Endometriosis. What is it? A lot of people, maybe most, haven’t even heard of endometriosis. In a lot of ways it is hard to really understand what it is and even harder for people to get what having this disease can do to a patient’s life. It can be difficult for even the most articulate patient to effectively communicate how devastating endometriosis can be on one’s life and ability to function. In this first of a series of posts I will review the medical definition. In the following posts I will discuss what it really means to have endometriosis.

Endometriosis is defined as the presence of endometrial tissue outside of the uterus, the organ in which the baby grows during pregnancy. When it is time to deliver, the uterus pushes the baby out. Most of the uterus is made up of muscle, but it is lined on the inside by a thin layer of glands called the endometrium. This layer provides the nutritional support to the developing fetus prior to the development of the placenta (also known as the after-birth).

During the first two weeks of the menstrual cycle the endometrial lining responds to the estrogen, growing in thickness. You can think of estrogen as "fertilizer" for the endometrial lining. The last half of the cycle, after ovulation occurs (about two weeks after the first day of bleeding during a period), the other female hormone, progesterone, helps to stabilize and mature the endometrial lining. If a woman is not pregnant, both the estrogen and progesterone levels drop at the end of the cycle, the endometrial lining sloughs off, and her period starts the next day.

The menses are made up of the endometrial lining, blood and clots. Most of the menses comes out through the vagina, but some flows back up through the fallopian tubes and remains inside of the body. Endometriosis is formed when some of this tissue implants and starts to grow. We don’t completely understand why some women develop endometriosis, and others do not. It is probably related to both the amount of tissue that a woman’s body has to get rid of each month, genetics, environmental exposure and her body’s inability to recognize or get rid of the endometrial tissue inside of the body cavity. Research to date points to a selective decrease in the immune system as a cause of endometriosis. Several studies, as well as our own personal experiences, also have shown a decrease in the natural "killer-cell" function (a type of white blood cell) in women who have endometriosis.

As discussed in the previous paragraph, in the vast majority of cases, retrograde menstruation (backward flow) with implantation of endometrial tissue is the cause of endometriosis. Endometrial tissue also can form or spread via other, less common mechanisms. Extra-pelvic endometriosis (endometriosis outside of the pelvic cavity) has been reported in virtually every organ of the body.

Most of these rare endometrial implants, and are formed as a result of the spread of endometrial tissue through the blood stream (hematogenous spread).

Endometriosis also has been reported in the lymph nodes (e.g., obturator lymph nodes). Thus, endometriosis can also spread through the lymphatic system.

Endometrial tissue can spread during surgery as well. The most common example of surgical spread is endometriosis that is found in the abdominal wall following a C-section. Endometriosis has even been reported in the belly button. This usually presents with bleeding from the navel with menses.

Finally, another theory states that endometriosis can form as a result of what is called coelomic metaplasia. Coelomic cells are those formed as a fetus, and metaplasia means one cell-type turns into another cell-type. In other words, in adult life some of the primitive cells turn into endometriosis. This probably accounts for the case of endometriosis reported in men. All of the men who had endometriosis were on estrogen therapy (most likely for prostatic cancer), which probably stimulated the transformation of the coelomic cells into endometriosis (Ladies, please do NOT slip your significant other a few birth control pills if he is not understanding of your pain!).

In conclusion, endometriosis is the presence of endometrial tissue outside of the uterus. In the vast majority of cases, the back flow of menstrual tissue through the fallopian tubes with implantation of endometrial tissue is probably the cause of endometriosis.

Much of this can be seen as dry and boring, but is the medical aspect of "What is Endometriosis." I will talk about the reality of what having endometriosis means for women and the impact on their lives in the next couple of posts.

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