Atypical forms of microscopic colitis: morphological features and review of the literature.

Microscopic colitis is defined as a syndrome of chronic watery diarrhea with a chronic inflammatory cell infiltrate in the colonic mucosa but without significant abnormalities at colonoscopy. It encompasses at least two histopathologic entities (ie, collagenous and lymphocytic colitis). The recognition and characterization of microscopic colitis has markedly changed the approach to the evaluation and management of chronic diarrhea. The histologic features of collagenous and lymphocytic colitis are well known to most pathologists. By considering the clinical history and symptoms, the pathologist should be able to reach the correct diagnosis in most cases. However, the spectrum of morphologic changes associated with watery diarrhea syndrome appears to be broader than originally thought. Morphologic changes more often associated with chronic inflammatory bowel disease or even chronic ischemic or infectious colitis have been noted in patients with clinically established microscopic colitis. The data presented in this article suggest that microscopic colitis is a heterogeneous entity, which includes both classic and "atypical" forms. Problems arise when cases do not fit the usual pattern or lack some of the findings that are expected. Pathologists should be aware of the presence of atypical forms of microscopic colitis.