Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

352 Small Animal Radiolo g y and Ultrasono graphy
Either pneumocolon or barium enema may be used to evaluate the colon. The most
difficult problem is being certain that normal colonic content is not mistaken for an intraluminal
or intramural mass. In most inflammatory colonic diseases, there is no radiographically
apparent change. Lymphocytic plasmacytic colitis may produce diffuse
thickening of the colon wall. In severe cases of ulcerative colitis, the colon wall will appear
thickened, and extensions of the barium column into the thickened wall may be
observed. 407 Inflammation may predispose to intussusception. The most common site for
intussusception in the dog is the ileocolic junction. In some cases, both the terminal ileum
and cecum may be involved. In most cases, there will be dynamic ileus of the small bowel
with significant distention of the small intestine with gas and fluid. Radiographically, there
may be an apparent tissue-dense mass within a gas-filled loop of colon on survey films. The
folded edematous intestinal mucosa produces a pattern that has been described as a coiled
spring when outlined by the gas-filled colon. More frequently, the diagnosis requires a contrast
study of the colon. The contrast must be administered retrograde, because the ileus
will prevent contrast that is administered orally from reaching the site of the obstruction.
The classic appearance is of a tube within a tube, or intussusceptum into intussuscipiens
(Fig. 3-126). A variant of the intussusception is an inversion of the cecum (see Fig. 3-125).
When this happens, the cecum inverts into the colon and causes some degree of blockage
at the ileocolic junction. 406 The shape of the cecum, which is seen as a tissue-dense mass
surrounded by air or barium within the lumen of the colon, permits recognition of this
lesion. The inverted cecum may not completely obstruct air or fluid passage. Therefore it
may not appear to be a distal small intestinal obstruction. Cecal inversion may be diagnosed
during an upper GI series.
Tumors and granulomas may produce intramural colonic lesions that may be evident
as thickening of the colonic wall or as tissue-dense masses protruding into the lumen.
These often can be recognized without the administration of contrast, thus relying on the
normal contrast afforded by the gas within the lumen of the colon. Adenocarcinoma may
produce a circumferential intramural lesion (Figs. 3-127 and 3-128). The colon may be
dilated with air, fluid, or feces cranial to the lesion. Rectal administration of air or barium
will outline the extent of the lesion when an endoscope cannot be passed through the area
of narrowing. The mucosa is often irregular and ulcerated. Strictures may also produce circumferential
narrowing; however, they usually have a smooth mucosal surface. In most
cases a biopsy is required to make a specific diagnosis.
Extramural masses may compress or displace the colon but usually do not distort the
mucosal margin. Prostatic enlargement may compress the colon, and stump pyometra may
produce a smooth indentation of the colon. Enlarged ileocolic lymph nodes can produce a
Fig. 3-126 A 1-year-old male Coonhound with vomiting and
anorexia for 3 days and a palpable midabdominal mass. Survey radiographs
revealed poor abdominal detail due to cachexia and a
dynamic ileus. The barium enema revealed dilated, gas-filled small
intestinal loops (white s). There is a tissue-dense mass within the
ascending and transverse large intestine (white i), which is outlined
by a thin line of barium (black arrows) surrounding it. Close scrutiny
revealed a slight depression in the border of the mass at its tip in the
transverse colon (open black arrow). Differential diagnoses include
intussusception, intracolonic neoplasm, or foreign material.
Diagnosis: Ileocolic intussusception.