Endometriosis is a gynecological condition that occurs when endometrial-like tissue – tissue similar to the type that lines the uterus – grows in areas outside of the uterus, particularly the ovaries and fallopian tubes, bowel, bladder, and pelvic lining.

It was first identified as a distinct medical condition in 1920 by the Canadian gynecologist Thomas Cullen. Advances in laboratory technology and surgical techniques in the decades that followed revealed rankings of disease subtypes and stages based on differences in spread and depth of implanted tissue.

This out-of-place tissue responds similarly to the uterine lining, thickening and disintegrating under the influence of hormones that govern the menstrual cycle. With no way for the body to remove the stray cells, surrounding tissues can become inflamed, often giving rise to sticky adhesions and stiff scar tissue that impedes movement.

What are the symptoms of endometriosis?

It's estimated that around one in 10 women, or around 200 million women, or people assigned female at birth, worldwide between the ages of 15 and 49, have the disorder, with 20 to 25 percent experiencing no obvious symptoms. Many people are diagnosed as a result of experiencing difficulty in falling pregnant.

For the remaining 80-odd percent, endometriosis can be a chronically painful condition, often presenting as strong cramping and severe pelvic discomfort. Symptoms can coincide with menstruation and may be accompanied by fatigue, nausea, and heavy periods.

The condition is generally described as progressive, experienced as heavy, painful periods during adolescence with symptoms gradually worsening over time, although this has proved difficult to track. Yet, the nature of the illness also makes it hard to generalize, with significant variations in its impact on different bodies.

What causes endometriosis?

It's not always entirely clear why some people get endometriosis, and others don't.

Studies have demonstrated the condition can run in families, suggesting genes might play a role in putting some people at greater risk.

In some cases, it has been thought to be the result of what's known as retrograde menstruation, a relatively common activity where uterine tissue progresses from the uterus up through the fallopian tubes. But this explanation has been criticized more recently, as many women experience retrograde menstruation and don't all have endometriosis – plus, it can't explain the occurrence of endometriosis in those who don't menstruate.

There is also the possibility that stray endometrial tissue that would usually be removed by the body's immune system sticks around longer due to an immune disorder.

How is endometriosis treated?

Endometriosis has no cure, and current treatments are limited. Today, therapies aiming to reduce endometriosis symptoms fall into three categories; hormonal, surgical, and complementary.

Hormonal treatments aim to induce a state that mimics pregnancy or menopause, tricking the body into holding off on growing new endometrial linings indefinitely.

In cases where the condition is severe, or the individual intends to become pregnant, excision surgery could be a better option. This involves a minimally-invasive laparoscopic procedure where a surgeon excises any offending scar tissue they can find.

Complementary options aren't intended to address the underlying causes of symptoms but could help those with the condition. This is especially important in severe forms where pain can become centralized. This might include psychological counseling or physiotherapy, as well as pelvic floor therapy, nutrition counseling, and neuromodulatory procedures.

Being a condition of such diversity, producing a range of symptoms, severities, and probably resulting from an array of underlying causes, there is no one treatment that suits all. What's more, knowledge of the condition has been impacted by a scarcity of evidence, not least thanks to an under-appreciation of the need to take period pain seriously in medical research.

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