Ultrasonography is invaluable in the diagnosis of the cause of colic in horses. The sonographic findings can aid the veterinarian in determining if the horse has a medical or surgical lesion. Diagnostic ultrasonography provides a window for noninvasive visualization of gastrointestinal viscera, which are otherwise difficult to examine. Transrectal ultrasonographic evaluation of abnormalities detected on rectal palpation can also be performed in adult horses to further clarify the rectal findings.

Normal Ultrasonographic findings in the equine gastrointestinal tract

Only large intestinal echoes are usually imaged in the intercostal spaces (ICS) and the flank in the adult horse. Occasionally small intestinal echoes are imaged between the stomach and spleen and in the caudal ventral abdomen of the adult horse. Both large and small intestinal echoes are usually imaged from the ventral abdomen in the foal, while primarily large intestinal echoes are usually imaged in the intercostal spaces (ICS) and the flank. The large intestinal echoes are recognized by their large semi-curved, sacculated appearance, except for the right dorsal colon. The right dorsal colon has a smoother nonsacculated appearance and is usually imaged from the right 14th – 10th intercostal spaces. The large intestinal wall is hypoechoic to echogenic with a hyperechoic gas echo from the mucosal surface and normally measures 3.5 mm or less in thickness. Peristaltic activity is normally visualized. The small intestinal echoes are recognized by their small tubular and circular appearance. The wall of the jejunum is hypoechoic to echogenic with a hyperechoic echo from the mucosal surface and is usually 3 mm or less in thickness. Some anechoic fluid and hyperechoic gas is often imaged in the lumen of the jejunum. Peristaltic waves are also normally visualized. The duodenum is imaged around the caudal pole of the right kidney and medial to the right liver lobe. It appears small circular (when sliced in its short axis) with a hypoechoic to echogenic wall, also < 3 mm in thickness, and has a fluid lumen. The duodenum usually appears partially collapsed and its peristaltic motion is easily visualized during real-time scanning. The gastric fundic echo is visualized in the left 9 - 12th ICS and is imaged as a large semi-circular structure medial to the spleen at the level of the splenic vein. In the neonate the stomach is also imaged from the ventral abdominal window, caudal to the liver. The gastric wall is hypoechoic to echogenic with a hyperechoic gas echo from the mucosal surface and normally measures up to 7.5 mm in thickness. Gastric rugal folds can be often be imaged in adult horses.

Herniation/Displacement

Abnormal positioning of the gastrointestinal viscera is difficult to diagnose ultrasonographically, unless the viscera are displaced into the scrotum, thoracic cavity or into an umbilical hernia. Displacement of the gastrointestinal viscera into the thoracic cavity through a diaphragmatic hernia can usually be diagnosed ultrasonographically by scanning the affected side of the thorax and cranial abdomen and looking for the rent in the diaphragm, as displacement of the overlying lung by the herniated viscera occurs. The approximate size of the diaphragmatic hernia can be estimated and the gastrointestinal viscera evaluated for the degree of bowel compromise. However, a diaphragmatic hernia could be missed ultrasonographically if it was located in the center of the diaphragm and the herniated viscera were not in contact with the thoracic wall. In horses with abdominal wall hernias or ruptures of the prepubic tendon, diagnostic ultrasonography can be used to measure the size of the defect, so an appropriately sized piece of mesh can be prepared preoperatively for implantation in horses with abdominal wall hernias. Furthermore the contents of the hernial sac and the presence of any adhesions can be identified and the hernial ring described.

Nephrosplenic ligament entrapment

Diagnosis of a nephrosplenic ligament entrapment is suspected ultrasonographically, based upon the inability to visualize the spleen or left kidney transabdominally and the visualization of ingesta and/or gas filled large bowel instead. The spleen is ventrally displaced. The most dorsal portion of the spleen that can be imaged has a straight horizontal dorsal border extending from the paralumbar fossa to the 10-12th intercostal space, at which point the colon is no longer visible due to the intervening lung. Dorsal to the spleen a bright hyperechoic reflection is imaged from the displaced or entrapped large colon. The sonogram can be used to see if treatment with phenylephrine, followed by lunging, or rolling the horse has successfully corrected the nephrosplenic ligament entrapment.