|Classification and external resources|
Microscopic colitis refers to two related medical conditions which cause diarrhea: collagenous colitis and lymphocytic colitis. Both conditions are characterized by the presence of chronic watery diarrhea, normal appearances on colonoscopy and characteristic histopathology findings of inflammatory cells.
People who develop microscopic colitis are characteristically, though not exclusively, middle-aged females. The average age of diagnosis is 65 but 25% of cases are diagnosed below the age of 45. Patients have a long history of watery diarrhoea, which may be profuse. Microscopic colitis is the diagnosis in around 10% of cases investigated for chronic non-bloody diarrhea.
Colonoscopic appearances are normal or near normal. Multiple colonic biopsies are taken in order to make the diagnosis. As the changes are often patchy, an examination limited to the rectum may miss cases of microscopic colitis, and so a full colonoscopy has been recommended.
A higher incidence of autoimmune diseases, for example arthritis, Sjögren's syndrome, and coeliac disease, has been reported in patients with microscopic colitis. Associations with various drugs have been found, especially proton pump inhibitors, H2 blockers, and non-steroidal anti-inflammatory drugs (NSAIDs). Bile acid diarrhea is found in 41% of patients with collagenous colitis and 29% with lymphocytic colitis.
Microscopic colitis is characterized by an increase in inflammatory cells, particularly 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. The key feature is more than 20 intra-epithelial lymphocytes per 100 epithelial cells. In lymphocytic colitis, these are the principal 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. The two types of microscopic colitis share many other features including epidemiology, risk factors and response to therapy which has led to the suggestion they are two subtypes of the same disease.
Lymphocytic and collagenous colitis have both been shown in randomized, placebo-controlled trials to respond well to budesonide. Budesonide, in formulations designed to be active in the distal intestine, is effective for both active disease and in the prevention of relapse.
Studies have been performed in both forms of microscopic colitis of treatment with a number of other agents including bismuth subsalicylate (Pepto-Bismol), mesalazine/mesalamine (with or without cholestyramine), systemic corticosteriods and probiotics. The evidence for their effectiveness is less good than that for budesonide.
The prognosis for lymphocytic colitis and collagenous colitis is good, and both conditions are considered to be benign. The majority of people afflicted with the conditions recover from their diarrhea, and their histological abnormalities resolve, although relapses can occur and maintenance treatment may be required.
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