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What is Endometriosis?

Endometriosis is tissue similar to the endometrium (the uterine lining) but is NOT the endometrium, despite what many misinformed websites and even doctors say! Getting the definition correct is crucial for disease understanding and treatment.


The following 2 definitions are from the Endo Girls Blog website article called Defining Endometriosis:

“Endometriosis is a systemic, inflammatory disease characterized by the presence of endometrial-like tissue found in extrauterine sites.” (Kennedy S. et al., 2005; Klemmt et al., 2018; Saunders et al., 2021)

Here’s another:

“Given the abundance of differential invasive, adhesive and proliferative behaviors between the native endometrium and the lesions of endometriosis (Delbandi et al., 2013), it is not merely just ‘rogue’ endometrium. The ectopic lesions of the disease resemble, but are not identical, to their eutopic counterparts (Ahn et. al., 2016). Studies have demonstrated that the tissues are functionally dissimilar (Zanatta 2010; Freger et al., 2021).”


Here is a quick overview from the Center for Endometriosis article Endometriosis: Understanding a Complex Disease:

“Defined correctly as the presence of tissue somewhat similar to the endometrium – endometrial-like – found in extrauterine sites, the aberrant processes involved in endometriosis give rise to pain, inflammation, development of endometriomas (“chocolate cysts”), fibrosis, formation of adhesions (fibrous bands of dense tissue), organ dysfunction and more. Alterations in certain biological processes of the endocrine and immune systems have also been observed with the disease, and endometriosis is embodied by a complexity of multiple immunologic abnormalities, endocrine alterations and unusual expression of adhesion molecules.”


Symptoms may include (but aren’t limited to):

  • debilitating menstrual pain
  • pelvic pain away from the menstrual cycle (which may be acute or chronic)
  • painful sex
  • painful bowel movements or bowel dysfunction
  • painful urination or urinary dysfunction
  • pain with use of a tampon
  • leg pain
  • lower back pain
  • shoulder pain
  • chest pain
  • lung collapse (primarily around menstruation)
  • coughing up blood
  • pain when breathing
  • IBS or digestive problems like diarrhea or constipation
  • bloating aka “endo belly”
  • food intolerances
  • infertility
  • brain fog
  • fatigue
  • and more

In 2018, the UK’s National Health Service named endometriosis in a list of twenty of the most painful conditions a person can get.


These endometriotic lesions shouldn’t be in the body. Pelvic endometriosis is the most common location, defined as lesions of the fallopian tubes, ovaries and local peritoneum. The local peritoneum consists of the pelvic sidewalls, posterior cul-de-sac (pouch of douglas), surface of the uterus, and the broad, round and uterosacral ligaments. [Source: Extra Pelvic Not Rare]. Extrapelvic endometriosis is endometriosis found elsewhere, and is estimated to affect 1 in 5 people with endometriosis. However, the true number of people with extra pelvic endometriosis is probably much higher, because the disease is often underdiagnosed and misdiagnosed. Additionally, we lack a lack of a classification system that encompasses all aspects of the disease, which makes it difficult to know the true numbers of people affected.

Apart from the reproductive system, endometriosis has been found in all 11 major body systems. The most common extrapelvic locations are the digestive, urinary, thoracic, and skin systems


There are generally three accepted forms of endometriosis, and some people suspect that the different forms have different origins. The forms are:

  • Superficial (lesions are less than 5mm below the peritoneal surface)

  • Ovarian Endometrioma (a cyst caused by endometriosis on the ovary; also known as chocolate cysts). Endometriomas are almost never found alone and are often an indicator that the person has more extensive pelvic and intestinal disease. Endometriomas are also known to have a higher rate of recurrence than other forms of endometriosis, even in the hands of an expert. Recurrence is when the lesions return after having been excised.

  • Deeply Infiltrating Endometriosis (when lesions are more than 5mm below the peritoneal surface). DIE is considered the most severe form of endometriosis, but all forms can cause horrible symptoms and pain.


Endometriosis is often staged using the American Society for Reproductive Medicine Classification System (ASRM). This categorizes endometriosis into 4 stages: stage I is minimal endometriosis, stage IV is the most severe. However, this staging system was designed with fertility in mind. It doesn’t correlate to the level of pain a person has, so someone with Stage I could have more pain than someone with Stage IV. Any stage of endometriosis can cause excruciating pain. Also, the staging system doesn’t take into account endometriosis in locations such as the bladder, bowels, vagina, cervix, and more locations. The ASRM system has been criticized for having too much focus on the ovaries and adhesions.

A new staging system was developed called the AAGL 2021 Endometriosis Classification, which was specifically designed to measure surgical complexity. Like the ASRM classification, the stage does not correlate to the patient’s pain, and the staging system doesn’t attempt to give fertility outcomes. However, there is hope that it will help in the future with surgical reimbursements and improve financial incentives to completely treat disease, especially in complex cases needing significant expert time and effort. The staging system is available as an app called AAGL Endo Classification which you can download to your phone. What I like about the app is that the surgeon goes through the different areas of (abdominal) endometriosis, determining if you have no endometriosis there or less/more than 3 cm. Anatomic locations include: peritoneal, L/R ovary, L/R fallopian tube, L/R uterer, bladder, cul-de-sac obliteration, vagina, retrocervical, rectovaginal septum, rectum/sigmoid colon, small bowel/cecum, appendix. Then after they can download a PDF for you to see the locations/depth of your endometriosis.

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