Microscopic colitis (MC) is characterized by chronic watery non-bloody diarrhoea with normal colonoscopy. Which two histological subtypes are recognised, and which is more common?

• A Collagenous colitis (collagen band >10 µm) and lymphocytic colitis (IEL >20/100); collagenous colitis is more common ✓
• B Lymphocytic colitis (intraepithelial lymphocytes >20/100 epithelial cells) and collagenous colitis (subepithelial collagen band >10 µm); lymphocytic colitis is more common
• C Eosinophilic colitis and granulomatous colitis; eosinophilic is more common
• D Crohn's colitis and ulcerative colitis; Crohn's is more common in microscopic colitis

Explanation

Microscopic colitis has two subtypes: collagenous colitis (CC), characterised by a thickened subepithelial collagen band >10 µm (normal <5 µm) on trichrome stain with surface epithelial damage; and lymphocytic colitis (LC), characterised by >20 intraepithelial lymphocytes per 100 epithelial cells. Collagenous colitis is slightly more common, particularly in middle-aged women, and strongly associated with NSAIDs, PPIs (especially lansoprazole), SSRIs, and ranitidine. Both subtypes have normal or near-normal endoscopic appearance, requiring targeted biopsies. Budesonide is the first-line treatment for both subtypes.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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