Lymphocytic, collagenous and other microscopic colitides: pathology and the relationship with idiopathic inflammatory bowel diseases.

SUMMARY: Collagenous colitis and lymphocytic colitis are the two major conditions characterized by chronic watery diarrhoea, without endoscopic or radiological lesions, but with histological abnormalities and therefore considered as "microscopic colitis". The histology of colonic biopsies shows inflammation of the mucosa, and either thickening of the subepithelial collagen band or an increase of lymphocytes in the surface epithelium. Different variant forms have been reported under separate names. These are probably not specific entities. The incidence of microscopic colitis is slightly less than the incidence of chronic idiopathic inflammatory bowel diseases (IBD). Microscopic colitis and IBD are clearly different entities. The relation between both entities is weak but double. Biopsy samples from patients with IBD may mimic the features of lymphocytic or collagenous colitis, both in the initial onset and during follow-up. In the large majority of these cases, endoscopy shows or has shown mucosal lesions. In rare cases, however, a double diagnosis was made. Certain patients, usually of older age, presented first with a microscopic, usually collagenous colitis and developed subsequently genuine ulcerative colitis.