Megacolon

Description

Megacolon is a state of permanently increased diameter of the large bowel. This change in intestinal structure leads to abnormal function, including reduced colonic motility and chronic constipation. The condition most commonly occurs in cats and dogs, but pigs can also be afffected. White foals suffering congenital colonic agangliosis, an autosomal recessive trait, may develop secondary megacolon.

In small animals, megacolon may be congenital or acquired, which may be idiopathic. Although well described as Hirschsprung's disease in human medicine, congenital megacolon is less understood in cats. It was first recognised in 1998¹, and is thought to be caused by aganglionosis of a colonic segment, leading to persistent smooth muscle contraction the affected region and dilation of the colon proximal to this. Acquired megacolon is more common than the congenital form, and in cats this is most often idiopathic. The true cause of "idiopathic" megacolon is thought to be an intrinsic defect in colonic smooth muscle function². Aquired megacolon can occur in both cats and dogs as a sequel to any disease or lesion that interferes with normal defecation: faecal retention caused dilatation of the colon and impairs colonic motility. Causes could include neuromuscular abnormalities (spinal cord disease, intervertebral disk disease, dysautonomia, trauma), metabolic disorders (severe dehydration, hypokalaemia), drug therapy (vincristine, anticholinergics, barium), mechanical obstruction (pelvic fracture malunion, foreign bodies, stricture, anal/rectal atresia) and conditions causeing dyschezia (anal sacculitus, perianal fistula, trauma preventing posturing, procititis). After megacolon has persisted for several months, it is unlikely that normal colonic motility will be restored after resolution of the underlying cause. In many cases, the aetiology of megacolon is not determined.

Signalment

Manx Cat. Source: Wikimedia Commons; Author: Jonik (2004)

Congenital megacolon is not well studied in cats and dogs but does appear to occure in both species. Any age, breed or sex of animal may develop aquired megacolon, but idiopathic megacolon is more common in middle-aged to older cats. There is also some evidence for an increased risk of megacolon in Manx cats.

Diagnosis

Clinical Signs

Cats with colonic impaction may be presented with a history of reduced frequency of defecation or failure to defecate for a period of time ranging from days to weeks. The animal may have been observed to make frequent but unsuccessful attempts to defecate and to spend a prolonged time in the litter box. The cat may also be presented because it is lethargic, inappetent, anorexic and intermittently vomiting. These latter signs may be due to the absorption of bacteria and toxins through the damaged mucosal barrier in the dilated colon. The vomiting reflex is also stimulated by distension of the colon and irritation of the mucosa. The animal may be in a crouched position indicative of abdominal discomfort. Although constipation is usually associated with absence of defecation, the impacted faeces can irritate the mucosa inducing secretion of fluid and mucus that leaks around the colonic mass to give 'paradoxical diarrhoea'. Chronic or persistent straining associated with constipation may lead to perineal hernia or rectal prolapse (Welches and others 1992).

References

1. Rosin, E et al (1988) Subtotal colectomy for treatment of chronic constipation associated with idiopathic megacolon in cats: 38 cases (1979-1895). Journal of the American Veterinary Medical Association 193, 850-853.
2. Washabau, R J and Stalis, I H (1996) Effects of cisapride on feline colonic smooth muscle function. American Journal of Veterinary Research 57, 541-546.