Learning Objectives

Knowing in which specific cases it’s worth taking the time for a complete abdominal ultrasound and what to hunt for in those cases.

Technical Considerations

A large curvilinear (2–6 MHz) transducer is required. Horses with a sport clip frequently don’t require additional clipping. In horses with thick hair, clipping is needed. The skin should be cleaned and soaked in alcohol. Coupling gel is applied.

A detailed abdominal ultrasound evaluation of the abdomen is indicated in cases with chronic colic, weight loss, fever of unknown origin and horses with other vague presenting complaints. As opposed to the fast decision-making required in acute colic, these cases benefit from a thorough detailed ultrasound examination. This lecture will cover the abnormal details to pay attention to which make a difference in case management.

Intestinal Wall Thickness and Layering

The normal wall thickness of the cecum, colon and most of the small intestine is less than 3 mm. The ileum has a more prominent muscular layer and can measure up to 5 mm. Diseases such as Inflammatory bowel disease (IBD) and alimentary lymphoma can cause thickening of the intestine. Ultrasound examination is not able to identify cells and as such can’t differentiate thickened bowel caused by IBD versus lymphoma. Colitis can also cause thickening of the colon wall, with the more specific right dorsal colitis imaged in the wall of the right dorsal colon. Although the right dorsal and right ventral colon have little sonographic difference, they can easily be differentiated by their relative position, provided there is no displacement. The right dorsal colon is found ventral to the liver in the right 11th to 13th intercostal space. In horses with this condition, a hypoechoic layer can be imaged, corresponding to submucosal edema, inflammatory cell infiltrates and granulation tissue which was confirmed on necropsy in a study of 5 cases.¹ Aside from this study, the association between abnormal wall layering and specific diagnoses has not been critically assessed. It makes sense that conditions such as IBD and lymphoma, which cause cellular infiltration of the submucosa, result in an echoic, thickened submucosal layer, whereas edema of the colon wall which occurs in colitis and colon torsion result in an anechoic wall.

Ileal muscular hypertrophy has been described as a cause of weight loss in the horse. In these cases, the hypoechoic muscular layer is markedly thickened on necropsy,² which was recently sonographically described in a case report as well³.

Other Gastro-Intestinal Abnormalities

More obvious findings such as intra-abdominal abscesses or masses can be rewarding findings in horses with a fever of unknown origin or weight loss. However, in a case series of intra-abdominal abscesses, ultrasound evaluation had a 38% sensitivity for detecting the abscess.⁴ Despite this, other clues such as increased peritoneal fluid can help raise the suspicion.

Abnormalities of the Liver

Liver disease can cause colic, fever, weight loss and depression. Ultrasonographic abnormalities include rounded margins which indicate liver enlargement, an increase in echogenicity which indicates cellular infiltration, a granulomatous response, fibrosis or hepatic lipidosis, distended biliary ducts which result in a “parallel channel sign” and decreased vascular markings.⁵ Choleliths and hepatoliths can also be imaged.

Abnormalities of the Kidneys

Small kidneys are generally associated with chronic renal failure, whereas enlarged kidneys can be associated with acute renal failure but also urolithiasis, pyelonephritis or neoplasia. Perirenal anechoic edema can be seen in horses with acute renal failure. Nephroliths can be imaged in the renal pelvis.

Abnormalities of the Spleen

Splenic abnormalities have been described in horses with weight loss, fever and depression. Splenic abscesses appear as hypoechoic masses. Splenic neoplasia, most commonly lymphosarcoma can result in a diffuse mixed echogenicity, or distinct which can be difficult to differentiate sonographically from other splenic neoplasia or even splenic abscesses and hematoma.⁵

In conclusion, a thorough sonographic evaluation of the equine abdomen provides valuable information in cases of chronic colic, weight loss, fever of unknown origin and other vague signs.

References

1.  Jones SL, Davis J, Rowlingson K. Ultrasonographic findings in horses with right dorsal colitis: five cases (2000–2001). J Am Vet Med Assoc. 2003;222(9):1248–1251.

2.  Chaffin MK, Fuenteabla IC, Schumacher J, Welch RD, Edwards JF. Idiopathic muscular hypertrophy of the equine small intestine: 11 cases (1980–1991). Equine Vet J. 1992;24(5):372–378.

3.  De Solís CN, Biscoe EW, Lund CM, Labbe K, Muñoz J, Farnsworth K. Imaging diagnosis-muscular hypertrophy of the small intestine and pseudodiverticula in a horse. Vet Radiol Ultrasound. 2015;56(2):E13–16.

4.  Arnold CE, Chaffin MK. Abdominal abscesses in adult horses: 61 cases (1993–2008). J Am Vet Med Assoc. 2012;241(12):1659–1665.

5.  Reef VB, Whittier M, Allam LG. Sonographic evaluation of the adult abdomen. Clin Tech Equine Pract. 2004;3(3):294–307.