Megacolon: the hard facts (Proceedings)
Constipation is defined as the infrequent or difficult evacuation of stool. It is a common problem in cats that 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.
OverviewConstipation is more prevalent than we recognize. Clients may perceive firm pellets as being "normal" and report them as such when queried by the veterinary team. If their cat defecates in the garden or if the litter pan is not cleaned on a daily basis, they may be unaware that their cat has excessively dry stool. 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 mucus or blood passed associated with irritative effects of impacted stool, and even, intermittently, diarrhea. Vomition is frequently associated with straining. Inappetence, 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.
Not only is constipation uncomfortable, it is a sign of cellular water deficit. When a cell is dehydrated and the water intake (from drinking and eating) of the cat is maximized, the kidneys have reclaimed as much water as they are capable of, then colonic contents are the last source of water to try to regulate hydration. Determining the actual character of a cat's feces, while unsavory, is important in assessing their overall condition. Asking the client if there has been a change in stool character may not elicit the information; if a cat has had hard stool for months, even if the client is aware that the stool is pelleted, the question will not produce this information. Asking the client to tell you if the stool is hard pieces, moist logs, semi-formed (cow patties) or "coloured water" will draw out the desired information.
Etiology and pathophysiology
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.
In Chagas' disease, trypanosomes destroy the enteric plexus; in the other three conditions, there is no deficiency of mural ganglia.
In cats with megacolon, congenital hypoganglionosis has only recently been identified in a case report of a single kitten1. Dysautonomia is uncommonly seen in North America, but should consider if concurrent ocular, esophageal, gastric, or lower urinary tract problems are noted2. Dr. Robert Washabau and his co-workers have studied idiopathic feline megacolon extensively and have identified that the underlying problem is characterized by abnormalities in smooth muscle function3-5.