Endometriosis

What Is It?

Endometriosis is a condition in which endometrial tissue, the tissue lining the inside of the uterus, grows in places outside of the uterine cavity. Although endometriosis can develop in many different locations, the most common sites are in the areas of the pelvis and abdomen, including the ovaries, the outside surface of the uterus, the membranes and ligaments of the pelvis and lower abdomen, the fallopian tubes, and the spaces between the bladder, uterus and rectum. Less commonly, patches of endometriosis grow within the wall of the rectum, bladder, intestines or appendix. Rarely, deposits of endometriosis are found in areas very far from the reproductive tract, such as the lung, arm, thigh and skin.

Areas of endometriosis tend to behave like the normal endometrium (uterine lining). Therefore, they can respond to the normal cyclic rise and fall of female hormones. They even may ooze blood at the time of menstruation, sometimes causing cyclic episodes of pelvic or abdominal pain. As they grow, areas of endometriosis also may interfere with a woman's fertility by covering or growing into the ovaries. These deposits can block the fallopian tubes directly, or block them indirectly by producing scarred tissue and adhesions (bands of fibrous tissue) that distort the normal anatomy of the reproductive tract. Small areas of endometriosis on the ovaries may merge into larger areas and form large fluid-filled cysts called endometriomas. Endometriomas sometimes are called chocolate cysts because they typically contain a thick, brownish mixture of blood and sloughed tissue that looks like chocolate.

Researchers have several different theories about the cause of endometriosis. According to one explanation, the condition develops when a backward (retrograde) flow of menstrual fluid carries bits of uterine tissue upward through the fallopian tubes into the pelvis instead of flowing downward toward the vagina. This seems to be a reasonable explanation for areas of endometriosis that are found around the uterus or ovaries.

However, the retrograde-flow explanation does not explain why endometriosis sometimes is found in distant sites such as the lung or skin. Endometriosis in sites far away from the pelvis may be explained by migration of endometrial cells directly through the bloodstream or lymph channels. An alternative explanation is that certain special types of cells found throughout the body may transform themselves spontaneously into endometrial cells and then join together to form large deposits of endometriosis.

Whatever the underlying cause of endometriosis, however, faulty immune defenses play some role in its development. The immune system apparently fails to identify and destroy endometrial-tissue deposits growing outside of the uterus where they do not belong, and may even promote the growth and proliferation of this tissue. Research is being done to understand the relationship between the immune system and occurrence of endometriosis.