Endometriosis: What exactly is it?
- Dr Natalie Hutchins
- Mar 6
- 5 min read
Updated: Mar 24
Endometriosis is an incredibly common condition; nearly 200 million women around the world are living with it.
Small patches of tissue similar to the tissue we have lining our wombs (endometrium), grows in areas outside of the womb. The most common place for this is in the pelvis where it can be found on any organ or structure within it, such as the ovaries, bladder, bowel, vagina, nerves and ligaments (see figure 1).
It can either be found superficially on these structures or it can invade these tissues more deeply, a bit like cancer can (although it is not a cancer). When it does this it is referred to as ‘deep infiltrating endometriosis’. These areas of tissue cause inflammation, which leads to pain and can lead to structures ‘sticking together’; called adhesions, which can distort a woman’s anatomy, prevent proper function and cause pain. Endometriosis can also implant on the nerves directly and lead to nerve dysfunction.
Figure 1: Places in the abdomen and pelvis that endometriosis can grow. From top to bottom: Diaphragm, liver, intestines, rectouterine pouch, uterus and ovaries, nerves and ligaments, bladder and vagina. It can grow anywhere really.
So, Endometriosis isn’t just period pain then?
No. Although there is a range in how severe a woman’s symptoms will be and each woman is different. Some women will only experience mild symptoms and some women only find out they have it when endometriosis is found unexpectedly during an unrelated operation.
However, in many women it is or eventually becomes, a chronic, systemic inflammatory condition. The patches of endometrial tissue can extend beyond the pelvis and abdomen to affect the diaphragm, lungs, pericardium (lining around the heart), and even the nasal cavity and brain (although this seems to be very rare).
How does the endometrial tissue get to these places?
Well, the first theory was that some of the blood and endometrial tissue that leaves the body as a period, flows backwards through the fallopian tubes and into the pelvis. This is known as the theory of retrograde menstruation. We know that this plays a part as we see more endometriosis in girls and women that have anatomical abnormalities that mean their period can’t flow out through the vagina, causing more to flow backwards into the pelvis through the fallopian tubes.
But this can’t be the full picture because we know that some amount of retrograde menstruation happens in nearly every woman. So, it’s likely that certain genetic traits and or problems with how a woman’s immune system works, means these cells are not cleared from the pelvis in the normal way and instead implant in areas they would not normally be found.
But that wouldn’t explain how it gets to areas outside the pelvis and abdomen?
Well, it could. For instance, when endometriosis is found in the wall of the abdomen, usually in a scar from a cesarean section, it is likely that the cells have implanted there from the pelvis during the surgery.
And a similar thing could be happening in the case of thoracic endometriosis. Our pelvic and abdominal organs are bathed in a small amount of fluid and this naturally moves in a clockwise direction, going up on the right side towards the liver. The abnormal endometrial cells can therefore be carried towards the liver and implant on the diaphragm just above it and from there make their way to the lungs. Some women with thoracic endometriosis have holes in their diaphragm and it is more common to have thoracic endometriosis in the right lung, which would fit with this theory.
Figure 2: Arrows showing natural movement of peritoneal fluid within the abdomen that is thought to carry the endometrium-like cells around the abdomen and pelvis
However, not all scientists agree and this theory wouldn’t make sense for endometriosis found in really distant areas like the brain for example. It might be that the endometrium-like cells get to these areas through the bloodstream or lymphatic system. Other theories are that normal cells in these areas transform from normal tissue to endometrium-like tissue or that pockets of abnormal tissue are there due to abnormalities occurring during development in utero. This would fit with why some girls are found to have endometriosis even before their period starts.
What role do our hormones play?
Whilst oestrogen is not a cause for endometriosis, it is a driver of the condition. The endometrium-like tissue behaves very similarly to the endometrial tissue within the womb; it grows in the first half of the cycle and breaks down when progesterone levels drop leading to inflammation.
This means that whilst many women will experience an improvement in their symptoms at the time of the menopause, menopause is not a cure. It can make management of the symptoms of the menopause with hormone replacement therapy more challenging.
Do environmental factors play a role too?
It’s likely that they do, yes. Endocrine disruptors are chemicals that can mimic the action of our body’s own natural hormones[1]. So, it would make sense that if endocrine disruptors can mimic the action of oestrogen, and we know oestrogen is a driver of endometriosis, that they could have a role to play. There is some evidence that environmental pollutants such as dioxins, phthalates, bisphenol A and heavy metals have a link to endometriosis[2]. These are found in varying amounts in the environment; from the air we breathe, to food, water and cosmetics.
Lifestyle factors may also play a role but the evidence for this is very limited. Factors such as low exercise, smoking, night shift work, psychological stress, a diet high in red meat and low in vegetables[3] have all been implicated. However, we know that all these factors that have enormous health benefits to women in other ways and so lifestyle change should always be considered..
Unfortunately, endometriosis is a complex condition and studying the impact of these environmental factors without introducing some form of bias into the study design will be very challenging. Hopefully, such studies will be done in the future but for now, the evidence we have for the influence of these factors on endometriosis is conflicting.
Take aways:
1. We don’t fully know why endometriosis happens but there are multiple theories. It is likely that multiple factors cause and drive it.
2. The most common site for endometriosis is the pelvis but it can implant on any organ or structure, but not all women will be affected in the same way.
3. Genetics do play a role but environmental and lifestyle factors may also be important.
External resources:
[2] Upson K. Environmental risk factors for endometriosis: A critical evaluation of studies and recommendations from the epidemiologic perspective. Curr Epidemiol Rep. 2020 Sep;7(3):149-170. doi: 10.1007/s40471-020-00236-3. Epub 2020 Aug 18. PMID: 33042729; PMCID: PMC7543974.
[3] Parazzini F, Chiaffarino F, Surace M, Chatenoud L, Cipriani S, Chiantera V, Benzi G, Fedele L. Selected food intake and risk of endometriosis. Hum Reprod. 2004 Aug;19(8):1755-9. doi: 10.1093/humrep/deh395. Epub 2004 Jul 14. PMID: 15254009.
