What is endometriosis?
Endometriosis is a chronic disease
Endometriosis is a chronic disease in which fragments of endometrial-like tissue are found outside the uterus.
These fragments are located inside the abdominal cavity, where they may graft onto the peritoneum, a thin membrane that covers all intra-abdominal organs (peritoneal or superficial endometriosis), or extend beyond this membrane, where they infiltrate various organs or anatomical structures (sub-peritoneal endometriosis).
These fragments of tissue, known as "endometriosis lesions", vary in size from a few dozen microns to several centimetres, and have the potential to grow and disseminate in flare-ups punctuated by menstrual periods.
General information on the endometrium
The endometrium is the mucous membrane lining the inner wall of the uterus.
Its monthly growth is designed to create the ideal conditions to receive an embryo in the event of fertilization. In the absence of fertilization, the endometrium is eliminated in the form of menstruation by the cervix, then the vagina, and the cycle begins again the following month in a physiological manner.
During menstruation, some of the blood flows backwards through the fallopian tubes into the abdominal cavity, where it decays over the following days.
The degradation of blood releases the iron contained in hemoglobin, which induces a local inflammatory process.
Moderate, short-lasting menstrual pain may be physiological, linked to uterine contraction to evacuate the endometrium, or to pelvic inflammatory phenomena linked to the degradation of blood in the abdominal cavity.
The origin of endometriosis
The mechanism of appearance of endometriosis lesions is partially known, and several theories attempt to explain it. These theories are probably complementary, and much scientific research is underway to try to explain the origin of the disease more clearly (Lagano AS, Martin DC et al, Int J Molec Science 2019).
Some lesions are probably linked to the reflux of menstrual blood through the fallopian tubes. In patients with endometriosis, there is probably abnormal attachment of endometrial cells to the peritoneum. This implantation theory has long dominated the scientific literature. It explains the very frequent appearance of disseminated endometriosis lesions in women with uterine malformations that impede or prevent the rapid evacuation of menstrual blood and increase tubal reflux. On the other hand, this theory does not explain the appearance of endometriosis in women with congenital absence of the uterus, in young adolescent girls, or the rare cases of endometriosis in men. It is clear that endometriosis cells are not simply cells transplanted from the uterus to the abdomen, and that other mechanisms are involved in the development of endometriosis lesions.
Others are probably the result of a transformation of normal cells (metaplasia) into endometriosis cells. This theory may explain the appearance of endometriosis cysts in the ovaries, as well as lesions appearing remotely in the pelvis.
Other lesions could originate in embryonic remnants, according to the mullerianosis theory. This theory could explain the high frequency of endometriosis lesions behind the uterus and on the utero-sacral ligaments, but not the presence of endometriosis lesions in the ovaries, sigmoid colon, appendix or on the diaphragm.
Finally, a recent theory emphasizes the epigenetic transformations occurring in normal endometrial cells, which would be at the origin of the appearance of intra-abdominal endometriosis lesions.
All these theories explain the mechanism behind theappearance of endometriosis lesions in certain patients, but none is likely to be able to explain the phenomenon in a comprehensive way.
The consequences of endometriosis lesions
The growth of endometriosis lesions leads to the appearance of increasingly voluminous lesions, which can invade and infiltrate the organs of the pelvis or abdomen: the ovaries, fallopian tubes, vagina, but also the colon, rectum, bladder, ureters, diaphragm or, more rarely, the sacral or sciatic nerves.
These endometriosis lesions can behave like real benign tumors, infiltrating and deforming organs and disrupting their normal functioning.
Evolution of endometriosis
Practitioners feel that the actual prevalence of endometriosis has been steadily increasing over recent decades, with no unanimous explanation for this phenomenon. (Scioscia M, Roman H et al, Hum Reprod 2019)
One explanation is the increasing number of menstrual periods in a woman's life. In fact, over the last few decades, the increase in the age of first pregnancy, the reduction in the number of pregnancies and the shorter duration of breastfeeding have mathematically reduced the length of physiological amenorrhea in a woman's life. Endometriosis, whose development is directly linked to menstruation, would therefore be more likely to progress.
Other hypotheses suspect a role for pollutants and endocrine disruptors in increasing the risk of onset and development of endometriosis. Fundamental research studies will undoubtedly one day enable us to determine this origin.
Most authors agree that the mechanism of onset and development of endometriosis is probably multifactorial.
Bibliography:
Scioscia, Roman et al, Hum Reprod 2019.
Lagana AS, Martin DC et al. Int J Molec Sciences 2019. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6888544/pdf/ijms-20-05615.pdf
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