Pathology



The pathology is of particular importance in IBD. It is the basis for the clinical manifestations described below and also for the distinction that is made between UC and Crohn’s disease.

Ulcerative Colitis. The acute lesion is a diffuse (without skip areas), superficial (mucosal and submucosal) inflammation that almost always in­volves the rectum (>95 per cent) and may extend from the rectum throughout the colon. With pancolitis the terminal ileum may be slightly involved (a “backwash ileitis”), but UC is es­sentially a colonic disease. Infiltrated with neutrophils, the epithelium is typically diffusely inflamed, granular, and friable and may show small superficial ulcerations or occasionally deep linear ulcers. Multiple microabscesses may de­velop around the crypts. When severely inflamed, the colon may become markedly distended (di­ameter >6 cm) with attenuation of its walls, a con­dition known as “toxic megacolon.” This is as­sociated with an immediate danger of perforation. In chronic UC gradual deposition of connective tissue and hyperplasia of the muscularis mucosae secondary to the continuous processes of injury and repair may produce a smooth, foreshortened colon with loss of normal haustral markings (the lead-pipe colon radiographically). Heaped-up, rounded patches of mucosa develop as outcrop-pings between areas of ulcerative injury; they are called pseudopolyps, since they are not neoplas­tic. Ultimately with long-standing disease the ep­ithelial cells may show dysplastic changes that are considered to be harbingers of malignancy. Unless carcinoma occurs UC rarely produces an obstructive lesion or results in fistula formation.

Crohn’s Disease. In Crohn’s disease, also termed regional enteritis, the inflammation is transmural (involving all layers of the bowel and the serosal surface), may be discontinuous (with skip areas of normal bowel between), and in­volves the rectum in fewer than 50 per cent of cases. In contrast to UC, it is not exclusively a colonic disease: approximately one third is co­lonic; one third is ileal; and one third is ileoco-lonic. Rarely the more proximal small bowel, the stomach, or even the mouth may be involved. With transmural and even mesenteric involve­ment, inflamed loops of bowel may become ad­herent to each other or to other organs, producing palpable masses, fistulas, or obstruction. The mu­cosa may appear grossly normal or exhibit a cob­blestone appearance. Deep linear ulcers may be present, most characteristically in the long axis of the bowel. The infiltrate in the thickened, stiff­ened walls contains not only neutrophils but also lymphocytes and* macrophages, with granuloma formation in about 50 per cent of cases. As noted, fistula formation, including perirectal disease, is common, but dysplasia of epithelial cells is not noted.