Endometriosis involves the large bowel in approximately 15 - 20% of cases. The sigmoid or rectosigmoid colon is usually involved. The endometrial implant on the bowel wall induces a fibrotic reaction and hyperplasia of the muscle layer. These changes lead to the formation of an extramucosal mass.

The colonic implants are often asymptomatic and are discovered incidentally during abdominal or pelvic surgery. Alternatively, patients may complain of pelvic or lower abdominal pain, alteration of bowel habit, or rectal bleeding. These symptoms may have a cyclic, usually monthly, nature. The diagnosis of colonic endometriosis should be suspected in any premenopausal woman who experiences episodic bowel symptoms in association with menstrual complaints.

Barium enema examination typically shows a single extramucosal mass associated with a crenulated mucosal pattern. The crenulations are caused by the fibrotic reaction to the endometrial implant. The mass is most commonly located on the inferior margin of the sigmoid colon, as in the above case, or on the anterior wall of the rectosigmoid colon. Film .2 is an example of a large endometrial implant involving the anterior wall of the rectosigmoid. Multiple mass lesions are occasionally seen. Films .3 and .4 are from the barium enema examination of another patient with colonic endometriosis. Film .3 shows a large extramucosal mass with a "saw-tooth" mucosal pattern indenting the inferior aspect of the sigmoid colon. This patient had further endometrial implants (Film .4) causing both a large mass (long arrow) in the ileocecal region and nodular lesions (short arrows) deforming the terminal ileum. C = inverted, mobile cecum.

Colonic endometriosis may be treated by hormonal therapy or, if necessary, by segmental resection of the involved bowel.