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Peritoneal Endometriosis

by Tamer Seckin, MD | Posted on June 10, 2020

by Tamer Seckin, MD Endometriosis Excision Surgeon / Seckin Endometriosis Center (SEC)

Peritoneal Endometriosis

What is peritoneal endometriosis?

The present classification of endometriosis is defined by the American Society of Reproductive Medicine for patients who have fertility issues. Therefore, it does not take into consideration the most important symptom of endometriosis, which is the pain. The current classification system is a morphological classification that focuses strictly on the anatomical changes of specific areas, particularly the fallopian tubes and ovaries. From an endometriosis surgeon’s perspective, however, this type of classification does not focus on the fibrosis and deep involvement of the tissue and organs lying behind the peritoneum, such as the bladder, bowels, diaphragm, appendix, etc. In short, outside of fertility, this classification system does little to describe the patient’s symptoms, which can include constipation, diarrhea, gas, bloating, bladder symptoms, painful sex, back pain, and leg pain. In this sense, endometriosis should be grouped as either peritoneal endometriosis, ovarian endometriomas, and deep endometriosis. This section looks to discuss peritoneal endometriosis, particularly how it should be defined based on the extension of the anatomy and the organs the endometriosis will involve across the abdominal lining.

Endometriosis is a disease that primarily involves the peritoneum, the lining of the abdomen. In other words, a majority of patients, particularly at a young age, who suggest symptoms of endometriosis, will have peritoneal endometriosis no matter what form of involvement or severity of the disease. This can explain the patient’s symptoms. Therefore, because of the anatomical importance of the peritoneum in endometriosis involvement, it is important to have a thorough understanding of what exactly is this vital organ.

The peritoneum and bowel mesentery have recently been accepted and classified as their own organs according to Gray’s Anatomy. This new classification of organs is a revolution as the peritoneum, while often thought of as just the lining of the abdominal cavity, has also been found to have key immune functions. It also is responsible for a constant secretion of fluids to allow for the slippery and autonomic texture and function of the small and large bowels to be mobile. Finally, its drainage of fluid to the hemidiaphragm and thorax area may help to explain the distribution of early endometriosis lesions.

Why is it so unique?

Peritoneal endometriosis is unique in that it can vary from multifocal and multicentric involvement and from early angiogenesis all the way to a full fibrotic nodule, which could be mild or diffuse. There may be one or two endometriosis lesions invading the peritoneum or a multitude of around 50 lesions. Peritoneal endometriosis can thus, be described as the most common form of endometriosis, in which lesions are typically more non-pigmented than pigmented. This can be particularly noted after putting in contrast that allows the lesions to be visualized with the aid of laparoscopic light.

What it stromal endometriosis?

Peritoneal lesions can also be cystic or fibrotic lesions, with peritoneal defects around central lesions, which will stain positive for stromal endometriosis. Stromal endometriosis is a concept unique to peritoneal endometriosis. It is when there are no glands but visible stroma of the endometrium that is often found adjacent to the main pigmented or central lesion. This is called multi-focal endometriosis if it is in the same area but dispersed, and multi-centric if it involves multiple areas, even across the peritoneum. The multicentric and multifocal nature of endometriosis has been defined for bowel involvement, but it is important to note in cases of peritoneal endometriosis.

What is the concept of “laterality” and how does it relate to a patient’s symptoms?

Interestingly, endometriosis lesions on the peritoneum can be mostly found on the pelvic sidewalls with deeper lesions noted on the uterosacral plate on each side (noting more of a left side predominance over right) and cul-de-sac. The patient’s symptoms are thus justified when they talk about “laterality.” This element of laterality is one of the most important caveats to a patient’s symptoms of pain, whether it is dysmenorrhea (painful menstruation), dyspareunia (painful sex), or dyschezia (painful bowel movements), as oftentimes this description can coincide with the anatomical location of the endometriosis lesions themselves. In this sense, it is important for endometriosis specialists to look particularly at psoas muscles on each side as there are nerves that travel above the muscle, which can often accumulate endometriosis lesions and be the site of the associated laterality of pain. This is also true for the hypogastric nerve, which is often focused on during surgery in cases of the deep involvement of endometriosis of the presacral area, lateral to the rectum. 

Where are typical surgical locations in which peritoneal endometriosis can be found?

When endometriosis is excised from the peritoneum, we are highly particular in defining and noting what anatomical area the endometriosis lesion was removed from. We carefully define and note where endometriosis lesions are on the pelvic sidewall, which includes:

  • Psoas muscle
  • Pelvic rim
  • The area under the ovary, which is the superior part of the ovarian fossa or ovarian cliff
  • Periureteral ovarian fossa (where the ovaries typically rest)
  • Where the hypogastric artery and hypogastric nerve dominates, posterior part of the:
  • Uterosacral ligament
  • Uterosacral plate
  • Uteroartery reflection
  • Urethral bridge
  • Posterior bold ligament
  • Anterior and upper rectum
  • Rectosigmoid colon
  • Cul-de-sac
    • Rectovaginal cul-de-sac
    • Rectovaginal septum (underneath peritoneum) and lateral extension above the pudendal nerve
    • Posterior cul-de-sac, which is behind middle rectal artery and pararectal cul-de-sac
    • Anterior cul-de-sac includes the uterovesical peritoneum (where bladder and uterus meet) and the anterior and posterior portion of the bold ligament.
  • Sigmoid colon
  • Appendix and periappendiceal (near the appendix)

What are pigmented vs. non-pigmented endometriosis lesions?

The appearance of peritoneal endometriosis is classified as pigmented endometriosis and non-pigmented endometriosis. Pigmented endometriosis is the classical definition of endometriosis which has been defined for years and many are familiar with. They are red, black, and a sort of “blueberry” color. The non-pigmented lesions also can be of typical appearance, but also can be more subtle with white color and clear contour that can make them highly difficult to identify within the peritoneum.

