The pelvic peritoneum is the thin lining of the pelvis which looks like plastic wrap. Endometriosis is most commonly found on the surface of the peritoneum. It is located in predictable patterns, due to the fact that it is laid down embryologically. Some female embryos are destined to have more patterned areas than others, the result of genetics, environment, and chance. The most common areas of involvement, in approximate order, are: 1. cul-de-sac; 2. Left uterosacral ligament; 3. Right uterosacral ligament; 4. Left broad ligament; 5. Right broad ligament. Endometriosis may remain superficial within the peritoneum, or it may be associated with deeper invasion in a particular area. The areas of the pelvis which are most likely to have invasion are (in declining order) the uterosacral ligaments, rectosigmoid colon, and bladder. Invasive endometriosis under the microscope looks like small islands of endometriosis surrounded by fibromuscular metaplasia. Metaplasia is when one type of tissue transforms into another type of tissue and is a reflection of just how plastic the human body is. Fibromuscular simply means that the metaplastic tissue surrounding invasive endometriosis has fibrous tissue as well as crude muscle-like fibers. Endometriosis can hurt whether it is superficial or invasive.
Most women with endometriosis do not have ovarian involvement, but when the ovaries are involved, it means that there is an increased chance of having bowel involvement, as well as an increased chance of having more pelvic areas of involvement, as well as an increased chance of invasive disease. So ovarian endometriosis virtually always means there is more (usually much more) endometriosis than just ovarian disease. However, many women with ovarian disease presenting with endometrioma cysts of the ovaries have had ultrasounds which can easily find these cysts. The physician's focus is frequently just on the cysts without realization that there is always more disease present than just the cysts. This means that if surgery occurs, only the cysts are treated, sometimes only by puncturing and draining them (which has been shown to fail in most cases - the cysts just fill up with bloody fluid again). Ovarian endometriomas are frequently surrounded by scar tissue which glues the ovaries to each other, the ovaries to the pelvic sidewalls, the rectum to the uterus, leading to a real mess.
And everywhere there are adhesions binding things together, there can be invasive disease. Invasive disease of the uterosacral ligaments can cause scar tissue which surrounds the adjacent ureters, pulling them into the mess. Rarely the ureters are invaded by endometriosis, which may require removal of a segment of ureter. When the rectum is stuck against the back side of the cervix and uterus, this is what is called obliteration of the cul-de-sac and carries a 70% chance that the rectal wall is invaded to some degree by endometriosis, which will require some type of bowel surgery to deal with. When the bladder is invaded, surgery will be required to remove the portion of bladder involved by the disease. Fortunately, this does not cause problems with bladder function.
Endometriosis can also invade the diaphragm, which is a thin muscle separating the abdominal cavity from the chest cavity. Because the diaphragm is so thin, symptomatic endometriosis is always full thickness, requiring removal of the areas of the diaphragm involved by the disease.
So when you stand back and look at it, endometriosis is THE most aggressive benign disease affecting humans. It can invade the bowel, ureter, bladder, diaphragm, pelvic nerves, and so on. Some unlucky patients have invasive disease in all these locations. Invasive endometriosis involving multiple organ systems is far more common than cancer involving multiple organ systems. If a cancer surgeon were to find this degree of cancer in a patient, they would simply close the patient up and recommend that their final affairs be put in order. This is why cancer surgeons are not necessarily good endometriosis surgeons - they very rarely operate on disease as severe as endometriosis can be. But an endometriosis surgeon can't stop simply because a patient may have severe disease everywhere.
A surgeon who is going to treat endometriosis completely must be prepared to operate across multiple organ systems, operate on extremely challenging disease presentations in those organ systems, do it with as little collateral damage as possible, and do it while enduring criticism from those physicians who do not understand endometriosis or pelvic pain's effect on women. This is a huge challenge that very few surgeons are willing to take on or should try to take on. Yet meeting this challenge is what an endometriosis surgeon must do. Few are called and even fewer are chosen.