What is Endometriosis?

Endometriosis is defined as the presence cells similar to those that line the uterus (endometrium) are found in an abnormal place (usually the pelvis) . The endometrium (lining) of the uterus usually spreads to the pelvis through the tubes and settles most commonly in the pelvis. Like the lining of the uterus, the endometrium grows under the influence of the major female hormone oestrogen. The most common sites in the pelvis are on and below the ovaries, and deep in the pelvis behind the uterus, called the Pouch of Douglas. Here the endometriosis grows on the ligaments behind the uterus and on the vagina and rectum. It also may grow on the bladder, appendix, abdominal wall and even sometimes in the upper abdomen. 

Studies suggest that endometriosis affects 1 in 10 women of reproductive age, with an estimated 176 million women worldwide having the condition. 

The endometrial cells:

  • are most commonly found on organs in the pelvis
  • may start to grow and form patches or nodules on pelvic organs or on the peritoneum (the inside lining of the abdomen and pelvis;
  • have the same cyclical/menstrual changes outside the uterus as inside the uterus
  • may bleed at the same time as your period (menstruation).

Endometrial cells found outside the uterus grow to form lesions or patches that bleed and leak fluid in response to your hormones at the time of the period. This leads to inflammation and scarring. These patches found outside the uterus cannot pass out of your body and remain in your pelvic cavity, on organs and other surfaces. On your ovaries, cysts called endometriomas can develop over time. These are sometimes called 'chocolate cysts' because of the darkish material they contain. On other surfaces the patches can form nodules (or lumps of endometriosis ).

There may be different forms or types of endometrios:

  • Superficial endometriosis: here, superficial areas of disease appear on the lining of the pelvis and abdomen (called the peritoneum). These do not invade deeply into the tissues, however can still cause significant pain.
  • Deep-infiltrating endometriosis: endometriosis lesions do invade deeply (more than 5mm) into the tissues and can lead to scarring and nodules. These can grow into nearby organs, such as the bladder, bowel , appendix and ovary
  • Ovarian disease: this may occur and is often considered a separate form of the disease. Here, burrowing lesions on and under the ovaries can lead to the development of 'chocolate cysts' or endometriomas
  • Adenomyosis: the endometrial cells grow inside the muscle of the uterus
  • Outside the pelvis: this is rare
  • Upper abdomen, eg on the diaphragm(most common area outside of the pelvis)
  • In the liver, nose, eye, lungs (these are rare)
  • Abdominal wall, often associated with previous operation scarring. (usually following caesarean section)

The most common sites of endometriosis

Retrograde menstruation which causes most endometriosis