The penetration and growth of endometrial tissue from the uterine lining into the myometrium (uterine muscle) is called adenomyosis or internal endometriosis. This disease may coexist with external endometriosis in which endometrial implants are located outside the uterus. The abnormally located endometrial tissue, like the normal endometrium, tends to bleed with the menses. The blood and debris may accumulate in these misplaced glands creating small fluid collections inside the uterine wall. This penetrating and functioning endometrial tissue may lead to swelling; the uterus may become larger and globular. Adenomyosis may present as a diffuse condition or it may be focal. In the latter, there are local areas of swelling, so-called adenomyomas, that may mimic other uterine masses.

Most commonly adenomyosis is mistaken for another common condition, uterine fibroids. There is however a fundamental difference between a fibroid (a distinct tumor) and adenomyoma. Each fibroid originates from one abnormal cell. Under the effect of estrogen this cell multiplies. The growing tumor may displace and compress tissues but it does not invade the surrounding uterine muscle because of this growth pattern of fibroids, it is possible to remove all of the tumor without removing any normal uterine tissue during myomectomy (surgical removal of fibroids). In contrast, adenomyoma is not a discrete tumor but rather a local swelling of the uterine wall as a result of the penetration of endometrial tissue. Therefore it is not possible to remove tissue affected by adenomyosis without actually removing the involved uterine muscle.

Symptoms

Adenomyosis may be present and cause no symptoms. When this condition presents with symptoms the typical triad is uterine enlargement, pelvic pain and heavy and abnormal menstrual bleeding. Pain, which is most common during menses (dysmenorrhea), may be severe cramping or knifelike. However, pain may be present any time during the cycle and not only during the period. Uterine enlargement may be generalized with a large globular uterus or it may present as localized "tumors". Periods may be very heavy and prolonged, with passage of clots. Heavy bleeding may lead to anemia. Later, abnormal bleeding may be present any time during the cycle besides the heavy periods.

The effect of adenomyosis on fertility and pregnancy is not clear. Adenomyosis may well lower fertility. The information available suggests that adenomyosis may be present in up to 17% of pregnant women over the age of 35.