What are the histologic features of idiopathic inflammatory bowel disease (IBD)?
• Chronic ulcerative colitis (UC): Grossly, there is diffuse involvement of rectosigmoid and left-sided colon, and proximal extent of the disease varies. Infections such as Cytomegalovirus, Salmonella, Shigella, and Clostridium difficile can complicate UC. Toxic megacolon is a fulminant acute complication of the disease. Histologically, the features of acute disease include cryptitis (neutrophilic infiltration in the crypt epithelium), crypt abscesses (neutrophils in the crypt lumens), and mucosal erosions and ulcers. The features of chronicity include architectural distortion of crypts (crypt dropout, bifid crypts, crypt branching), mucin depletion (loss of goblet cells), Paneth cell metaplasia, basal plasmacytosis, increased eosinophils, and prominent lymphoid aggregates. These changes are diffuse except in the resolving phase, in which these may be focal (they should not be confused with Crohn’s disease). Fibrosis is unusual in UC, in contrast to Crohn’s disease. The differential diagnosis, especially in the acute disease process, includes infection, ischemic colitis, and Crohn’s disease.
• Quiescent colitis: Histologically, mucosal atrophy (short crypts, loss of crypts, and crypt distortion), thickened muscularis mucosae, and normal inflammatory component in the lamina propria appear. Inflammatory pseudopolyps can be seen in longstanding cases.
• Backwash ileitis: Some patients with pancolitis demonstrate backwash ileitis, and the biopsy sample shows acute disease without features of chronicity.
• Crohn’s disease : Colon biopsy samples show variable morphologic findings. Some foci may appear normal and the others show aphthous ulcers, cryptitis, glandular distortion and loss, and occasionally granulomas. Transmural inflammation is characteristic of Crohn’s disease and distinguishes Crohn’s from UC. The rectum is usually spared. The resection (done in complicated cases) specimen shows segmental involvement with skip areas, linear ulcers, cobblestoning, strictures, fissures and fistulas, inflammatory pseudopolyps, serosa with creeping fat, and a firm pipelike bowel resulting from fibrosis. Involvement of terminal ileum shows villous blunting and increased inflammation in the lamina propria.