Introduction: non-infective colitis encompasses those causes of colitis not due to pathogenic organisms. Ulcerative colitis and Crohn's colitis are considered under their relevant sections.

Aetiology and pathology: the different types of non-infective colitis will be considered below, together with their pathological features and treatment.

Ischaemic colitis

This is the most common form of colonic ischaemia, and presents mainly in the elderly. The classical presentation is with cramping abdominal pain followed by rectal bleeding. The bleeding is normally dark red in colour, and the attacks often settle spontaneously after a few hours. The most commonly affected area of the colon is the splenic flexure. It is in this region that the ‘watershed’ between the superior mesenteric and inferior mesenteric arteries exists. The marginal artery of Drummond acts as a conduit between these vessels, but may be absent or underdeveloped in 5% of the population.

A number of precipitant causes exist, but the net effect is the same: decreased blood flow to the affected area. Atherosclerosis of the mesenteric vessels, low cardiac output states (including cardiopulmonary bypass), vasculitis and sickle cell are all common precipitants. Recurrent self-resolving attacks predispose to structuring of the affected bowel, which may necessitate resection. In the acute phase if the patient becomes shocked or develops signs of peritonism then an emergency colectomy may become necessary. If an underlying cause is identified, then this should be treated as this may prevent or reduce the incidence of future attacks.

Microscopic colitis

This consists of two distinct types of colitis: collagenous and lymphocytic. In collagenous colitis there is an increased thickness of the subepithelial collagen and increased numbers of lymphocytes.