Endometriosis describes a condition where pieces of womb lining tissue or endometrium are deposited outside the womb, in the pelvis or abdomen, for example. The endometrial tissue may occur on the fallopian tubes, ovaries, peritoneum, gut, rectum or vagina.
Endometrial lesions are stimulated by hormones and may bleed during menstruation in the same way the normal womb lining does and accumulating blood can cause swelling, inflammation and pain. The adhesions or internal scar tissue the condition causes, can also bind organs together, such as the ovaries and fallopian tubes, also causing pain.
Pain caused by organ binding may occur on a daily basis rather than subsiding once menstruation has finished.
Appearance of an endometriotic lesion
An endometrial lesion has similar features to normal endometrium when viewed under the microscope, with the endometrial stroma and epithelium present along with glands that are stimulated by hormones.
Older lesions may not have glands but have deposits of hemosiderin, a complex of ferritin and other material that is used as iron storage in cells rather than in circulating blood. While some endometrial lesions are very small and almost invisible, others may be large, obvious patches.
Endometrial lesions that develop on the ovary are blood-filled sacks that can grow with each menstrual cycle until they become as large as a grapefruit. Blood that has been inside these cysts for a long time darkens and can look like chocolate. The lesions are sometimes therefore referred to as "chocolate cysts."
The cysts maybe removed using surgery, which is considered the most effective way to permanently remove the tissue and relieve pain.
Reviewed by Sally Robertson, BSc