Abdominal Ultrasound in the Horse

Ultrasonographic evaluation of the abdomen is extremely useful in characterizing certain components of the disease process in the foal or horse with colic. The findings of the abdominal ultrasound can distinguish surgical and medical colic patients. It is a very useful non-invasive diagnostic tool and can be used to guide other techniques such as abdominocentesis and liver biopsies. It can be carried out trans-abdominally or trans-rectally. The latter scenario is useful for confirming andormalities palpated on prior rectal examinations. This technique is limited in large horses as the ultrasound beam cannot penetrate further than 30 cm and some images can be obstructed by the gas within the ultrasound lumen interfering with the beam. In general, a 3.0-MHz or 5.0 MHz zector scanner is used. The 3.0-MHz scan head can penetrate deeper than the 5.0-MHz head but with a loss of clarity of the image. In adult horses, images of the large intestine can be achiever by scanning between the intercostal space, flank and the ventral abdomen. It may be possible to see a portion of the jejunum by scanning the midventral abdomen in some horses. In foals, it is possible to see both large and small intestine by scanning the ventral abdomen.

The horse should be restrained in stocks ideally. The desired region is clipped and a generous quantity of coupling gel is applied. The liver can be viewed from both left and right sides, however if there is a reduction in hepatic mass then it will be easier to view it from the left side, between the 7th and 9th intercostal spaces. The hepatic architecture, size, location and dimensions of the hepatic veins and bile ducts should be noted. The spleen can be visualised from the left hand side between the 8th and 17th intercostal spaces. The same parameters should be noted.

The fundus of the stomach can be imaged from the left side, ventral to the diaphragm and lung,medial to the spleen, between the 9th and 12th intercostal spaces. The wall of the stomach is hypoechoic to echoic and can be up to 7.5 mm thick.

The duodenum can be imaged on the right side of the horse, starting at the 10th intercostal space, medial to the right lobe of the liver. continuing caudally alongside the right dorsal colon up to the caudal pole of the right kidney. In the short-axis (cross-sectional) it appears as an oval structure with a wall thickness of 3 mm or less. The duodenum often appears collapsed with periodic waves of peristalsis seen in real-time on the ultrasound image. The jejunum is not often visualised in adult horses. It may be seen in some horses when scanning the mid-ventral left abdomen, running next to the stomach. Comparatively, it is easily visualised in foals running along the ventral abdomen. The small intestines appear as small tubular structures in the longitudinal-axis and circular in the short-axis. The ileum can be visualised by scanning trans-rectally in the adult horse as it lies in the dorsocaudal abdomen. It is a more muscular part of the small intestine as has a wall thickness of 4 - 5 mm. Periodic waves of peristalsis can be visualised in every part of the small intestine.

The large intestine echoes have a characteristic large semicircular, sacculated appearance with a normal wall thickness of up to 3 mm. Images can be achieved by scanning the flank and between the intercostal spaces. The caecum is best imaged in the right paralumbar fossa. The right dorsal colon can be scanned between the 10th and 14th intercostal spaces, ventral to the liver.

In the normal horse, there should only be a small volume of anechoic peritoneal fluid which will lie in the cranioventral abdomen.

References

• Meuller E, Moore J. N (2008) Classification and Pathophysiology of Colic, Gastrointestinal Emergencies and Other Causes of Colic, in Equine Emergencies- Treatments and Procedures, 3rd Edition, Eds Orsini J. A, Divers T.J, Saunders Elsevier, pp 39-40
• Rose R.J, Hodgson D.R (2000) Examination of the Alimentary Tract, Alimentary Tract, Manual of Equine Practice, 2nd Edition, Saunders Elsevier, pp 287-288