Is "microscopic" peritoneal endometriosis invisible?

Murphy et al. [1] supported the concept of microscopic endometriosis by finding unsuspected foci of disease in 25% of random biopsy specimens of visually normal peritoneum in 20 patients with endometriosis. From these observations came the conclusions that microscopic peritoneal disease is invisible and cannot be destroyed surgically.

This study duplicates that of Murphy et al. [1] , except that light microscopy (LM) rather than scanning electron microscopy (SEM) was used to investigate biopsy specimens. The results refute the existence of invisible, microscopic endometriosis.

Materials and methods

Normal peritoneal biopsy specimens for this study were taken from the posterior pelvic peritoneum of 33 patients undergoing laparoscopic excision of abdominal peritoneum. The posterior pelvic peritoneum was studied because this is the area of the pelvis most commonly involved by disease in all age groups [2], and it was surmised that microscopic foci of invisible endometriosis should be most prevalent on the peritoneum of the cul-de-sac, uterosacral ligaments, or broad ligaments.

A 10mm operating laparoscope (Richard Wolf Medical Instruments Corp., Rosemont, IL) was advanced into the pelvis so that the objective lens was within 2cm of the peritoneal surfaces examined, thus gaining a modestly magnified view of the peritoneum. Peritoneum was judged likely to be devoid of endometriosis by the following criteria: (1) surface perfectly smooth with no surface texture irregularities or specular light reflections; (2) no abnormal vascular patterns such as centrifugal neovascularity or vascular clusters; (3) transparency, with no associated color; (4) no suggestion of subperitoneal cystic structures, and (5) no superficial fibrosis. Peritoneum meeting all of these criteria was eligible for study.

The biopsy technique involved tenting up the study peritoneum with a 3mm Wolf grasping forceps, number 8379.03, and then using 3mm Wolf scissors, number 8380.02, to excise the peritoneum as close to the grasping forceps as possible. This resulted in a specimen approximately 2mm in maximum dimension, which was placed in formalin and processed in the routine fashion for LM. At least eight serial step sections were made of each specimen. Endometriosis was diagnosed by the presence of endometrial glands and stroma.


None of the study specimens was positive for endometriosis. Twenty-four of the patients had endometriosis elsewhere in the pelvis, whereas nine did not. Among the 24 patients with endometriosis elsewhere, the study peritoneum in three each had a small cystic space which was not endometriosis.

Among the nine patients without endometriosis elsewhere in the pelvis, no studied peritoneum had any cystic space. The abnormal peritoneum removed from five of these patients contained glands "consistent with" endometriosis. Stroma was lacking, however.


This report indicates that it is possible by the use of both modest peritoneal magnification and the criteria of normal peritoneum defined in this study to distinguish peritoneum lacking endometriosis with 100% precision. If microscopic disease were truly invisible, it would be impossible to avoid it if enough biopsy specimens were taken of "normal" peritoneum in enough patients. This study population of 33 patients is larger than a previous study [1], so inadequate numbers of patients should not be a factor in the disparate findings.

There seem to be two possible explanations for the difference between the study finding a 25% incidence of endometriosis in visually normal peritoneum [1] and this study finding none: discriminatory threshold of the laboratory or discriminatory threshold of the clinician selecting tissue for the laboratory.

Regarding laboratory observations by LM or SEM, there should be no decided advantage of either. The SEM illustrations of two groups [1, 3] show lesions between 100 and 500μm in dimension, while the LM even at low power can easily resolve an erythrocyte gym in diameter. The step-sectioning of each specimen in this study makes it unlikely that endometriosis was missed by the laboratory in 33 patients.

The difference, therefore, may lie in the discriminatory threshold of the clinicians. Near-contact laparoscopy and defined criteria for normal peritoneum were used in this study but apparently not by Murphy et al. [1]

Visualization of the bottom of the pelvis can be hampered at laparotomy by blood, fluid, poor exposure, inadequate light, or lack of magnification. These are less problematic at laparoscopy. The visual manifestations of endometriosis are protean [4] and can easily blend with normal peritoneum. Much disease will be missed if black powderburn lesions only are sought.

In the diagnosis, treatment, and study of endometriosis, identification of peritoneum lacking disease is as important as identification of disease itself. Errors of identification either way will confound our understanding of the disease.


This study fails to uphold the concept of invisible, microscopic endometriosis. Peritoneum devoid of disease exists, can be described, and can be consistently found.

Acknowledgment: The author wishes to thank Dan Martin, M.D., for his critique of the methodology of this study.


  1. Murphy AA, Green WR, Bobbie D, Dela Cruz ZC, Rock JA: Unsuspected endometriosis documented by scanning electron microscopy in visually normal peritoneum. Fertil Steril 46:522, 1986.
  2. Redwine DB: The distribution of endometriosis in the pelvis by age groups and fertility. Fertil Steril 47:173, 1987.
  3. Vasquez G, Cornille F, Brosens IA: Peritoneal endometriosis: scanning electron microscopy and histology of minimal pelvic endometriotic lesions. Fertil Steril 42:696, 1984.
  4. Stripling MC, Martin DC, Chatman DL, Vander Zwaag R, Poston WM: Subtle appearance of pelvic endometriosis. Fertil Steril 49:427, 1988.