Equine ultrasonography: how can we benefit?


Ultrasonography is a diagnostic imaging technique for visualizing internal body structures. Visualization of structures such as tendons, muscle, blood vessels as well as internal organs for possible pathology and lesions may provide valuable information to a veterinarian. Not only does it provide information from a diagnostic point of view, but accurate interpretation of ultrasonographic pictures may also provide valuable supporting information to treatment options, response to treatment and may aid as a prognostic indicator in certain disease processes.

Ultrasound refers to sound waves at a frequency undetectable to the human ear. Ultrasound images are made by sending a pulse of sound waves into the tissues using an ultrasound transducer (probe). As the sound waves moves through tissue, they are reflected (echoes), and are recorded and displayed as images to the operator.

Many different types of images can be displayed using ultrasonography. The most well-known image is the B-mode image which displays the acoustic impedance of a two dimensional cross-section of tissue. Other types of images that can be displayed are blood flow and the motion of tissue over time. What differentiates ultrasonography from other methods of imaging is that it provides a real-time image, if portable it can be used for bedside imaging, it’s fairly affordable and has proven to be a safe, non-invasive and useful diagnostic imaging modality with an increasingly wide application.

How can ultrasonography be of use in equine medicine?
One of the most common applications of ultrasonography in the horse is evaluating the horse with colic. Although the value of ultrasonography in the adult horse has been questioned, it is still regarded as invaluable in a horse with colic. Limitations include the size and depth of the abdomen, partial enclosure of the abdomen by the ribs and the degree of gas distension sometimes present. Despite these limitations ultrasonography provides extensive information that cannot be achieved in any other way and can aid the veterinarian in determining whether a horse has a medical or surgical lesion. It provides a more accurate interpretation and detection of small intestinal strangulating lesions requiring surgery and non-strangulating lesions (Fig 1).

Figure 1: Distended loops of small intestine (9cm x 6 cm diameter) visible ultrasonographically. This type of image can be seen in horses with a small intestinal impaction or enteritis

Progression of small intestinal abnormalities can be monitored and appropriate diagnostic information can be obtained, resulting in earlier surgical intervention. Intestinal wall thickness indicative of colitis (Fig 2) can also be detected ultrasonographically.


Figure 2: Note the increased wall thickness in the picture above. This picture is a perfect example of right dorsal colitis (thickened colon wall visible below the liver on the right side of the abdomen.
Other gastrointestinal conditions readily detectable are secondary gastric distension (Fig 3), nephro-splenic entrapments (Fig 4), peritonitis (infection and inflammation within the abdominal cavity), heamo-abdomen (blood within the abdomen), abnormal growths or masses (neoplasia and abscesses) and intestinal adhesions.


Figure 3: Severe gastric distension.


Figure 4: Ultrasonographic presentation of nephrosplenic entrapment in the horse. In a normal horse the spleen and kidney is usually next to each other, however, with a left dorsal displacement of the large colon (nephrosplenic entrapment), the large colon can be visualised next to the spleen, as in this picture.


Ultrasonography can also prove invaluable in the musculoskeletal and respiratory organ systems. The extent of tendon injuries and response to treatment can be diagnosed and monitored ultrasonographically. Location of foreign bodies is another example of the usefulness of ultrasound (Fig 5).

Figure 5: Ultrasound picture of the gluteal region of a horse with a shotgun pellet acting as a foreign body causing a chronic draining tract. Note the little shadow casted by the foreign object.

Sonographic evaluation of the equine respiratory tract is useful for diagnostic, therapeutic and prognostic evaluation of the peripheral lung parenchyma or pleural disorders. It is considered superior to thoracic radiography for detecting effusions, consolidation of the lung, abscesses and tumours. Thoracic ultrasonography has been used successfully in the diagnosis, and need for therapeutic intervention, in foals affected by Rhodococcus pneumonia. One limitation of thoracic ultrasonography is that a lesion situated deeper within the lung tissue may not be detected if situated underneath normal aerated lung tissue. The reason for this is that air or gas reflects the ultrasound beam making it impossible to see deeper within the lung tissue.

Unlike adult horses, the size of foals makes them more suitable for scanning, overcoming some of the limitations in adult horses. Ultrasonography is advised in all septicaemic foals and for conditions such as omphalitis (umbilical infection), which is not always visible externally, pneumonia and gastrointestinal disorders. Most umbilical infections respond to medical intervention alone, and response to treatment with antibiotics can be monitored by performing repeat ultrasonographic evaluations.

In summary, ultrasound is a very useful diagnostic modality readily available. Subtle changes may require more experienced interpretations but in general can provide a veterinarian with valuable information. It should always be interpreted with the clinical picture of the horse, and can complement other diagnostic procedures in making a definitive diagnosis.

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