Megacolon & Chronic Constipation

Megacolon describes a colon that has become permanently dilated and unable to contract effectively, so faeces accumulate and harden into an immovable mass. It is far more common in cats than in dogs, and it usually develops as the end point of repeated bouts of constipation.

Normal defecation depends on rhythmic contractions of the colon’s smooth muscle, coordinated by its nerve supply. In idiopathic megacolon — the most common form — this machinery fails for reasons that are still not fully understood, and the colon stretches like an over-used elastic band until it can no longer recoil. Other cats develop megacolon secondarily: a pelvis that healed in a narrowed position after a road accident, chronic dehydration from kidney disease, or persistently low potassium can all set the cycle in motion.

The signs build gradually. Owners often notice their cat making frequent, unproductive trips to the litter box, crying or hunching while straining, and producing only small, rock-hard pellets. As the impaction worsens the cat may vomit, stop eating and become lethargic, with a firm, sausage-like mass palpable through the belly wall.

Management depends on severity. Mild, early cases are controlled with moisture-rich food, dietary fibre, the stool-softener lactulose, and prokinetic medication such as cisapride that encourages the colon to contract. Badly impacted cats need to be unblocked at the clinic with enemas and gentle manual evacuation under anaesthesia. When the colon is irreversibly damaged and medical treatment no longer works, a subtotal colectomy — surgical removal of most of the colon — is highly effective and allows the great majority of cats to live comfortably afterward, typically passing softer stools for the rest of their lives.