Cummings School of Veterinary Medicine, Tufts University, North Grafton, MA, USA
In this presentation, disorders such as intussusception, mechanical ileus, inflammatory bowel conditions, ulceration, and dehiscence are described. All these conditions along with the differential diagnoses are discussed in light of the clinical context and other available imaging modalities.
The main ultrasonographic feature of an intussusception is the multilayered appearance of the wall (concentric rings) representing the wall layers of the intussusceptum and intussuscipiens. The pattern varies somewhat with the length of the bowel segment involved, the duration of the process and the orientation of the scan plane relative to the axis of the intussusception. The intussuscipiens (outer bowel segment) is often edematous and hypoechoic, whereas the intussusceptum may appear normal in thickness and layering. The invaginated bowel can also be associated with invaginated mesenteric fat, inflammatory pseudocysts, enlarged lymph nodes or occasionally tumors in older dogs.
Mechanical and functional ileus refers to obstructive conditions of the gut. Mechanical ileus can be associated with GI foreign bodies, intussusception, localized peritonitis/adhesion, and neoplasia.
Foreign bodies (FB) are characterized by their size, shape, location, and internal architecture.
Irregularly shaped FB are difficult to identify ultrasonographically unless they cause a complete obstruction with fluid accumulation proximal to the obstruction site. In acute obstructive conditions, the peristaltic activity is often increased. FB are often highly reflective objects associated with strong acoustic shadowing, but on occasions, they can be transonic, or layered.
Wall thickening is the most common finding of inflammatory diseases, but this finding is not specific. The severity of the thickening does not appear to be useful for distinguishing different inflammatory diseases. Symmetry, the extent of the wall thickening and layer identification are used to distinguish inflammation from neoplasia. Inflammation is usually characterized by extensive and symmetric wall thickening with preserved layering, while neoplasia is often associated with localized, asymmetric wall thickening with disrupted layering.
In gastritis, diffuse or localized wall thickening may be identified. Gastric ulcers may be identified as discrete, central mucosal defects with the accumulation of microbubbles. Hypomotility is commonly observed in gastritis and ulcerated gastric cancer.
Thickening of the intestinal wall is the most common ultrasonographic finding in inflammatory bowel diseases.
Inflammation is characterized by extensive and symmetric wall thickening while often retaining layer identification.
Enteritis may produce different ultrasonographic appearances depending on the localization, duration and severity of the inflammatory condition. Common inflammatory changes such as lymphocytic plasmocytic enteritis (LPE) can be detected by mild to moderate thickening of one or several intestinal segments. Mildly thickened bowel usually can be identified by comparison to other intestinal segments of the same animal. In addition, the affected segment(s) can appear hypomotile and "rigid" as a small amount of fluid and/or ingesta floats in the lumen. As in people, the early stages of LPE primarily affect the mucosa and submucosa. The prominent mucosa is unevenly increased in echogenicity, and the demarcation between the mucosa and submucosa may be indistinct. The presence of bright mucosal speckles or perpendicular lines can be observed in some inflammatory bowel disease. The linear hyperechoic lines within the mucosa, aligned perpendicular to the lumen axis often represent dilated lacteals. This finding is commonly associated with protein-losing enteropathy and lymphangiectasia, and uncommonly seen in infiltrative tumors. The submucosa can appear thickened and uneven. In some conditions, the muscularis layer is moderately thickened. This condition may correspond to an idiopathic hypertrophy of the smooth muscle layer. In these instances, a "daisy-like" pattern of the affected segments of bowel can be seen. This pattern describes the circular, convoluted appearance of the mucosa seen in transverse section. Smooth muscle thickening can be present in chronic enteritis, particularly in cats, but this finding is not specific and also can be present in other disorders, such as mechanical obstruction secondary to foreign material or tumoral infiltration. Mild to moderate regional reactive lymph nodes enlargement is often encountered in inflammatory GI diseases.
Corrugated intestines appear as undulated bowel segments. This nonspecific finding can be seen in association with regional inflammation such as enteritis, pancreatitis, peritonitis, or abdominal neoplasia, or bowel ischemia.
In cases of severe inflammatory changes encountered with lymphoplasmacytic, eosinophilic, or granulomatous enteritis, edema, hemorrhage and fibrosis can severely disrupt the wall layering and be associated with mass lesions mimicking a tumoral process.
In perforation secondary to foreign-body migration, deep ulceration, or postoperative dehiscence, the affected wall is thickened and hypoechoic, and there is focal loss of layering. At times, a hyperechoic tract can be seen dissecting the wall, and the adjacent mesentery/omentum is significantly increased in echogenicity because of focal steatitis or peritonitis. Fluid accumulation is often noted near the perforation or dehiscence site, and free peritoneal gas can sometimes be detected as short, bright, linear interfaces associated with comet-tail artifacts.