Microscopic colitis

Overview
Microscopic colitis refers to two medical conditions which cause diarrhoea: collagenous colitis and lymphocytic colitis. Both conditions are characterised by the following triad of clinicopathological features:
 * 1) Chronic watery diarrhoea;
 * 2) Normal colonoscopy;
 * 3) Characteristic histopathology.

Clinical features
Patients are characteristically, though not exclusively, middle-aged females. They present with a long history of watery diarrhoea, which may be profuse. There is a higher incidence of autoimmune diseases, for example arthritis, Sjögren's syndrome, and coeliac disease, in patients with microscopic colitis. There are reports of associations with multiple drugs, especially non-steroidal anti-inflammatory drugs (NSAIDs).

Colonoscopy is normal or near normal. The changes are often patchy, so multiple colonic biopsies must be taken in order to make the diagnosis. A full colonoscopy is required, as an examination limited to the rectum will miss cases of microscopic colitis.

Pathology
The hallmark of microscopic colitis is an increase in inflammatory cells (i.e. lymphocytes) in colonic biopsies with an otherwise normal appearance and architecture of the colon. Inflammatory cells are increased both in the surface epithelium ("intraepithelial lymphocytes") and in the lamina propria. In lymphocytic colitis, these are the only abnormal features.

In collagenous colitis, the features of lymphocytic colitis are present, with in addition the presence of a thickened subepithelial collagen layer which may be up to 30 micrometres thick.

Treatment
No single treatment is accepted as the standard and measuring response is difficult. Often a trial of anti-diarrhoeals is followed by anti-inflammatory drugs.

Lymphocytic colitis is thought to respond well to mesalazine and collagenous colitis to budesonide.

Prognosis
The prognosis for lymphocytic colitis and collagenous colitis is good and both conditions are considered to be benign. The majority of people afflicited with the conditions recover from their diarrhoea and their histological abnormalities resolve.