Collagenous colitis is one of the two conditions under the umbrella of “microscopic colitis,” the second condition is lymphocytic colitis. The etiology is not clear and multiple factors may play a role, including: smoking, medications, bile acid malabsorption, infection, genetic predisposition and immune system imbalance.

Patients are usually older Caucasian females (female to male ratio can be as high as 9:1) presenting with watery, non-bloody chronic diarrhea. Colonoscopy is usually normal; however non-specific changes, mucosal hyperemia and ulcerations can be seen in some patients. Gastroenterologists who suspect microscopic colitis based on the typical presentation may take random biopsies.

Microscopic examination shows colonic mucosa with thickened subepithelial collagen plates of 10 microns or more. The collagen thickening is patchy, and it is best seen with trichrome stain. A helpful diagnostic feature is finding small vessels entrapped in the collagen band. Surface epithelium shows an increase in intraepithelial lymphocytes, however because the surface epithelium in collagenous colitis is very fragile, it often sloughs off during specimen processing. The surface epithelium denudation is a feature that helps the pathologist suspect this entity when first looking at the slides at low magnification. There is mild lymphoplasmacytic inflammatory infiltrate within the lamina propria, but usually no neutrophils. Although collagenous colitis is a chronic colitis, architectural distortion of the glands is not present.

Differential diagnoses include: lymphocytic colitis, inflammatory bowel disease, irritable bowel syndrome, scleroderma, amyloidosis, ischemic colitis, radiation colitis and mucosal prolapse.