chocolate cyst
Endometriosis is a condition in which normal endometrial tissue is located somewhere outside the uterus. The misplaced endometrial tissue is commonly within the pelvis (ovaries, uterine ligaments, pouch of Douglas, fallopian tubes, rectovaginal septum) and is often multifocal. Less commonly, it is foundin remote sites, such as lymph nodes, lungs, heart, skeletal muscle, or bone. Commonly, endometriosis appears as an ovarian cyst, which, when ruptured, oozes a dark brown, hemorrhagic material; such cysts are called “chocolate cysts.” Endometriosis is totally benign. However, it causes problems because it undergoes cyclic bleeding, like normal endometrium. Depending on the location, it may cause scarring, pain, or sterility.

So how does the endometrium get out of the uterus?

Well, that’s still a mystery. There are three main theories, none of which is alone entirely satisfactory:

1. The regurgitation theory. Menstrual flow backs upthrough fallopian tubes, with subsequent implantation of endometrial tissue in the peritoneum. Retrograde flow occurs pretty commonly in normal women, and menstrual endometrium is viable (if injected into the peritoneum, it lives). This is a nice theory for endometriosis in the pelvic cavity.

2. Metaplastic theory. Endometrial tissue arises from endometrial differentiation of coelomic epithelium. This could be true (embryologically, endometrium arises from coelomic epitheilium), and would be especially likely to cause endometriosis in sites such as the ovary.

3. Vascular/lymphatic dissemination theory. Endometrial cells somehow work their way into the vascular/lymphatic system. This hypothesis would explain how endometriosis can occur in distant sites, such as lymph nodes or lungs.

Perhaps all three theories are correct, either concurrently, or at different times in different patients.