Histologic features of microscopic colitis
• Microscopic colitis encompasses collagenous and lymphocytic colitis. Both of these conditions present as chronic watery diarrhea, are associated with autoimmune diseases, and show a near normal endoscopic examination. Histologically, collagenous colitis shows thickened subepithelial collagen layer that has irregular edges, is infiltrated by a few lymphocytes and eosinophils, and has dilated vessels. A few IELs may be seen. The collagen band can be highlighted by trichrome stain. The differential diagnosis also includes ischemic colitis, NSAID-associated injury, IBD, diverticular disease, radiation injury, mucosal prolapse, and amyloidosis.
• Lymphocytic colitis shows increased IELs more on the surface epithelium. Both of the conditions show increased chronic inflammation in the lamina propria with increased eosinophils seen in collagenous colitis. An association between lymphocytic colitis and celiac disease is well known.
• Irritable bowel syndrome: Histologically, the biopsy samples do not show significant abnormality in these cases and appear normal.
• Radiation colitis: The histologic finding mimics ischemic colitis and shows enlarged nuclei and cells with hyalinization of lamina propria and vessel walls with scattered atypical stromal cells.
• Eosinophilic colitis: Microscopically, abundant eosinophils in the mucosa extending into submucosa are seen with minimal architectural distortion, if any.
• Diversion colitis: Mild cryptitis is seen on microscopic examination. Follicular lymphoid hyperplasia may be seen. The condition reverses on treatment with short-chain fatty acids.
• Pouchitis: This is a complication following ileal-pouch anal anastomosis for refractory UC. The pattern of inflammation mimics UC and there are no specific histologic criteria to distinguish recurrent UC from nonspecific inflammation of the pouch. Comparison with the biopsy samples from the nonpouch portion of the ileum may help.
• Diverticular disease–associated colitis: It is seen in the areas around diverticular orifices. Histologically, the findings are similar to those seen in IBD. Correlation with endoscopic findings and clinical history is essential.
• Melanosis coli: The biopsy sample shows numerous brown pigment–laden macrophages (lipofuscin) in the lamina propria. These are negative for staining with iron stain. There are no significant acute or chronic changes in the biopsy sample.
• Endometriosis: The common site in GI tract is the sigmoid colon. The biopsy sample shows endometrial glands and stroma with hemorrhage or hemosiderin pigment. Any or all of the components may be present.