[Microscopic colitis: histopathological review with a clinicopathological correlation]

Microscopic colitis is a clinicopathological entity which, in addition to typical symptoms such as watery diarrhea, is characterized by its specific histopathology. Since colonoscopy yields normal findings, microscopic colitis belongs in a histological domain. The term encompasses two forms: lymphocytic and collagenous colitis. Histologically, lymphocytic colitis shows an increase in intraepithelial lymphocytes of more than 20 lymphocytes per 100 surface colonocytes, while collagenous colitis is characterized by a thickened subepithelial collagen layer of more than 10 µm. Specific stains help in the quantification of both. Since microscopic colitis does not always affect the entire colon and the number of intraepithelial lymphocytes varies physiologically, obtaining stepwise biopsies of the colon (with information on location where possible) is recommended. A thickened collagen layer is relatively specific for collagenous colitis, whereas intraepithelial lymphocytosis is also found in other diseases. Therefore, to make a correct diagnosis, it is important to correlate histological findings with clinical symptoms, including the main symptom of watery diarrhea.