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 non-bloody watery diarrhea, normal appearances on colonoscopy and characteristic histopathology findings of inflammatory cells. Signs and symptoms The main symptom is persistent non-bloody watery diarrhea, which may be profuse. People may also experience abdominal pain, fecal incontinence, and unintentional weight loss. Microscopic colitis is the diagnosis in around 10% of cases investigated for chronic non-bloody diarrhea. Associated conditions A higher incidence of autoimmune diseases, for example arthritis, Sjögren's syndrome, thyroid disorders, and celiac disease, has been reported in people with microscopic colitis. Associations with various drugs have been found, especially proton pump inhibitors, H2 blockers, selective serotonin reuptake inhibitors (SSRIs), and non-steroidal anti-inflammatory drugs (NSAIDs). Bile acid diarrhea is found in 41% of patients with collagenous colitis and 29% with lymphocytic colitis. Additionally, cigarette smoking has been identified as a significant risk factor of microscopic colitis. Diagnosis Colonoscopic appearances are normal or near normal. As the changes are often patchy, an examination limited to the rectum may miss cases of microscopic colitis, and so a full colonoscopy is necessary. Multiple colonic biopsies are taken in order to make the diagnosis. Histological features of colonic biopsies indicating microscopic colitis are: greater than 20 intraepithelial lymphocytes per 100 epithelial cells and, additionally, 10-20 μm of a thickened subepithelial collagen band in collagenous colitis. Inflammation of the lamina propria, with mainly mononuclear cells, may be observed in collagenous colitis. Pathology 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. These are the principal features of lymphocytic colitis. An additional distinguishing feature of collagenous colitis is a thickened subepithelial collagen layer, which may be up to 30 micrometres thick, that occurs in addition to the features found in lymphocytic colitis. The fact that the two types of microscopic colitis share many features including epidemiology, risk factors and, response to therapy has led to the suggestion that they are actually subtypes of the same disease. Differential diagnosis Differential diagnoses, which should be ruled out, include bile acid diarrhea, lactose malabsorption, celiac disease, Crohn's disease, ulcerative