Feline megacolon: The hard facts (Proceedings)
Constipation is defined as the infrequent or difficult evacuation of stool. It is a common problem in cats, may be acute or chronic and does not inherently imply a loss of colonic function. Often the underlying cause is dehydration and is readily managed by supportive hydration, by oral, nutritional or parenteral means. When a cat has intractable constipation that is unresponsive to therapy or cure, this is referred to as obstipation. Obstipation implies permanent loss of function. When obstipation results in dilatation of the colon or hypertrophy of the colon, then the condition is described as megacolon.
Dilated megacolon is the end-stage condition of idiopathic colonic dysfunction. The resulting disease has diffuse colonic dilatation and hypomotility. Hypertrophic megacolon is a result of pelvic fracture malunion and stenosis of the pelvic canal or another obstructive mechanism including neoplasm, polyp, or foreign body. Colonic impaction is the accumulation of hardened feces in the pelvic colon and is the consequence of constipation, obstipation or megacolon. It does not, in itself, imply loss of function or reversibility of the problem. This distinction is critical in considering treatment plans as well as prognosis.
In humans, there are two recognized forms of megacolon:1) Congenital aganglionic megacolon (Hirschsprung's Disease): During embryologic development, it is normal for the neural crest cells, which develop into the enteric neuronal plexus (Meissner's submucosal and Auerbach's myenteric) network. Congenital aganglionic megacolon is a disease in which the migration of neural crest cells arrests before reaching the anus resulting in a segment of the distal bowel lacking enteric neuronal coordination. This results in functional obstruction and colonic dilatation proximal to the affected segment. Distension of the colon may reach a diameter of 15-20 cm. As the colon distends, there is hypertrophy of the wall; eventually, if the distension outstrips the hypertrophy, thinning occurs which may result in rupture. Impacted feces may also, at any stage of the disease, cause mucosal inflammation and shallow ulceration.
2) Acquired megacolon is a condition of any age and may be from
a. Chagas' disease, a trypanosomal infection
b. Obstruction by neoplasm or inflammatory stricture
c. Toxic megacolon from ulcerative colitis of Crohn's disease involving the colon
d. Functional psychosomatic illness.
In Chagas' disease, trypanosomes destroy the enteric plexus; in the other three conditions, there is no deficiency of mural ganglia.
In cats with megacolon, there is little evidence to show any deficiency in enteric neuronal network as histologic studies fail to show significant abnormalities in density or morphology of ganglia. An extrinsic neural defect involving the parasympathetic or sympathetic nerves is unlikely, as one would expect concurrent problems with the lower urinary tract. Dr. Robert Washabau and his cohorts have worked extensively on feline megacolon and have studies to support that the underlying problem is characterized by abnormalities in smooth muscle function.
Constipation, obstipation and megacolon may be seen in cats of any age, breed and gender, however middle aged (mean 5.8 years), male (70%) domestic short-haired (46%) cats appear to be more at risk.
Cats are presented because of a client's observation of reduced, absent or painful, elimination of hard stool. Cats may pass stool outside the box as well as in it, may posture and attempt to defecate for prolonged periods or may return to the box to try repeatedly to pass stool, unsuccessfully. There may be mucous or blood passed associated with irritative effects of impacted stool, and even, intermittently, diarrhea. Vomition is frequently associated with straining. Inappetance, weight loss, lethargy and dehydration become features of this condition if unresolved. Dilated megacolon is preceded by repeated episodes of recurrent constipation and obstipation. In the cat with hypertrophic megacolon, there may be a known history of trauma resulting in pelvic fracture.