Ultrasonography of the Gastrointestinal Tract
College of Veterinary Medicine, Washington State University, Pullman, WA, USA
Ultrasonography can be applied to a significant amount of the gastrointestinal tract from the caudal pharynx and esophagus in the neck to the liver, stomach, and large and small intestines within the abdominal cavity. Depending on location and depth required to image the specific structure of interest, probes of multiple frequencies may be utilized. Superficial structures, such as the esophagus, and highest detail of intestinal surfaces may be best imaged using higher-frequency (7.5 MHz or greater) probes. Evaluation of the abdominal cavity as it pertains to the gastrointestinal tract includes evaluation of the liver, stomach, small intestine, and large intestine. The author performs the transabdominal ultrasound examination progressing in a cranial-to-caudal fashion on each side. The exam starts cranially at the third intercostal space, just caudal to the elbow, and progresses in a caudal fashion while scanning dorsal to ventral in each intercostal space from the diaphragm margin ventrad.
Cranially on the left side, the liver is visible adjacent to the spleen. Moving caudally, the stomach is visualized adjacent to the spleen. The stomach is evaluated for contents and size. The normal stomach in the adult horse usually extends over four to five intercostal spaces. Normal small intestine has minimal visible structure. It may be visible with variable motility just ventromedial to the spleen on the left side. The colon is visible ventromedial to the spleen on the left side. On the right side, the large colon is visible in the cranial abdomen. Progressing caudally, the liver is visible ventral to the margin of the lung fields over most of the mid-abdomen. Horses in their mid teens and older may have atrophy of the liver and it may not be readily visualized with ultrasound. The duodenum is visible ventromedial to the liver from the twelfth intercostal space to just ventral to the right kidney. The right dorsal colon is ventromedial. Ventral–medial to the duodenum is the right dorsal colon. The cecum will be visible in the right paralumbar fossa and extends ventrally along the costal arch and can be distinguished by its lateral vasculature.
On occasion, transrectal ultrasonographic examination may be useful in further characterizing abdominal masses, rectal masses, or intestinal surfaces.
The gastrointestinal anatomy within the abdominal cavity can be dynamic, especially in colic, where abnormal intestinal motility can be involved. Because of this, the author emphasizes the importance of serial sonographic evaluation over time to determine if anatomic changes have occurred, such as position of the colon or amount of gastric or small intestinal distension.
The esophagus (Figure 24.1) can be imaged from just caudal to the pharynx in the upper neck on the left side along most of its course to the distal neck. It is located just dorsal to the trachea and ventral to the jugular vein and carotid artery. The muscularis appears hypoechoic, whereas the lumen appears echogenic.
The normal stomach (Figures 24.2, 24.3) is imaged adjacent to the spleen on the left side of the abdomen.
Gastric impaction should be considered in animals with a prolonged clinical history of inappetence, mild colic signs, and sonographic evidence of solid gastric luminal content visible over more than five intercostal spaces. Gastric distention (Figures 24.4, 24.5, 24.6) from solid ingesta, fluid, or gas is a concern when the stomach extends beyond approximately five rib spaces.
The duodenum (Figures 24.7, 24.8) can be imaged along its course on the right side. It is first seen cranially as it appears between the left liver lobe and the right dorsal colon, and then progresses caudad to a point ventral to the right kidney.
Duodenal distension (Figure 24.9) can occur with distal small intestinal obstruction, postoperative ileus, or duodenal stricture and obstruction, as well as varying degrees of thickening and distension with enteritis or proximal enteritis syndrome.
Duodenitis (Figure 24.10) and duodenal stricture (Figure 24.11) can produce gastric outflow obstruction in foals. The thickened segment of duodenum can often be imaged along its course on the right side of the abdomen. Ultrasound can be useful in making the initial diagnosis, as well as monitoring medical therapy and aid in evaluation of postoperative patients that had gastrojejunostomy to relieve the gastric outflow obstruction.
Normal jejunum (Figure 24.12) usually has a nondescript appearance on ultrasound. It is usually visible just ventral and medial to the spleen in the caudal left abdomen around the level of the costal arch.
Small intestinal distension (Figures 24.13, 24.14, 24.15, 24.16, 24.17, 24.18, 24.19, 24.20, 24.21, 24.22, 24.23) can indicate strangulating or non-strangulating obstruction, enteritis, or altered motility such as occurs with postoperative ileus. Clinical history, severity of clinical signs, and occasionally other diagnostic aids, such as abdominocentesis, are required to differentiate between these types of lesions prior to confirmation by exploratory surgery.