What is Endo?

Para leerlo en español, clic aquí

Remember, this is educational information only and not medical advice (and I am not a doctor nor an expert on endo). Always check with your qualified medical professional before making any changes to your treatment plan.

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).

  • “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.”


These endometriotic lesions shouldn’t be in the body. While endometriosis is most commonly found on the reproductive system, it has been found in all 11 major body systems. Patients with endometriosis may have it on the ovaries, peritoneum (lining of the abdomen cavity and organs), bowel, bladder, uterosacral ligaments, rectovaginal septum, lung, diaphragm, or other parts of the body.


There are generally three accepted forms of endometriosis: 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), and 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. 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, even in the hands of an expert.


Endometriosis has glands and stroma, but doesn’t generally have blood vessels. Endometriosis glands secrete an inflammatory substance, which can irritate the surrounding tissue and destabilize nearby capillaries, causing bleeding in the surrounding tissues. This bleeding and inflammation can cause an immune response, the formation of fibrosis and adhesions, and pain. Endometriosis lesions can make their own estrogen via the aromatase enzyme. Endometriosis is often associated with changes in the surrounding tissues, such as neovascularity (where completely new blood vessels form), angiogenesis (where new blood vessels form from existing blood vessels) and neurogenesis (nerve growth). Studies have also demonstrated that endometriosis can have progesterone resistance, meaning that that the cells don’t respond to progesterone like they normally would.


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.

For more info