Endometriosis can result in either infertility or pelvic pain (especially during the menstrual flow). Typically, symptoms leading to the diagnosis of endometriosis occur during the reproductive years (yet rare cases of endometriosis in females prior to puberty or after menopause and even rarer cases of endometriosis in males have been reported).
Endometriosis occurs when endometrial glands and stroma (normally tissue that lines the uterine cavity) implant and grow outside the body of the uterus. Most often, endometriosis is located in the pelvis near or on reproductive structures (such as ovaries, the uterus, the fallopian tubes). However endometriosis can occur in virtually any tissue of the body, including distant sites like the lung, the knee and the skin.
A common misunderstanding involves the relationship between pelvic pain and infertility when each (independently) occurs in association with endometriosis. A woman with incapacitating pelvic pain due to endometriosis may have normal fertility. Likewise, a woman with infertility and endometriosis may have absolutely no pelvic pain. The apparent lack of association between the stage of endometriosis and pelvic pain is thought to be due to variability in the distribution of peritoneal nerves (if an endometriotic implant is next to a sensory nerve then there may be perception and pain, the number and distribution of these nerves throughout the pelvis may vary enormously between women).
Endometriosis can only be diagnosed with certainty by direct visualization (or tissue sampling), so that surgery is needed to diagnose pelvic endometriosis. Nonsurgical tests are nonspecific, including (1) ultrasonography (no specific findings are diagnostic of endometriosis, persistent nonfunctional ovarian cysts may have a characteristic appearance of an endometrioma but these are not diagnostic); (2) MRI (no specific findings are diagnostic of endometriosis, research suggests a 60% sensitivity and specificity for MRI as a diagnostic test for endometriosis); and (3) CA 125 serum concentration (found to be slightly elevated in women with endometriosis, modified sensitive CA 125 assays have been proposed as a way of monitoring the progression of disease in women with known endometriosis, nonspecific for diagnosis since CA 125 is elevated with any pelvic peritoneal inflammatory process including infections, tumors, active menstrual flow).
Endometriosis appears to depend on estrogen for growth. Endometrium (the tissue that normally lines the uterine cavity and that forms endometriotic lesions) is a very complex type of tissue since it is one of the only tissues in the human body that changes (grows, modifies its structure, alters its production of molecular messengers) in response to relative concentrations of circulating steroids. Regulation of (endometrial) cellular behavior by estrogen is thought to be the reason why endometriosis is rarely found in low estrogen states (prior to puberty, after menopause, following removal of the ovaries).