Collagenous colitis

**Definition:** Collagenous colitis is characterised by chronic watery diarrhoea, normal radiological and endoscopic appearance of the colon, and a specific histopathological feature consisting in the presence of a subepithelial collagen band (10 mm or more) adjacent to the basal membrane, together with epithelial lymphocyte infiltration and chronic inflammation of the lamina propria.