In reviewing the causes of endometriosis, it is important to first understand the regular menstrual cycle and how hormones in the body affect the menstrual cycle and the uterus itself.
- The endometrium is the inner layer of uterine tissue that is shed during menstruation.
- The thickness of the endometrial layer is related to the egg-producing (ovulatory) cycle and the hormonal levels that regulate this cycle.
- The endometrium is at its thinnest immediately following menstruation and thickens during the first two weeks of the menstrual cyle.
- Once the release of the egg (ovulation) has occurred, the endometrial tissue becomes rich in glands.
- The whole process prepares the uterus for the attachment of a fertilized egg. If implantation does not occur, the endometrial layer is shed, and bleeding, known as menstruation (a period), begins.
- Endometriosis occurs when growth of this endometrial tissue develops outside the uterus. This growth usually occurs within the pelvic region on the ovaries and other pelvic structures, such as the bladder and colon, but it may also occur within the abdominal cavity and as far away as the lungs, arms, legs, and even the brain.
- Hormone levels affect the course of endometriosis.
- Because the levels of hormones that affect endometriosis are related to the menstrual cycle, it is uncommon for women to develop endometriosis before menstrual cycles begin or after menopause.
- Endometriosis is also noted to be less severe when hormone levels are more constant. These conditions include pregnancy and other times when there is lack of a menstrual cycle.
Several theories may explain how endometriosis develops as follows:
- One popular theory focuses on a potential process known as retrograde menstruation. Retrograde menstruation can be thought of as backward flow during a period. This is also known as the implantation theory.
- Menstrual products, including endometrial cells, may escape into the body through the Fallopian tubes and are deposited onto internal structures such as the ovaries, the bladder, and portions of the large intestine.
- These cells, once deposited, are able to respond to progesterone and estrogen in much the same way as normal endometrial tissue within the uterus.
- The growth of this misplaced endometrial tissue can cause distortion of abdominal and pelvic structures and causes the development of adhesions (scars) within the abdominal and pelvic cavities.
- Endometrial tissue can be found on the outside of the uterus, the space between the uterus and the colon known as the posterior cul-de-sac, the supporting ligaments of the uterus, the ovaries, the urinary bladder, and other internal structures.
- However, it is unlikely that retrograde menstruation alone is the cause of endometriosis, since retrograde menstruation has been shown to occur commonly in many women. Other causative factors may play roles in determining which women develop endometriosis.
- Another theory, also known as coelomic metaplasia, suggests that a layer of cells surrounding the ovaries and other cells within the pelvic region are able to change into endometrial cells that are much the same as normal endometrial tissue. It is not certain what causes this development, but evidence suggests irritation by retrograde menstrual flow or infections may be the culprit.
- Transfer of endometrial tissues by a surgical procedure might be the cause for endometriosis implants seen in surgical scars (for example, episiotomy or Cesarean section scars).
- The rare cases of endometriosis that develop in the brain or other distant organs are likely due to the spread of endometrial cells via the bloodstream or lymphatic system.
- Some studies have shown alterations in the immune response in women with endometriosis, suggesting that abnormalities in the immune system may play a role in the development of the condition.
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