Messin' with intestines: ultrasonography of the GI tract (Proceedings)
Sonographic evaluation of the gastrointestinal tract is a routine part of the diagnostic investigation of gastrointestinal disorders. Improved visualization of the GI tract has been achieved due to technologic advances in both ultrasound machines and with the development of higher frequency transducers. The main limitation of gastrointestinal ultrasonography is the presence of gas (inside or outside the gastrointestinal tract). Contrarily, if the lumen of the bowel is filled with fluid, mucus, or food, evaluation of the walls is improved. Wall layering and thickness as well as relative motility can be evaluated in different segments of the intestines. Five ultrasonographic layers can be identified throughout the GI tract. From the lumen to the serosal surface, one can visualize the hyperechoic mucosal interface, the hypoechoic mucosa, the hyperechoic submucosa, the hypoechoic muscularis layer, and the hyperechoic subserosa and serosa. The mucosal layer is normally thicker than the muscular layer, but these can become similar in thickness during peristalsis. The duodenum is easily differentiated from the jejunum and ileum by its fixed location along the right lateral abdominal wall. The duodenal walls are slightly thicker (3-6mm) than the jejunum (2-5mm).
When interrogating a suspected abnormal GI segment, wall thickness, layering, symmetry at the lesion site, extent of the lesion, GI contents, GI motility, and affected regional tissues should be noted. The most common ultrasonographic sign of GI disease is wall thickening. However, this finding is nonspecific and has been reported in both inflammatory and neoplastic diseases. Additionally, accumulation of luminal fluid should alert one to a motility disturbance and careful evaluation for a GI lesion in this area is indicated.
Studies in the past documented differences in the ultrasonographic appearance of the GI tract with non-specific enteritis compared to intestinal neoplasia. Neoplasia usually shows loss of wall layering, more severe wall thickening (>1.5cm), more commonly had focal distribution, and draining jejunal lymph nodes are larger (>1.9cm). Enteritis results in mild to moderate diffusely distributed intestinal wall thickening, without loss of layering. Corrugation of the intestines has been seen with enteritis, but also can be associated with pancreatitis, peritonitis, thrombosis/ infarction and protein losing enteropathy. Intestinal wall thickness, though increased with enteritis, is insensitive and a nonspecific indicator of the presence and differentiation of inflammatory bowel diseases. Recent studies have identified other ultrasonographic changes in the bowel which help to differentiate between specific causes of enteritis with relatively high sensitivity and specificity. Mucosal echogenicity may be a better parameter for detecting inflammatory disease of the intestines in dogs with chronic enteropathies. Changes to the normal hypoechoic echogenicity of the mucosal layer can be used to predict likely etiologies of intestinal diseases. With food responsive enteropathies (due to food allergies), the normally hypoechoic mucosal layer was unaltered. A diagnosis of protien losing enteropathy with associated lymphangiectasia was highly correlated with increased mucosal echogenicity. With this diagnosis, the increased mucosal echogenicity appeared as linear hyperechoic mucosal striations, oriented perpendicular to the bowel lumen. Hyperechoic mucosal speckles are sensitive for the presence of inflammatory disease, yet they are non-specific for differentiating disease category. Interestingly, the increased mucosal echogenicity, regardless of underlying disease, did not resolve with treatment.Parvovirus enteritis has also shown to have very specific ultrasonographic changes when compared to the normal gastrointestinal tract of puppies. Parvovirus causes significant mucosal thinning of the duodenum and jejunum related to villus sloughing and mucosal erosion/ ulceration. Mucosal thinning is present with concurrent overall normal wall thickness. Contrarily, other causes of enteritis result in increased overall wall thickness. Additional findings with parvovirus enteritis include diffuse fluid filling of the small and large intestines without normal peristalsis, duodenal hyperechoic mucosal speckling, duodenal and jejunal corrugation, and generalized indistinct wall layering.