Clinical meaning
Microscopic colitis is a chronic inflammatory condition of the colon characterized by chronic watery, non-bloody diarrhea with a grossly normal-appearing colonic mucosa on colonoscopy — diagnosis requires histological examination of colonic biopsies, hence the name 'microscopic.' It is classified into two histological subtypes: collagenous colitis, defined by a thickened subepithelial collagen band (greater than 10 micrometers versus normal less than 5 micrometers) with increased intraepithelial lymphocytes and surface epithelial damage; and lymphocytic colitis, characterized by increased intraepithelial lymphocytes (greater than 20 per 100 epithelial cells) without significant collagen band thickening. The pathophysiology involves loss of colonic epithelial barrier integrity and dysregulated mucosal immune response to luminal antigens. In collagenous colitis, the abnormal collagen deposition (primarily type VI collagen) beneath the surface epithelium is produced by activated myofibroblasts and impairs water and electrolyte absorption, contributing to secretory diarrhea. In lymphocytic colitis, CD8+ intraepithelial T lymphocytes infiltrate the epithelium and release pro-inflammatory cytokines (interferon-gamma, TNF-alpha) that increase epithelial permeability and stimulate chloride secretion, producing a net secretory state. Both subtypes share an association with autoimmune conditions (celiac disease, thyroid disease, rheumatoid arthritis),...