Classification of Endometriosis
Endometriosis should be classified using a "classification toolbox" approach that combines the revised American Society for Reproductive Medicine (r-ASRM) system with the Enzian classification for deep disease and the Endometriosis Fertility Index (EFI) when fertility outcomes are relevant. 1
Anatomical Classification by Location
Endometriosis is classified into three main anatomical phenotypes based on location and depth of invasion: 2
Superficial peritoneal endometriosis: Endometrial-like tissue implants on the peritoneal surface of pelvic organs, representing the most common form encountered during laparoscopy 2
Ovarian endometriomas: Cystic lesions within the ovaries, present in approximately 40% of patients with endometriosis 3
Deep infiltrating endometriosis (DIE): Lesions extending deeper than 5 mm under the peritoneal surface or those involving or distorting bowel, bladder, ureter, or vagina 1, 2
Specific Anatomical Distribution
The most common sites of endometriotic lesions in order of frequency are: 4
- Ovaries (67% of patients) 4
- Uterosacral ligaments (46% of patients) 4, 3
- Ovarian fossa (32%) 4
- Pouch of Douglas (30%) 4
- Bladder (21%) 4
- Rectosigmoid intestine (14-15% overall; 45% of surgical patients have intestinal infiltration on histology) 4, 3
Critical pitfall: Left-sided lesions predominate for all locations except ovarian superficial implants and fallopian tube involvement. 4
Disease Severity Classification Systems
The r-ASRM Classification
The r-ASRM system stages endometriosis into four categories based on surgical findings: 5, 6
- Stage I (Minimal): Score 1-5 points
- Stage II (Mild): Score 6-15 points
- Stage III (Moderate): Score 16-40 points
- Stage IV (Severe): Score >40 points
Major limitations of r-ASRM: 1, 2
- Very poor correlation with pain symptoms and quality of life 1, 2
- Poor correlation with fertility outcomes 1
- Does not adequately describe deep endometriosis 1, 2
- Poor predictive accuracy for treatment outcomes 1
Why r-ASRM persists despite limitations: Its longevity, universal familiarity, widespread use in literature, and incorporation into other classification systems justify its continued use as a baseline descriptive tool. 1
The Enzian Classification
When using r-ASRM, you must employ the Enzian classification concurrently when deep endometriosis is present to provide complete operative description. 1, 2
The Enzian system specifically addresses retroperitoneal structures and deep infiltrating disease that r-ASRM inadequately captures. 1, 6
Limitation: Enzian has poor correlation with symptoms and infertility, and limited prognostic value. 1
The Endometriosis Fertility Index (EFI)
The EFI is a validated tool specifically for predicting fertility outcomes following surgical staging. 2, 6
Key advantage: The EFI likely works because it includes important clinical variables that affect pregnancy likelihood independent of endometriosis presence. 1
Multifocal Disease Pattern
Endometriosis typically presents as multifocal disease rather than isolated lesions. 3
- The mean number of DIE lesions per patient is 4 3
- Multifocal disease is observed in the majority of patients 3
- When intestinal DIE is present, the mean number of intestinal lesions per patient is 1.3 3
Critical Clinical Caveats
Do not assume symptom severity correlates with anatomical extent of disease—the r-ASRM staging has very poor correlation with pain and quality of life. 1, 2 Small but evolutive lesions may have larger clinical impact than large fibrous stable lesions. 7
Do not overlook deep infiltrating disease when using standard classification systems—supplement r-ASRM with Enzian classification when DIE is suspected. 1, 2
Comprehensive Disease Definition
Beyond anatomical classification, endometriosis should be understood as: 1
- An inflammatory disease process characterized by endometrial-like tissue outside the uterus associated with pelvic pain and/or infertility 1, 2
- A systemic disease with multifactorial etiology including genetic factors with possible epigenetic influences 1, 2
- A hormone-responsive condition with estrogen-dependence and progesterone resistance 1, 2
- A proliferative disease with chronic inflammatory characteristics 1, 2