Introduction to Endometriosis


Introduction to Endometriosis

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Endometriosis: The Basics

Be sure to also visit our Comprehensive Fact Sheet on Endometriosis

The correct clinical definition of endometriosis is ‘a disease characterized by the presence of endometrial-like tissue found outside the uterus.’ What this means is, lesions comprised of tissue that somewhat resembles - but is not the same as - the endometrium is located elsewhere, causing various pain and symptoms. Diagnosis of endometriosis is considered uncertain unless surgery is performed and biopsy-proven diagnosis of the lesions can be obtained.


Usually the lesions are located in the pelvis, but they can also found in other places. Endometriosis has been identified on every abominopelvic organ, including on the ovaries, tubes, ligaments and ureters and on, around or even in the bowel, bladder and other organs. Endometriosis is also found in remote locations, though this is less common, such as on the diaphragm or lungs. Even far remote locations like the back of the knee and the brain have been detailed with endometriosis, although these cases are extremely rare.

The location of the lesions, the depth to which they penetrate the tissue they rest upon, and their overall size contribute to the unique nature of this disease. Although there are many differing opinions concerning aspects of endometriosis, one thing everyone agrees on is that every case of endometriosis is different from every other case. We may find similar cases, but none that are exactly alike.  There are 4 stages of endometriosis: minimal, mild, moderate, severe. However, stage was primarily developed to assess fertility and has no bearing on pain or symptoms. A person in stage 1 can suffer in the extreme while a person with stage 4 does not experience any pain (though higher stages often present with painful, fibrotic and deep disease).

WHAT does it do?

The magnitude of endometriosis pain depends in part on where it is and how much you have. A spot (focus) of endometriosis may stay small and relatively inactive for many years. However, even the tiniest lesion can cause incapacitating pain if it irritates a nearby nerve. Additionally, the disease creates an inflammatory response. This means increased blood supply and congestion; release of multiple immune response cells (like T cells, plasma cells; as well as arachidonic acid, cytokines, and TNF (tumor necrosis factor). All these factors and more are part of the local response to lesions. Larger areas of disease can become locally invasive as they respond to hormone stimulation. The tissue surrounding the lesion can begin to break down and bleed. The body's natural reaction is to try to cover this raw area with scar tissue (also called adhesions). But if active endometriosis becomes trapped beneath adhesions, pain and pressure can result. A large walled-off area (frequently an ovary) can lose its central blood supply. Then degeneration and destruction of the localized blood can create a cystic mass called an endometrioma. An endometrioma can be quite small, like a pea. They can also grow very large. We have seen endometriomas the size of softballs. Invasion of nearby structures including the bowel, bladder and ureters can occur. Advanced endometriosis can result in a pelvis frozen with adhesions. This means that organs designed to float freely within the pelvis are stuck together. Then, any movement of any one of those structures (such as ovarian movements during ovulation, movements during sexual intercourse, or moving material through the bowel) can result in enormous pain.

WHAT does it feel like?

This question has no universal answer. We have cared for advanced-stage patients who had no pain, and cared for patients with minimal disease who were incapacitated by pain. We have seen every degree of misery between those points. Pain is the most common symptom. Other symptoms may include:

  • painful menstruation

  • pelvic pain apart from menses

  • backache

  • painful intercourse or pain associated with sexual activity

  • painful bowel movements

  • fatigue

  • bloating

  • constipation

  • menstrual diarrhea

  • pain with exercise

  • painful pelvic exams

  • painful urination

  • sciatic pain

Of course, not everyone has every symptom and some will have those not listed; these are among the most common ones.