A condition whereby the endometrium (the inner layer of the uterus) grows into the myometrium (the muscular outer layer); it is also called "internal endometriosis". Adenomyosis may result in a swelling of the uterine wall which appears to be a fibroid, but unlike a fibroid, which is a distinct tumor which can be removed, an adenomyoma cannot be extracted without also removing muscle tissue.
Although benign, adenomyosis can cause rather severe symptoms such as very painful menstrual cramps and prolonged and heavy periods; the uterus may swell up to four times its normal size. (One source refers to adenomyotic uteri as "boggy" and enlarged, but I have no idea what a boggy uterus might look like.)
Unfortunately, diagnosing adenomyosis is difficult because it requires microscope examination of uterine tissue, though MRI and ultrasound can sometimes help reveal the condition. Usually, though, adenomyosis is only discovered after a major surgery such as Caesarean section or hysterectomy.
The cause of endomyosis is unknown. Most women with adenomyosis also have other uterine pathologies; up to 80% also have fibroids, and a smaller but significant percentage also have endometriosis (endometrial tissue outside the uterus). The co-existence of these other conditions make it more difficult to diagnose the adenomyosis. Adenomyosis generally affects women over 30 who have previously given birth, and estimates of prevalence range from 15% to over 65% of women in this age group.
Most women with adenomyosis are asymptomatic, and those with symptoms find they usually resolve after menopause, which is a good thing, for treatment options aren't great. GnRH (gonadotropin releasing hormone) agonists can be used for a short period, but they cause menopausal symptoms such as hot flashes and osteoporosis, and so are not prescribed long-term, and regrowth of the abnormal tissue usually occurs rather quickly once the medication has stopped. So the only real treatment at the moment is hysterectomy, a rather drastic measure, though a partial hysterectomy, removing only the diseased area, has been used with some success.