Diverticulitis is inflammation of an abnormal pouch (diverticulum) in the intestinal wall. These pouches are usually found in the large intestine (colon). The presence of the pouches themselves is called diverticulosis.

Causes, incidence, and risk factors:

Small protruding sacs of the inner lining of the intestine (diverticulosis) can develop in any part of the intestine. They are most common in the colon, specifically the sigmoid colon. These sacs, called diverticula, occur with increasing frequency after the age of 40. When they become inflamed, the condition is known as diverticulitis. Diverticula are thought to develop as a result of high pressure or abnormal pressure distribution within the colon. High pressure exerted against the colon wall causes pouches of the intestinal lining to bulge outwards through small defects in the colon wall.

Diverticulosis is very common, being found in more than half of Americans over age 60. Only a small percentage of these people will develop the complication of diverticulitis. Diverticulitis is caused by inflammation, or (sometimes) a small perforation within a diverticulum. If the perforation is large, stool in the colon can spill into the abdominal cavity, causing an abscess or peritonitis.

Direct risk factors for diverticulitis are not known. Risk factors for diverticulosis may include older age or a low-fiber diet.