Microscopic Colitis

Microscopic colitis is a chronic inflammatory bowel disease characterized by chronic, non-bloody, watery diarrhea in the setting of a grossly normal-appearing colonic mucosa on colonoscopy. The diagnosis can only be confirmed through microscopic examination of colonic mucosal biopsies, which is the distinguishing feature that gives the condition its name. There are two histological subtypes: collagenous colitis and lymphocytic colitis. In collagenous colitis, there is a thickened subepithelial collagen band (greater than 10 micrometers, compared to the normal thickness of less than 5 micrometers) beneath the surface epithelium of the colon, accompanied by an inflammatory infiltrate in the lamina propria. This collagen band disrupts normal fluid and electrolyte absorption across the colonic mucosa, leading to secretory diarrhea. In lymphocytic colitis, the subepithelial collagen band is normal in thickness, but there is a significant increase in intraepithelial lymphocytes (greater than 20 lymphocytes per 100 epithelial cells, compared to the normal value of fewer than 5 per 100). These intraepithelial lymphocytes are predominantly CD8-positive T cells that damage surface epithelial cells and impair their absorptive function. Both subtypes share a common pathogenesis involving...