Microscopic colitis.

Microscopic colitis (MC) is the common denominator for lymphocytic and collagenous colitis (CC). It is now recognized as a relatively frequent cause of diarrhea that equals the prevalence of inflammatory bowel disease. Patients are typically middle-aged women, but disease may occur at every age. Patients with MC report watery, non-bloody diarrhea in the absence of endoscopic and radiologic abnormalities. Lymphocytic colitis is characterized by an increased number of intraepithelial lymphocytes, and CC by a thickened subepithelial collagen band, whereas in both an increased mononuclear infiltration of the lamina propria is found. The pathogenesis of MC is largely unknown, but may relate to autoimmunity, adverse reactions to drugs or (bacterial) toxins, and abnormal collagen metabolism in the case of CC. Budesonide is so far the only drug that has proven efficacy in randomized controlled trials both for the induction and maintenance of remission. Patients who are nonresponsive, dependent or who experience side effects on budesonide may benefit from thiopurine or anti-TNF treatment, but these options are still experimental. The long-term prognosis of MC is good; it does not appear to predispose to malignancies and can in some cases be self-limiting. Further research and randomized clinical trials are required to expand our understanding of the natural course and the pathogenesis of MC.