Ultrasound of the Nonacute Abdomen: Gastrointestinal Tract

1. Introduction

Ultrasonography is invaluable in the diagnosis of a wide variety of gastrointestinal diseases in horses and determining if the horse has a medical or surgical lesion. The quick evaluation of the abdomen in horses with severe colic (FLASH examination) is an examination of selected high-yield areas of the abdomen that can be done in the emergency setting in less than 15 minutes by individuals who are not extensively trained in diagnostic ultrasound.¹  The windows for the FLASH scan of the emergency abdomen include the left middle third of the abdomen, the renosplenic window, the gastric window, the ventral abdomen, the duodenal window, the right middle third of the abdomen, and the right cranioventral thorax.¹  In postoperative ponies, bowel handling during exploratory celiotomy caused minimal changes in bowel wall thickness, contractility, amount of distention, luminal contents, and peritoneal fluid.²  Therefore, abnormal gastrointestinal motility, size or contents; bowel wall thickness, and/or changes in the quantity or echogenicity of the peritoneal fluid are clinically significant in a postoperative patient. A complete sonographic examination is also very useful in evaluating the horse with chronic colic, chronic weight loss, or when other abdominal disease is suspected. Sonographic evaluation of the abdomen is also valuable for following the clinical progress in horses with a wide variety of gastrointestinal diseases.

The sonographic examination of the gastrointestinal tract can be performed with a microconvex transducer, a high-frequency linear transducer for evaluation of bowel wall thickness close to the body wall, and a transrectal transducer for evaluating any abnormalities detected on rectal examination. A large lower-frequency convex transducer is useful for the FLASH examination, in which time is of the essence in making the diagnosis and the subtleties of the image are less important, or for evaluating gastrointestinal structures that are further away from the transducer. The standard linear transrectal transducer can be used and placed over any structure that is abnormal on rectal palpation to further define the abnormality but is limited by the depth of penetration of the transducer and the ability to place the transducer directly over the abnormality. A small microconvex transducer can also be used transrectally and has the advantage of a wide field of view and being able to direct the transducer towards the abnormality without having to place the transducer directly over the abnormality.

2. Normal Ultrasonographic Findings in the Equine Gastrointestinal Tract

The large intestinal echoes are recognized by their large semicurved, sacculated appearance, except for the right dorsal colon which has a smoother nonsacculated appearance and is imaged consistently in the right 11th and 12th intercostal spaces in both normal horses and horses with right dorsal colitis.³ The large intestinal wall is hypoechoic to echogenic with a hyperechoic mucosal gas echo and measures ≤3.5 mm in the adult horse. Peristaltic activity is normally visualized. The cecum has larger contractions and a trend towards smaller sacculations compared with the large colon in normal horses.⁴ The small intestine has a small tubular and circular appearance with a hypoechoic to echogenic wall that usually measures ≤3 mm in the equine adult. 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 and circular with a hypoechoic to echogenic wall, also measures ≤3 mm, and appears partially collapsed with its peristaltic motion easily visualized. Jejunal, cecal, and colonic motility is decreased with fasting.⁵ The gastric fundic echo is visualized in the left 9th to 12th intercostal space and is imaged as a large semicircular structure medial to the spleen at the level of the splenic vein. The gastric wall is hypoechoic to echogenic with a hyperechoic gas echo from the mucosal surface and normally measures ≤7.5 mm. All the wall thicknesses in normal adult ponies are significantly thinner than in the normal adult horse.²

Large intestinal motility was significantly reduced (most marked in the aboral left ventral colon) in stabled horses compared with pastured horses.⁶ Xylazine in fasted horses decreased jejunal and cecal motility.⁵ Romifidine has been shown to result in decreased motility (nonpropulsive contractions) of the jejunum, cecum, and left ventral colon.⁷ Both feeding status and sedation need to considered when evaluating GI motility in horses.⁵

3. Gastrointestinal Diseases

Right Dorsal Displacement of the Large Colon

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. Right dorsal displacements have traditionally been difficult to definitively diagnose. Horses with a colonic displacement and an elevated gamma-glutamyl transferase (GGT) are most likely to have a right dorsal displacement of the large colon (RDDLC).⁸ The success of treating horses with right dorsal displacements has been reported to be 64% in a recent study.⁹ In this study, right dorsal displacement was diagnosed by the identification of a gas distended colon oriented horizontally across the abdomen on rectal palpation and the sonographic finding of a large gas filled large colon.⁹ The detection of abnormally located large colon mesenteric vessels, distinct from cecal vessels, along the right lateral abdomen, dorsal to the costochondral junction in at least 2 intercostal spaces (between intercostal space 10 to 16) is consistent with a surgical diagnosis of right dorsal displacement of the large colon (Fig. 1).¹⁰ This finding was not seen by these investigators in other types of surgical colics. Although these investigators did not see these abnormally located mesenteric vessels in the right lower abdominal area in horses with a large colon volvulus, this remains a possibility.¹⁰ 
 

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