While the exact cause of endometriosis remains unknown, many theories have been presented to better understand and explain its development. These concepts do not necessarily exclude each other.
- Estrogens: Endometriosis is a condition that is estrogen-dependent and thus seen primarily during the reproductive years. In experimental models, estrogen is necessary to induce or maintain endometriosis. Medical therapy is often aimed at lowering estrogen levels to control the disease. Additionally, the current research into aromatase, an estrogen-synthesizing enzyme, has provided evidence as to why and how the disease persists after menopause and hysterectomy.
- Retrograde menstruation: The theory of retrograde menstruation, first proposed by John A. Sampson, suggests that during a woman's menstrual flow, some of the endometrial debris exits the uterus through the fallopian tubes and attaches itself to the peritoneal surface (the lining of the abdominal cavity) where it can proceed to invade the tissue as endometriosis. While most women may have some retrograde menstrual flow, typically their immune system is able to clear the debris and prevent implantation and growth of cells from this occurrence. However, in some patients, endometrial tissue transplanted by retrograde menstruation may be able to implant and establish itself as endometriosis. Factors that might cause the tissue to grow in some women but not in others need to be studied, and some of the possible causes below may provide some explanation, ''e.g.'', hereditary factors, toxins, or a compromised immune system. It can be argued that the uninterrupted occurrence of regular menstruation month after month for decades is a modern phenomenon, as in the past women had more frequent menstrual rest due to pregnancy and lactation. Sampson's theory certainly is not able to explain all instances of endometriosis, and it needs additional factors such as genetic or immune differences to account for the fact that many women with retrograde menstruation do not have endometriosis. In addition, at least one study found that endometriotic lesions are biochemically very different from transplanted ectopic tissue, which casts doubt on Sampson's theory.
- Müllerianosis: A competing theory states that cells with the potential to become endometrial are laid down in tracts during embryonic development and organogenesis. These tracts follow the female reproductive (Mullerian) tract as it migrates caudally (downward) at 8–10 weeks of embryonic life. Primitive endometrial cells become dislocated from the migrating uterus and act like seeds or stem cells. This theory is supported by foetal autopsy.
- Coelomic Metaplasia: This theory is based on the fact that coelomic epithelium is the common ancestor of endometrial and peritoneal cells and hypothesizes that later metaplasia (transformation) from one type of cell to the other is possible, perhaps triggered by inflammation.
- Genetics: Hereditary factors play a role. It is well recognized that daughters or sisters of patients with endometriosis are at higher risk of developing endometriosis themselves; for example, low progesterone levels may be genetic, and may contribute to a hormone imbalance. There is an about 10-fold increased incidence in women with an affected first-degree relative. One study found that in female siblings of patients with endometriosis the relative risk of endometriosis is 5.7:1 versus a control population.
- Transplantation: It is accepted that in specific patients endometriosis can spread directly. Thus endometriosis has been found in abdominal incisional scars after surgery for endometriosis. It can also grow invasively into different tissue layers, ''i.e.'', from the cul-de-sac into the vagina. On rare occasions endometriosis may be transplanted by blood or by the lymphatic system into peripheral organs such as the lungs and brain.
- Immune system: Research is focusing on the possibility that the immune system may not be able to cope with the cyclic onslaught of retrograde menstrual fluid. In this context there is interest in studying the relationship of endometriosis to autoimmune disease, allergic reactions, and the impact of toxins. It is still unclear what, if any, causal relationship exists between toxins, autoimmune disease, and endometriosis.
- Environment: There is a growing suspicion that environmental factors may cause endometriosis, specifically some plastics and cooking with certain types of plastic containers with microwave ovens. Other sources suggest that pesticides and hormones in our food cause a hormone imbalance.
- Birth Defect: In rare cases where imperforate hymen does not resolve itself prior to the first menstrual cycle and goes undetected, blood and endometrium are trapped within the uterus of the patient until such time as the problem is resolved by surgical incision. Many health care practitioners never encounter this defect, and due to the flu-like symptoms it is often misdiagnosed or overlooked until multiple menstrual cycles have passed. By the time a correct diagnosis has been made, endometrium and other fluids have filled the uterus and fallopian tubes with results similar to retrograde menstruation resulting in endometriosis. The initial stage of endometriosis may vary based on the time elapsed between onset and surgical procedure.
Cause of pain
The way endometriosis causes pain is the subject of much research. Because many women with endometriosis feel pain during or around their periods and may spill further menstrual flow into the pelvis with each menstruation, some researchers are trying to reduce menstrual events in patients with endometriosis.
Endometriosis lesions react to hormonal stimulation and may "bleed" at the time of menstruation. The blood accumulates locally, causes swelling, and triggers inflammatory responses with the activation of cytokines. It is thought that this process may cause pain.
Pain can also occur from adhesions (internal scar tissue) binding internal organs to each other, causing organ dislocation. Fallopian tubes, ovaries, the uterus, the bowels, and the bladder can be bound together in ways that are painful on a daily basis, not just during menstrual periods.
Further Reading
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