Causes

Endometriosis occurs among women all over the world, but researchers have been unable to determine its cause. A combination of genetic, biologic, and environmental factors appear to work together to trigger the initial process, produce implantation, and cause subsequent reseeding and spreading of the implants.

Initial Cause and Distribution of Endometriosis

Retrograde Menstruation. One explanation for the development of endometriosis implants involves retrograde menstruation. This occurs during a woman's period, when menstrual tissue flows backward through the fallopian tubes rather than out through the vagina. Early theorists suggested that, in some cases, the redistributed uterine tissue attached and grew in areas outside the uterus, forming endometriosis implants. This theory does not fully explain endometriosis, however. Many women have some retrograde menstruation, but not all of them develop endometrial cysts. Consequently, other factors must explain why uterine tissue becomes implanted and grows in areas outside the uterus.

Lymphatic Transport. This theory suggests that endometriosis first develops when uterine tissue is separated and transported to other organs through the lymphatic system or the bloodstream.

Candida. There is absolutely no evidence that endometriosis is caused by candida (commonly called yeast infection), as claimed in some consumer publications.

Causes of Persistence and Growth of Endometriosis

There are two basic mysteries surrounding the persistence and growth of endometriosis:

• Why do endometrial implants survive the attack by the immune system, which is typically launched against any foreign presence in the body?
• How do these endometrial travelers develop new blood vessels and implant themselves in other locations?

Impaired Immune System. Some research is focused on possible immune disorders in women with endometriosis. One theory proposes that women with endometriosis have fewer natural killer (NK) cells, which are factors in the immune system important for surveillance. In their absence, the immune system is weakened and may allow endometrial tissue to invade and take root. A recent study suggests that other types of immune system cells are also underactive in women with endometriosis, allowing the woman's body to tolerate the implanted tissue.

Some evidence suggests that endometriosis represents an autoimmune condition, in which the immune system launches an attack on its own cells and tissue. Much of the evidence rests on the relatively high incidence of other inflammatory autoimmune disorders (such as multiple sclerosis, rheumatoid arthritis, and lupus) that occur in women with endometriosis. It is unclear, however, how this response relates to endometriosis itself and whether endometriosis should be treated as an autoimmune condition.