear similar to that seen in the adult patient—with profoundly fluid-distended segments of small intestine occasionally with sedimentation of particulate material to the ventral or dependent aspect. With the hernia, small intestinal segments may also be evident within the vaginal tunic. Small intestinal disorders including enteritis can be easily identified in the foal. The ultrasound finding of fluid distension of the small intestinal lumen along with variable motility and variable thickening (2 to 3 mm) of the small intestinal wall concurrent with fever and leucopenia is supportive of the clinical diagnosis of enteritis. The small intestinal wall may often be less distinct with enteritis due to inflammatory cell infiltrates and variable edema of the wall.

Colic associated with small intestinal obstruction from intussusceptions occur more commonly in young foals, often secondary to enteritis or dysmotility secondary to birth asphyxia. Serial ultrasound examinations may be necessary to identify the intussusception, which is classically described as a “target lesion” with the concentric rings of the intussuscepted intestinal wall. Occasionally, the acute onset of rotaviral enteritis will result in variable signs of colic and inappetence, sometimes before the appearance of diarrhea. Abdominal ultrasound can be useful in identifying liquid contents of both the small and large intestines, which may be indicative of impending diarrhea.

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