How are non-pigmented endometriosis lesions identified?

Non-pigmented endometriosis is a highly important form to note as it is only visible through such techniques as aqua blue contrast. This is because the contrast when injected behind the peritoneum eliminates the reflection of overpowered light from the underlying red and white color that comes from the tissue muscle and other surrounding structures.


Peritoneal is highly difficult to diagnose as it is not visible upon imaging examines. Neither ultrasound technology or the more thorough MRI exam, show signs of endometriosis involving the peritoneum. Laparoscopic visualization with excision surgery and an accompanying pathology report is the only way to formally diagnose peritoneal endometriosis. However, upon physical examination, there are ways to assess for suspected peritoneal involvement. A pelvic exam, particularly accompanied by a rectovaginal assessment, can help an experienced gynecologist identify points of tenderness that can point to peritoneal endometriosis.

What is angiogenesis and how is it helpful in identifying early endometriosis?

One key identification point that indicates the development of early endometriosis is angiogenesis. Vascular change and formation of new, or enlarging capillary beds are always part of the inflammatory process. Usually, these changes migrate upward from deeper layers up to the surface of the peritoneum as new blood vessels form. This process of new blood vessel formation is called angiogenesis. Different formations of angiogenesis, including elevation, spiral formation, and budding (also called sprouting) may be identified easily with the Aqua Blue™ inspection technique. Increased vascularity along with new nerve formation might explain the unparalleled pain in cases of early to late endometriosis.

Peritoneal endometriosis and its relationship with the bodies immune system

Nearly all peritoneal endometriosis lesions excised have been found to have inflammation, which means that the body is resisting and fighting off the foreign glands and stroma of the anatomical dislocation of the endometrium tissue. Thus there is evidence towards an immune response attempting to reject these endometriosis lesions. In fact, the most common sign and finding of excised lesions is this inflammation, which a pathologist will describe upon a biopsy report. It is not uncommon to see histiocytes, iron deposits, and infiltration of chronic inflammatory cells within the endometriosis lesions found in the peritoneum. These lesions can also have fibroblastic regeneration abilities as well. In addition, these lesions can often stain positive for stroma and estrogen receptors. Thus, this is an area where stem cells and metaplasia can occur, all of which point toward a classic wound healing mechanism. However, the wounds being the endometriosis lesions, are never healing without surgical intervention. From this histological perspective, endometriosis can be thus thought of as a wound that never heals and flares up with menstruation and estrogen changes.

Therefore, peritoneal endometriosis is where endometriosis is defined as not the traditional stroma gland of the endometrium, but more importantly by the lesions noted inflammation and scarring, which can explain a patient’s symptoms. It is thus important for a surgeon to clean these areas and remove lesions as fully as possible. This will ensure that a pathologist can clearly identify the lesions and define whether there are any abnormalities in the cell's nuclei that could indicate cancerous changes and more importantly, formally diagnose and confirm the patient has endometriosis. 

Our Approach

Why do we feel non-pigmented endometriosis lesions are of particular importance?

These non-pigmented lesions, are not only morphologically cystic in appearance, vesicular, and protruding into the peritoneum through small vegetations (abnormal growths), but they are also microscopically can infiltrate even deeper, affecting key nerves and other structures. This topic is thus of uttermost importance as we often feel that peritoneal endometriosis is the most ignored element in recognizing endometriosis. Many doctors do not recognize the importance of the peritoneum in endometriosis involvement or are not comfortable in addressing the issue due to its proximity to major vital organs and true removal of these lesions would necessitate vast surgical skill and experience on the part of the surgeon. But the main issue lies in recognizing the lesion itself and fully understanding the anatomy of the peritoneum and the surrounding organs.

Patients’ symptoms often correlate with the organs and peritoneal location that these non-pigmented endometriosis lesions are sitting within. If these lesions are sitting on the pelvic sidewall, then it is common for patients to experience back pain and leg pain during their periods, which is pain following the nerve path of the psoas region that innervates the vulva part of the thighs or the sciatic nerve in the back of the thigh. It is in the delicate removal of lesions from the peritoneum where the concept of excision surgery becomes important when treating patients, as many times both the endometriosis and abnormalities within the peritoneum may not reveal classical signs of pigmented endometriosis, even under a microscope. This is why we rely on a combination of excision surgery with aqua blue contrast to treat our endometriosis patients, as we believe each and every patient deserves the most meticulous care that we can provide.

How is our care in treating peritoneal endometriosis so unique?

Our experience in excising more than 20,000 peritoneum specimens, including more lesions of non-pigmented nature, is highly key in ensuring our minimally invasive surgery is as thorough as possible. By utilizing our patented aqua blue contrast, we are able to identify these lesions, excise them, and ship them to a pathologist for a full biopsy report. In addition, following excision, we ensure reconstruction, repair, and restoration of the peritoneum anatomy in order to ensure organ function. This is no easy feat and is only made possible through decades of experience coupled with a team surgical approach, in which we have GI and urology specialists on hand if there is any concern of effecting said organs. Through our precise and meticulous methods, we have found that peritoneal endometriosis is indeed present in nearly patients who are affected by endometriosis. Thus we know and understand the importance of carefully operating in this area to ensure the highest chance for relief of symptoms for our patients.


  1. Endometriosis Foundation of America

  2. Börner C et al,. “Pain Mechanisms in Peritoneal Diseases Might Be Partially Regulated by Estrogen.” Reproductive Sciences SAGE Journals, 017 Jan 1:1933719117715126.

  1. Seckin, T., The Doctor Will See You Now: Recognizing and Treating Endometriosis. 2016.

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