J. Barry David

Hagyard Equine Medical Institute, Lexington, KY, USA

The ultrasound machine is one of the most useful pieces of equipment in the neonatal intensive care unit (NICU). The indications to use ultrasonography are numerous and the images in this chapter were generated while investigating the potential causes of fever, colic, tachycardia, tachypnea, anemia, hypoproteinemia, azotemia, and abnormal heart or lung sounds, in the NICU patient. Other uses for ultrasound in neonates include the diagnosis of ocular problems, arthropathies, tendon and ligament injuries, and the investigation of swelling in various locations.

The neonate’s size and lack of body fat makes it easier to get quality ultrasound images as compared to an adult horse. In addition, due to their small size, one can use ultrasound probes with a higher frequency, which will also improve image quality. Generally, the use of a 5–10 mHz curvilinear probe, with alcohol as a contact medium, will produce excellent diagnostic images for most of the examinations on foals. An exception for this is echocardiography where sector probes remain the gold standard to obtain quality, diagnostic cardiac images. All considered, neonates are amenable to ultrasound, no matter what probe the practitioner has. A 5 mHz linear probe can produce quality diagnostic images during thoracic and abdominal examinations.

The medical/physical condition of the neonate will dictate the position of the patient during the examination. Ultrasonography of a standing foal provides the clinician with images of the internal organs that are in the most recognized orientation; but frequently the neonatal patient is recumbent. If a foal is recumbent, the image of the pulmonary tissue can change significantly. The down-side lung on a recumbent neonate typically has sonographic changes that may or may not be associated with pathology; the changes may be the result of perfusion/ventilation mismatch. Conversely, a recumbent neonate’s upper lung appearing sonographically abnormal is significant. The orientation of the abdominal organs in a recumbent foal will change slightly compared to the standing foal, but minimal differences in the echogenic characteristics of the organs are seen between the standing and recumbent foal. The position of the heart during ultrasound examination of the recumbent neonate may preclude the sonographer from obtaining standard diagnostic views, as opposed to a standing foal. However, most of the time, an accurate diagnosis of the cause of a particular murmur can be made in a recumbent foal. When scanning the foal’s heart, the hair over the cranioventral thorax, on and behind the triceps muscle, may need to be clipped, but many times the foal’s haircoat is thin enough to scan without clipping. Generally, only alcohol is required as a contact medium, but ultrasound gel may facilitate obtaining quality cardiac images. While scanning the heart, the ultrasound probe should be placed between the cranial intercostal spaces, with the longitudinal plane oriented parallel to the rib surfaces. The neonate’s intercostal spaces are narrow, which may make the acquisition of diagnostic images challenging.

Ideally, a recumbent foal will be maintained in a sternal position to aid in preventing the development of pneumonia of the dependent lung, but in some cases (e.g., seizure activity, rib fracture), maintaining sternal recumbency is not possible. When the foal must remain in lateral recumbency during the examination, assess the sonographic appearance of the independent or upper lung. After the upper lung is scanned, the foal should be turned, and after a short period of time, the other lung can be scanned. Clipping a foal’s chest for thoracic ultrasonography is generally not necessary and the use of alcohol, as a contact medium, is most practical. The clinician should perform a complete and thorough examination of each side of the foal’s chest. The entire lung field should be investigated by placing the probe dorsally, in each intercostal space, with the longitudinal plane of the probe parallel to the rib surface, and slowly moving it ventrally until the diaphragm is visualized. The more cranial regions of the lung can be imaged directly through the triceps muscle. The scanning technique to search for rib fractures is identical to scanning lungs, except now the probe is placed directly on the rib surface. The diaphragm is an important structure to identify as it separates the pleural cavity from the peritoneal cavity. Being able to identify the diaphragm will allow the clinician to determine whether an abnormal accumulation of fluid is in the chest or abdomen. It will also be important to visualize the diaphragm to make a diagnosis of diaphragmatic hernia.

When scanning a neonate’s abdomen, clipping the foal’s hair is usually not necessary to obtain clear, diagnostic images. As with the thorax, alcohol is a practical, rapidly applied contact medium. When scanning the lateral aspects of the abdomen, the technique is the same as scanning the thorax; place the probe in the intercostal space, with the probe’s longitudinal plane parallel to the rib surface and move slowly ventrally. Organs that will normally be visualized on the left lateral abdomen in the neonate include the liver, spleen, left kidney, stomach, and portions of the intestinal tract. Visualizing the stomach full of milk may be a normal finding if the foal has just nursed and is not demonstrating signs of colic. Organs visible on the right lateral side of the neonate’s abdomen include the right dorsal colon, duodenum, liver, and right kidney. The bladder, ventral and dorsal colons, small colon, small intestine, internal umbilical structures, and often the kidneys can be visualized from the ventral abdomen. Although the orientation of the organs may be altered by recumbency, the differences between images obtained in the standing neonate, compared to a recumbent one, are generally not significant. The exception to this statement is if the foal has an accumulation of fluid in the abdomen. Abdominal fluid is easier to identify and quantify when the neonate is in the standing position. The following images provide some examples of neonatal ultrasonography.

Thoracic Cavity