Endometriosis is a condition where tissue similar to the lining of the womb (uterus) is found elsewhere in the body. It is most commonly found in the pelvis (in or on the ovaries, behind the uterus, in tissue that holds the uterus in place and on the bowel and the bladder) but it can be found in other locations such as the chest cavity, lungs, abdominal wall scars and the navel. Endometriotic tissue may be located on the surface of organs and on the lining of the abdominal cavity (peritoneum). It may infiltrate deeper into the structures or may form cysts within the ovaries. These cysts are known as 'chocolate cysts' or endometriomas, which may be identified on an ultrasound scan.
Every month, if a woman is not pregnant, the lining of the womb breaks down and is shed as a menstrual period. The endometriotic tissue (endometriosis) goes through a similar cycle and bleeds during menstruation. This may cause pain during periods, inflammation and scarring. The scarring can cause organs or structures to stick to each other and this may distort the normal anatomy. Anatomical distortion of the pelvic organs may cause difficulty in becoming pregnant. Deep endometriosis tissue and scarring can sometimes form small lumps and these are called 'endometriotic nodules'. These nodules can be very tender and painful to touch. Scarring and nodules of endometriosis may cause some women to experience pain during sexual intercourse.
Who is affected by endometriosis?
Any woman in the reproductive period of her life can be affected from teenage years to the menopause. Endometriosis usually improves with the onset of menopause.
What causes endometriosis?
The exact cause is not known. It is possible that there are multiple contributing factors and that these factors may be inherited in some families.
The theories on how endometriosis develops include:
- retrograde menstrual flow: menstrual blood flows backwards down the fallopian tubes into the pelvis. Cells from the lining of the womb (endometrial cells) within this blood may implant onto the surface of structures in the pelvis and continue to grow there giving rise to endometriosis. This theory best explains why endometriosis is commonly found in the pelvis.
- lymphatic or vascular spread: endometrial cells may be carried to different parts of the body via the lymphatic drainage or blood vessels. This is probably how distant site endometriosis develops.
- metaplasia: this is a term used to describe a process during which cells in the body may undergo change into endometrial cells. Examples include cells lining the inside of the abdomen and cells from the ovary.