Endometriosis

Endometriosis is a chronic, estrogen-dependent inflammatory condition in which tissue resembling the lining of the uterus (endometrium) grows outside the uterine cavity, most often on the pelvic peritoneum, ovaries, and structures within the pelvis. It is a common cause of pelvic pain, painful menstruation, and subfertility in women of reproductive age.

Definition

Endometriosis is the presence of endometrium-like epithelium and stroma outside the uterus, accompanied by a chronic inflammatory response, typically affecting pelvic sites and behaving as an estrogen-dependent disorder.

Scope

This entry covers endometriosis as a benign clinical entity: its definition, leading hypotheses about how ectopic endometrial-like tissue arises and persists, its inflammatory and estrogen-dependent character, and its principal presentations. It is a reference overview and does not provide treatment recommendations.

Core questions

• How does endometrium-like tissue come to grow outside the uterine cavity?
• Why is endometriosis described as an estrogen-dependent, chronic inflammatory disease?
• How do the main disease forms — superficial peritoneal lesions, ovarian endometriomas, and deep infiltrating endometriosis — relate to symptoms such as pain and subfertility?

Key concepts

• Ectopic endometrium-like tissue
• Chronic pelvic inflammation
• Estrogen dependence
• Superficial peritoneal, ovarian endometrioma, and deep infiltrating subtypes
• Dysmenorrhea, dyspareunia, and chronic pelvic pain
• Association with subfertility

Key theories

Retrograde menstruation (Sampson's theory)

Sampson proposed that viable endometrial fragments flow backward through the fallopian tubes during menstruation and implant on pelvic surfaces, seeding ectopic lesions. It remains the most widely cited hypothesis for pelvic endometriosis, though it does not by itself explain why only some women develop the disease.

Mechanisms

The most widely cited mechanism is retrograde menstruation, in which endometrial tissue passes back through the fallopian tubes and implants on pelvic surfaces; however, because retrograde menstruation is common while endometriosis is not, additional factors are invoked. These include a permissive local immune and inflammatory environment, locally increased estrogen production within lesions, and progesterone resistance, which together favor the survival, growth, and inflammatory activity of ectopic tissue. The resulting chronic inflammation, fibrosis, and adhesions are linked to pain, while distortion of pelvic anatomy and altered ovarian and tubal function contribute to subfertility.

Clinical relevance

Endometriosis is a major cause of chronic pelvic pain, dysmenorrhea, dyspareunia, and subfertility, and it can substantially affect quality of life. It is relevant across gynecology and reproductive medicine, and definitive diagnosis has historically required surgical visualization. This entry describes the condition for reference purposes and is not a basis for individual diagnostic or treatment decisions.

Epidemiology

Endometriosis is estimated to affect roughly 10% of women of reproductive age, with higher proportions among those presenting with chronic pelvic pain or subfertility. Estimates vary with the population and the diagnostic approach, and diagnosis is frequently delayed because symptoms overlap with other conditions.

History

Endometriosis was characterized in the early twentieth century, and John Sampson's 1927 description of peritoneal endometriosis and the retrograde-menstruation hypothesis became a foundational reference for the field. Subsequent work reframed the disease as an estrogen-dependent, chronic inflammatory disorder with molecular features such as local estrogen production and progesterone resistance, as synthesized in later reviews.

Debates

Why do only some women with retrograde menstruation develop endometriosis?

Retrograde menstruation is common in menstruating women, yet endometriosis is not universal, so the theory alone is insufficient; immune, inflammatory, hormonal, and genetic factors are proposed to explain why ectopic tissue establishes and persists in some individuals.

Key figures

• John A. Sampson
• Serdar E. Bulun
• Krina T. Zondervan

Seminal works

• sampson-1927
• bulun-2009
• zondervan-2020

Frequently asked questions

Is endometriosis a form of cancer?

No. Endometriosis is a benign, chronic inflammatory condition. It involves endometrium-like tissue growing outside the uterus rather than malignant tissue, although it can cause significant pain and subfertility.

What is retrograde menstruation?

It is the backward flow of menstrual tissue through the fallopian tubes into the pelvis. Sampson's theory proposes that this tissue can implant and grow, and it remains the most cited explanation for pelvic endometriosis, though it does not fully explain why only some women are affected.