Microscopic colitis: pathologic considerations, changing dogma.

Microscopic colitis as an entity was first recognized in 1976, and has become one of the most frequent diseases to exclude on colonic mucosal biopsies. In some pathology practices, up to 30% of colonic biopsies received are from patients in whom microscopic colitis is the clinical question. In this review, the evolution of the terminology and early studies describing the pathology of microscopic colitis are discussed. The pathology of lymphocytic and collagenous colitis is reviewed in detail, including common diagnostic pitfalls, and what is currently known about the pathogenesis of these diseases. The differential diagnosis of microscopic colitis includes other idiopathic inflammatory bowel diseases (Crohn's and ulcerative colitis), infections, and drug reactions. The distinction between these entities and microscopic colitis is discussed in detail. Finally, recent studies have revealed new histopathologic changes in microscopic colitis that challenge the currently held concepts of how microscopic colitis fits into the spectrum of inflammatory bowel diseases.