Chapter 10 Constipation
Reviews of idiopathic megacolon have emphasized the importance of considering an extensive list of differential diagnoses (e.g., neuromuscular, mechanical, inflammatory, metabolic/endocrine, pharmacologic, environmental, and behavioral causes) for the obstipated cat (Box 10-1; reviewed in reference 1). One review of published cases suggests that 96% of cases of obstipation are accounted for by idiopathic megacolon (62%), pelvic canal stenosis (23%), nerve injury (6%), or Manx sacral spinal cord deformity (5%).2 A smaller number of cases are accounted for by complications of colopexy (1%), and colonic neoplasia (1%); colonic hypo- or aganglionosis was suspected in another 2% of cases. Inflammatory, pharmacologic, and environmental/behavioral causes were not cited as predisposing factors in any of the original case reports. Endocrine factors (e.g., obesity and hypothyroidism) were cited in several cases, but were not necessarily impugned as part of the pathogenesis of megacolon. It is important to consider an extensive list of differential diagnoses in an individual animal, but it should be kept in mind that most cases are idiopathic, orthopedic, or neurologic in origin. Behavioral (e.g., stress) and/or environmental (e.g., competition for the litter box) factors may play an important role in the development of the disorder, but haven’t been very well characterized in retrospective or prospective studies.
Differential Diagnosis of Constipation in the Cat
Colonic smooth muscle: idiopathic megacolon, aging
Spinal cord disease: lumbosacral disease, cauda equine syndrome, sacral spinal cord deformities (Manx cat)
Hypogastric or pelvic nerve disorders: traumatic injury, malignancy, dysautonomia
Submucosal or myenteric plexus neuropathy: dysautonomia, aging
Intraluminal: foreign material (bones, plant material, hair), neoplasia, rectal diverticula, perineal hernia, anorectal strictures
Perianal fistula, proctitis, anal sac abscess, anorectal foreign bodies, perianal bite wounds
Metabolic and Endocrine
Metabolic: dehydration, hypokalemia, hypercalcemia
Endocrine: hypothyroidism, obesity, nutritional secondary hyperparathyroidism
Pathophysiology and Mechanisms
Megacolon develops through two pathologic mechanisms: dilation and hypertrophy. Dilated megacolon is the end stage of colonic dysfunction in idiopathic cases. Cats affected with idiopathic dilated megacolon have permanent loss of colonic structure and function. Medical therapy may be attempted in such cases, but most affected cats eventually require colectomy. Hypertrophic megacolon, on the other hand, develops as a consequence of obstructive lesions (e.g., malunion of pelvic fractures, tumors, foreign bodies). Hypertrophic megacolon may be reversible with early pelvic osteotomy or it may progress to irreversible dilated megacolon if appropriate therapy is not instituted.3
Constipation and obstipation are earlier manifestations of the same problem. Constipation is defined as infrequent, difficult, painful evacuation of feces but does not necessarily imply a permanent loss of function. Many cats experience one or two episodes of constipation without further progression. Intractable constipation that has become refractory to cure or control is referred to as obstipation. Obstipation implies a permanent loss of function. A cat is assumed to be obstipated only after several consecutive treatment failures. Recurring episodes of constipation or obstipation may culminate in the syndrome of megacolon.
The pathogenesis of idiopathic dilated megacolon involves functional disturbances of colonic smooth muscle. Megacolonic smooth muscle develops less isometric stress in response to neurotransmitter (acetylcholine, substance P, cholecystokinin), membrane depolarization (potassium chloride), or electrical field stimulation, when compared to healthy controls.4,5 Differences have been observed in longitudinal and circular smooth muscle from descending and ascending colon. No significant abnormalities of smooth muscle cells or of myenteric neurons have been observed on histologic evaluation. Feline idiopathic megacolon is a generalized dysfunction of colonic smooth muscle, and treatments aimed at stimulating colonic smooth muscle contraction generally improve colonic motility. The lesion may begin in the descending colon and progress to involve the ascending colon over time.6
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 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
Evaluation of the Patient
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.
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.