Endometriosis is a condition where there is ectopic endometrial tissue outside the uterus. A lump of endometrial tissue outside the uterus is described as an endometriomaEndometriomas in the ovaries are often called “chocolate cysts”. Adenomyosis refers to endometrial tissue within the myometrium (muscle layer) of the uterus.



The exact cause of endometriosis is not clear, but there are several theories. No specific genes have been found to cause endometriosis; however, there does seem to be a genetic component to developing the condition.

One notable theory for the cause of ectopic endometrial tissue is that during menstruation, the endometrial lining flows backwards, through the fallopian tubes and out into the pelvis and peritoneum. This is called retrograde menstruation. The endometrial tissue then seeds itself around the pelvis and peritoneal cavity.

Other possible methods for endometrial tissue exiting the uterus have been proposed: 

  • Embryonic cells destined to become endometrial tissue may remain in areas outside the uterus during the development of the fetus, and later develop into ectopic endometrial tissue. 
  • There may be spread of endometrial cells through the lymphatic system, in a similar way to the spread of cancer. 
  • Cells outside the uterus somehow change, in a process called metaplasia, from typical cells of that organ into endometrial cells.


Pathophysiology of the Symptoms

The main symptom of endometriosis is pelvic pain. The cells of the endometrial tissue outside the uterus respond to hormones in the same way as endometrial tissue in the uterus. During menstruation, as the endometrial tissue in the uterus sheds its lining and bleeds, the same thing happens in the endometrial tissue elsewhere in the body. This causes irritation and inflammation of the tissues around the sites of endometriosis. This results in the cyclical, dull, heavy or burning pain that occurs during menstruation in patients with endometriosis.

Deposits of endometriosis in the bladder or bowel can lead to blood in the urine or stools. 

Localised bleeding and inflammation can lead to adhesions. Inflammation causes damage and development of scar tissue that binds the organs together. For example, the ovaries may be fixed to the peritoneum, or the uterus may be fixed to the bowel. Adhesions can also occur after abdominal surgery. Adhesions lead to a chronic, non-cyclical pain that can be sharp, stabbing or pulling and associated with nausea.

Endometriosis can lead to reduced fertility. Often it is not clear why women with endometriosis struggle to get pregnant. It may be due to adhesions around the ovaries and fallopian tubes, blocking the release of eggs or kinking the fallopian tubes and obstructing the route to the uterus. Endometriomas in the ovaries may also damage eggs or prevent effective ovulation.



Endometriosis can be asymptomatic in some cases, or present with a number of symptoms:

  • Cyclical abdominal or pelvic pain
  • Deep dyspareunia (pain on deep sexual intercourse)
  • Dysmenorrhoea (painful periods)
  • Infertility
  • Cyclical bleeding from other sites, such as haematuria

There can also be cyclical symptoms relating to other areas affected by the endometriosis:

  • Urinary symptoms
  • Bowel symptoms

Examination may reveal:

  • Endometrial tissue visible in the vagina on speculum examination, particularly in the posterior fornix
  • A fixed cervix on bimanual examination
  • Tenderness in the vagina, cervix and adnexa



Pelvic ultrasound may reveal large endometriomas and chocolate cysts. Ultrasound scans are often unremarkable in patients with endometriosis. Patients with suspected endometriosis need referral to a gynaecologist for laparoscopy.

Laparoscopic surgery is the gold standard way to diagnose abdominal and pelvic endometriosis. A definitive diagnosis can be established with a biopsy of the lesions during laparoscopy. Laparoscopy has the added benefit of allowing the surgeon to remove deposits of endometriosis and potentially improve symptoms.



The American Society of Reproductive Medicine (ASRM) has a staging system for endometriosis. It is worth being aware of this staging system; however, it is not mentioned in the NICE guidelines, and does not necessarily predict the symptoms or the difficulty in managing the condition. NICE recommend documenting a detailed description of the endometriosis rather than using a specific staging system. The ASRM staging system grades from least to most severe:

  • Stage 1: Small superficial lesions
  • Stage 2: Mild, but deeper lesions than stage 1
  • Stage 3: Deeper lesions, with lesions on the ovaries and mild adhesions
  • Stage 4: Deep and large lesions affecting the ovaries with extensive adhesions



Helpful guidelines for the management of endometriosis are the RCOG Green-top guideline 41 on chronic pelvic pain (2012), the ESHRE guidelines on endometriosis (2013) and the NICE clinical knowledge summaries (2020).


Initial management involves:

  • Establishing a diagnosis
  • Providing a clear explanation
  • Listening to the patient, establishing their ideas, concerns and expectations and building a partnership
  • Analgesia as required for pain (NSAIDs and paracetamol first line)


Hormonal management options can be tried before establishing a definitive diagnosis with laparoscopy:

  • Combined oral contractive pill, which can be used back to back without a pill-free period if helpful
  • Progesterone only pill
  • Medroxyprogesterone acetate injection (e.g. Depo-Provera)
  • Nexplanon implant
  • Mirena coil
  • GnRH agonists


Surgical management options:

  • Laparoscopic surgery to excise or ablate the endometrial tissue and remove adhesions (adhesiolysis)
  • Hysterectomy

Laparoscopic treatment may improve fertility. Hormonal therapies may improve symptoms but not fertility.


Explanation of Treatment Options

Cyclical pain can be treated with hormonal medications that stop ovulation and reduce endometrial thickening. This can be achieved using the combined oral contraceptive pill, oral progesterone-only pill, the progestin depot injection, the progestin implant (Nexplanon) and the Mirena coil.

The cyclical pain tends to improve after the menopause when the female sex hormones are reduced. Therefore, another treatment option for endometriosis is to induce a menopause-like state using GnRH agonists. Examples of GnRH agonists are goserelin (Zoladex) or leuprorelin (Prostap). They shut down the ovaries temporarily and can be useful in treating pain in many women. However, inducing the menopause has several side effects, such as hot flushes, night sweats and a risk of osteoporosis.

Laparoscopic surgery can be used to excise or ablate the ectopic endometrial tissue. In women where there is chronic pelvic pain due to adhesions, surgery can be used to dissect the adhesions and attempt to return the anatomy to normal. 

Hysterectomy and bilateral salpingo-opherectomy is the final surgical option. During the procedure, the surgeon will attempt to remove as much of the endometriosis as possible. Importantly, this is still not guaranteed to resolve symptoms. Removing the ovaries induces menopause, and this stops ectopic endometrial tissue responding to the menstrual cycle.

Infertility secondary to endometriosis can be treated with surgery. The aim is to remove as much of the endometriosis as possible, treat adhesions and return the anatomy to normal. This improves fertility in some but not all women with endometriosis.


Last updated June 2020
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