Ultrasonographic Investigation of the Intestinal Obstruction in the Dog and Cat
Michail N. Patsikas, DVM, PhD, DECVDI
Assistant Professor, Radiology Section, Clinic of Surgery, Faculty Veterinary Medicine, Aristotle University of Thessaloniki
Intestinal obstruction (ileus) can occur in any part of the intestinal tract but most often develops in the small intestine as a result of its narrower lumen. The obstruction can be anatomical or functional and the clinical signs vary with the site of the lesion, the degree of luminal obstruction, and the severity of the cardiovascular compromise.
Intestinal obstruction suspected by clinical and laboratory findings can be confirmed by radiography. Plain abdominal radiographs may show excessive bowel distention with gas usually proximal to the obstruction and fluid filled intestinal loops. A contrast study using barium sulfate suspension typically reveals bowel distension proximal to the obstruction. The barium-impregnated polyethylene spheres (BIPS) are successfully used to detect physical (complete and partial) and functional intestinal obstruction. However, in most of the cases the etiology of the obstruction is not evident on plain radiographs while in some cases the lack of normal peristalsis prevents contrast media from reaching the point of obstruction.
Ultrasonography is considered a useful imaging modality for the investigation of the intestinal obstruction. Advantages of ultrasonography compared to the conventional radiography in the diagnosis of intestinal obstruction include: the lack of ionizing radiation with the technique, no need for anaesthesia, reduction in the time required, observation of the intestinal motility, visualization of the intestinal wall layers and examination of adjacent structures such as lymph nodes, pancreas, liver and spleen that are not usually visible radiographically. Ultrasonographic evaluation of the intestine requires ventral abdominal hair clipping and fasting for 12 hours. A real time sector scanner with a 5 and/or 7.5-MHz transducer is usually used.
Transverse, longitudinal and oblique scans of the intestine are required with the animal position depending upon operator preferences. Except for the proximal duodenum, the other portions of the small intestine cannot be specifically identified ultrasonographically because of the lack of anatomic landmarks. The target-like appearance of the cecum and the air-filled colon are easily distinguished.
Five layers are normally visible ultrasonographically in the wall of the small intestine: the hyperechoic mucosal surface, hypoechoic mucosa, hyperechoic submucosa, hypoechoic muscularis, and hyperechoic serosa. The main ultrasonographic patterns of the luminal content of the intestine are the mucus, the fluid and the gas pattern. The mucus pattern is characterized by hyperechoic lumen without acoustic shadowing. The fluid pattern is observed as anechoic lumen and the gas pattern appears as intraluminal hyperechoic interfaces casting a "dirty" shadow. In the dog, the duodenal wall is 4-5 mm, and the rest of the intestinal wall usually 2-3 mm thick. In the cat, duodenum is 2.5-3 mm, and the remaining intestine about 2mm thick. The average number of contractions is 4-5 per minute for the duodenum, and 1-3 for the rest of the bowel. Motility is rarely noted in the descending colon.
The most consistent ultrasonographic findings in mechanical obstruction are the segmental fluid-filled dilated intestinal loops with increased or decreased peristaltic activity, pendulous movement of the ingesta, the presence of a foreign body or invaginated intestinal loops in the distended bowel and the presence of akinetic intestinal loops together with free abdominal fluid accumulation. In chronic mechanical and in paralytic ileus there is a decrease or even absence of the intestinal contractions and usually a generalized intestinal dilation. The causes of the mechanical ileus are: a) intraluminal foreign objects as linear and nonlinear foreign bodies; b) intramural masses as neoplasia, granuloma, haematoma and masses of parasites; c) extramural compression as hernias, adhesions, intussusceptions and volvulus; d) congenital lesions as atresia. However, intestinal foreign bodies, intussusceptions and intestinal tumours are the most common causes of intestinal obstruction in dogs and cats.
The ultrasonographic appearance of the intestinal foreign bodies is depended mainly on their acoustic impedance. Foreign bodies that transmitted sound and those with characteristic shape that produce strong acoustic shadowing may be recognizable. The bowel segment with a linear foreign body has a typical "accordion-like" appearance in longitudinal sections. However, many foreign bodies are not detectable so that ultrasonography is able to reveal intestinal foreign bodies in approximately half of the cases.
Ultrasonography is a sensitive and specific method in detection of intestinal intussusceptions in dogs and cats. The multiple hyperechoic and hypoechoic concentric rings around an echogenic core in transverse sections and the multiple hyperechoic and hypoechoic parallel lines in longitudinal sections are characteristic ultrasonographic patterns of this condition.
Intramural masses usually cause incomplete obstruction of the intestine. Intestinal tumours are the most common cause and can be readily detected ultrasonographically as asymmetric thickening of the intestinal wall and disruption of its layered appearance. Differentiation between intestinal tumours and inflammatory bowel diseases as granulomas and transmural granulomatous enteritis is usually difficult, although inflammation is usually characterized by extensive symmetric wall thickening without loss of the layered appearance. In some cases, however, gas accumulation in the loops may prevent the clear visualization of the affected part of the intestine.
Hernias, containing intestinal loops are easily detectable by ultrasound. Colour and pulsed wave Doppler technique may be used to estimate the viability of the entrapped intestine. Intestinal adhesions and volvulus usually cannot be detected by ultrasound.
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