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.

Acquired megacolon also occurs as a sequel to any lesion or disease that prevents normal defecation for a prolonged period of time. If megacolon has persisted chronically for several months or longer, the chance of reversing the motility dysfunction following the correction of an underlying cause is unlikely.Chronic retention of faeces and distension of the colon irreversibly damages colonic motility to cause acquired megacolon. Idiopathic megacolon is seen primarily in cats, and it has recently been suggested this condition may involve changes to normal colonic smooth muscle function. In many cases, the aetiology of megacolon is not determined.

There are many potential underlying caused for chronic faecal retention and thus acquired megacolon. In addition to the abnormalities of the caudal spinal cord responsible for congenital megacolon,

Signalment

Cats may suffer idiopathic megacolon, and both cats and dogs may develop the acquired form. Congenital megacolon is not well studied in cats and dogs. Any age or sex of animal may 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.

Idiopathic megacolon

has no age, breed or sex predisposition.

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.