Neoplasia

In dogs, colonic tumors are more common than gastric or small intestinal tumors. The mean age of dogs affected with colonic neoplasia is variably reported between 7 and 11 years of age.⁷ Most colonic tumors of dogs are malignant and include adenocarcinomas, lymphosarcomas, and gastrointestinal stromal tumors (leiomyosarcoma, neurofibrosarcoma, fibrosarcoma, and ganglioneuroma).^8–17 Leiomyosarcomas are the most common (91%) of the gastrointestinal stromal tumors.^10–13 Most colonic neoplasia develop in the descending colon and rectum, although leiomyosarcomas more frequently develop in the cecum.^10,12 Local tumor invasion apparently occurs at a slower rate with canine colonic neoplasia, and metastasis to distant sites is relatively uncommon. Benign colonic neoplasia (e.g., adenomas, adenomatous polyps, and leiomyomas) also occur, although they are less common than malignant tumors. Malignant transformation of adenomatous polyps to carcinoma in situ and invasive adenocarcinoma has been demonstrated in the dog just as it has in humans.^7,18,19 Extramedullary plasmacytomas are an uncommon tumor of the gastrointestinal tract, but many of these occur in the large intestine and rectum.^20,21 All of the aforementioned tumors are associated with signs of inflammation and obstruction (e.g., hematochezia, tenesmus, and dyschezia). Carcinoids (rare 5-hydroxytryptamine [5-HT] secreting tumors) are occasionally associated with diarrhea because of the effects of 5-HT on secretion and motility.

In cats, adenocarcinoma (46%) is the most common large intestinal tumor, followed by lymphosarcoma (41%) and mast cell tumors (9%).^22–24 The mean age of cats affected with colonic neoplasia is 12.5 years. The descending colon (39%) and the ileocolic sphincter (28%) are the most common sites of colonic neoplasia in the cat. Unlike colonic tumors in dogs, feline colonic tumors have a high rate (63%) of metastasis and, of course, metastasis is associated with decreased survival time. Metastatic sites include colonic lymph nodes, mesenteric lymph nodes, liver, spleen, bladder, urethra, omentum, mesocolon, lungs, duodenum, and peritoneum.

Intussusception

Intussusception is an invagination of one segment of the gastrointestinal tract into the lumen of an adjoining segment. The intussusceptum is the invaginated segment of the alimentary tract, whereas the intussuscipiens is the enveloping segment. Invagination may occur in an antegrade (aborad) or retrograde (orad) direction, but is most commonly in the antegrade direction. Any portion of the alimentary tract may be involved, but enterocolic intussusceptions account for almost two-thirds of published cases in dogs and cats. Enterocolic intussusceptions can be further divided into three types: cecocolic (or cecal inversion), with the inverted cecum forming the apex²⁵; ileocolic, with the ileum forming the apex; and ileocecal, with the ileocecal junction forming the apex.²⁶ Of these three forms of enterocolic intussusception, the ileocolic intussusception is most frequently encountered in clinical practice. A number of conditions are reported to predispose to intussusception, including intestinal parasitism, viral enteritis, foreign bodies, and masses, but in dogs and cats most intussusceptions are idiopathic.^27–29

History

Constipation, obstipation, and megacolon may be observed in cats of any age, sex, or breed; however, most cases are observed in middle aged (mean = 5.8 years) male cats (70% male, 30% female) of Domestic Shorthair (46%), Domestic Longhair (15%), or Siamese (12%) breeding.² Affected cats are usually presented for reduced, absent, or painful defecation for a period of time ranging from days to weeks or months. Some cats are observed making multiple, unproductive attempts to defecate in the litter box, while other cats may sit in the litter box for prolonged periods of time without assuming a defecation posture. Dry, hardened feces are observed inside and outside of the litter box. Occasionally, chronically constipated cats have intermittent episodes of hematochezia or diarrhea caused by the mucosal irritant effect of fecal concretions. This may give the pet owner the erroneous impression that diarrhea is the primary problem. Prolonged inability to defecate may result in other systemic signs, including anorexia, lethargy, weight loss, and vomiting.

Physical Examination

Colonic impaction is a consistent physical examination finding in affected cats. Other findings will depend upon the severity and pathogenesis of constipation. Dehydration, weight loss, debilitation, abdominal pain, and mild to moderate mesenteric lymphadenopathy may be observed in cats with severe idiopathic megacolon. Colonic impaction may be so severe in such cases as to render it difficult to differentiate impaction from colonic, mesenteric, or other abdominal neoplasia. Cats with constipation as a consequence of dysautonomia may have other signs of autonomic nervous system failure, such as urinary and fecal incontinence, regurgitation because of megaesophagus, mydriasis, decreased lacrimation, prolapse of the nictitating membrane, and bradycardia. Digital rectal examination should be carefully performed with sedation or anesthesia in all cats. Pelvic fracture malunion may be detected on rectal examination in cats with pelvic trauma. Rectal examination might also identify other unusual causes of constipation, such as foreign bodies, rectal diverticula, stricture, inflammation, or neoplasia. Chronic tenesmus may be associated with perineal herniation in some cases. A complete neurologic examination with special emphasis on caudal spinal cord function should be performed to identify neurologic causes of constipation, for example, spinal cord injury, pelvic nerve trauma, and Manx sacral spinal cord deformity.

Laboratory Evaluation and Tests

Although most cases of obstipation and megacolon are unlikely to have significant changes in laboratory data (e.g., complete blood count, serum chemistry, urinalysis), nonetheless these tests should be performed in all cats presented for constipation. Metabolic causes of constipation, such as dehydration, hypokalemia, and hypercalcemia, may be detected in some cases. Basal serum T₄ concentration and other thyroid function tests should also be considered in cats with recurrent constipation and other signs consistent with hypothyroidism. Although hypothyroidism was documented in only one case of obstipation and megacolon, obstipation is a frequent clinical sign in kittens affected with congenital or juvenile-onset hypothyroidism.² Constipation could also theoretically develop following successful treatment of feline hyperthyroidism.

Abdominal radiography should be performed in all constipated cats to characterize the severity of colonic impaction, and to identify predisposing factors such as intraluminal radiopaque foreign material (e.g., bone chips), intraluminal or extraluminal mass lesions, pelvic fractures, and spinal cord abnormalities. Radiographic findings of colonic impaction cannot be used to distinguish between constipation, obstipation, and megacolon in idiopathic cases. First or second episodes of constipation in some cats may be severe and generalized but may still resolve with appropriate treatment.

Ancillary studies may be indicated in some cases. Extraluminal mass lesions may be further evaluated by abdominal ultrasonography and guided biopsy, whereas intraluminal mass lesions are best evaluated by endoscopy. Colonoscopy may also be used to evaluate the colon and anorectum for suspected inflammatory lesions, strictures, sacculations, and diverticula. Barium enema contrast radiography may be used if colonoscopy is not possible. Both colonoscopy and barium enema contrast radiography will require general anesthesia and evacuation of impacted feces. Cerebrospinal fluid analysis, computed tomography (CT) or magnetic resonance imaging (MRI), and electrophysiologic studies should be considered in animals with evidence of neurologic impairment. Finally, colonic biopsy or anorectal manometry is necessary to diagnose suspected cases of aganglionic megacolon.