Crohn disease is characterized by chronic inflammation of the digestive tract, usually the terminal portion of the small intestine. The cause of Crohn disease is unknown. Apart from the greater tendency for fistulas to form and for the wall of the intestine to thicken until the channel is obstructed, it is only distinguishable from ulcerative colitis by microscopic findings. In Crohn disease, the maximum damage occurs beneath the mucosa, and lymphoid conglomerations, known as granulomata, are formed in the submucosa. Crohn disease attacks the perianal tissues more often than does ulcerative colitis. Although Crohn disease and ulcerative colitis are not common, they are disabling.

A combination of immunosuppressive and anti-inflammatory drugs, including corticosteroids and aminosalicylic acid compounds, are used to treat Crohn disease. The drugs are effective both in treating acute episodes and in suppressing the disease over the long term. Depending on the circumstances, hematinics, vitamins, high-protein diets, and blood transfusions are also used. Surgical resection of the portion of the large bowel affected is often performed. The entire colon may have to be removed and the small intestine brought out to the skin as an ileostomy, an opening to serve as a substitute for the anus. In ulcerative colitis, as opposed to Crohn disease, the rectal muscle may be preserved and the ileum brought through it and joined to the anus.

Cancer

In the Western world, colon cancer is more common than is stomach cancer, and it occurs about equally in both sexes. Risk factors for colon cancer include age, diets that are high in fat and low in fibre, a personal or family history of cancer, and the presence of polyps or ulcerative colitis. Symptoms are highly variable, the main feature being blood in the stools, but this may be detectable only by chemical testing. Cancers compress the colonic lumen to produce obstruction, they attach to neighbouring structures to produce pain, and they perforate to give rise to peritonitis. Cancers also may metastasize to distant organs before local symptoms appear. Nevertheless, the prognosis for patients with this cancer is considerably better than it is for cancer of the stomach. Some patients require a colostomy, in which an opening is made from the colon to the skin, where the fecal contents are extruded. After the colon has been removed partially, it is possible to join the terminal ileum or the remnant colon directly to the anal canal. A reservoir also can be fashioned out of the terminal ileum and placed inside the rectum muscle from which the inflamed mucosa has been removed. This functions as a normal rectum, and with retained sphincters at the anus, can render the patient continent, although there usually are three or four bowel movements daily.

Polyps

The tendency of some persons to form polyps, benign growths on the inner wall of the colon, is strikingly exemplified in the rare disorder known as familial polyposis, in which the colon may be studded with hundreds or thousands of small polyps. Because a colon that produces so many polyps eventually produces cancers as well, the colon should be removed surgically as soon as the diagnosis is made. The rectum may be left, but a visual examination of the residual mucosa must be made twice yearly to detect signs of early cancerous change. Another peculiar form of polyp is the villous adenoma, often a slowly growing, fernlike structure that spreads along the surface of the colon. It can recur after being locally resected, or it can develop into a cancer.

Anal disorders

Anorectal disorders related to defecation are more common in the Western world than elsewhere. Whether this distribution is related to diet, exercise, or personal hygiene is not clear. These disorders usually take the form of fissures (cuts or cracks in the skin or mucous membrane) at the junction of the anal mucous membrane with the skin between the thighs. If such fissures become chronically infected and resistant to treatment by sitz baths and local medication, they may require surgical correction. Anal fistulas sometimes occur as complications of serious bowel disease, as in tuberculosis or Crohn disease of the bowel, or in certain parasitic diseases. A more general disorder is the enlargement of veins of the rectum and anus to form external or internal hemorrhoids. Many adults in the Western world have such venous enlargements, but only a small number suffer serious symptoms from their presence. Hemorrhoids protrude, are associated with anal itching and pain, and bleed, especially when they come in contact with hard stools. These symptoms generally can be controlled by conservative measures, but occasionally they persist or cause so much distress that surgical removal of the enlarged and dilated veins is necessary.