What is Endometriosis and Why Does It Cause Pain During Menstruation?
Endometriosis is an inflammatory disease where endometrial-like tissue grows outside the uterus, causing pain during menstruation because these misplaced tissue deposits respond to hormonal fluctuations just like normal endometrium—they bleed, swell, and trigger inflammation with each menstrual cycle, but unlike uterine lining, this blood and tissue have no way to exit the body, leading to irritation of surrounding tissues and nerves. 1, 2
Understanding the Disease
Endometriosis affects approximately 10% of women during their reproductive years, translating to 176 million women worldwide and 9 million in the US. 2, 3, 4 The condition is now recognized as a systemic inflammatory disease rather than just a localized pelvic problem. 2, 5
Key Pathophysiologic Features
- Endometrial-like tissue grows on or extends below the peritoneal surface, most commonly on pelvic organs and tissues 1
- These lesions are estrogen-dependent and progesterone-resistant, making them highly responsive to menstrual hormones 2, 6
- The tissue has a proliferative nature and creates chronic inflammation with fibrosis 1
- Deep endometriosis is defined as lesions extending deeper than 5 mm under the peritoneal surface or involving bowel, bladder, ureter, or vagina 1, 2
Why Pain Occurs Specifically During Menstruation
The pain mechanism is directly tied to the hormonal cycle:
- Hormonal responsiveness: The ectopic endometrial tissue responds to the same estrogen and progesterone fluctuations that affect normal uterine lining 2, 6
- Cyclical bleeding: During menstruation, these lesions bleed just like the uterine endometrium, but the blood becomes trapped in the pelvic cavity with no exit route 4, 7
- Inflammatory cascade: The trapped blood and tissue breakdown products trigger intense local inflammation, causing pain that peaks during menses 1, 5
- Nerve sensitization: Chronic inflammation leads to pain sensitization and altered gene expression in the brain, creating both peripheral and central pain mechanisms 5, 6
Important Clinical Caveat
Not all endometriosis pain is limited to menstruation. While 90% of patients report pelvic pain, this includes dysmenorrhea (menstrual pain), nonmenstrual pelvic pain, dyspareunia (painful intercourse), dyschezia (painful bowel movements), and dysuria (painful urination). 1, 2, 3 The systemic inflammatory nature means pain can occur throughout the cycle as the disease progresses. 5, 6
Clinical Presentation Spectrum
- Variable presentation: Ranges from completely asymptomatic to severe symptoms interfering with daily activities 1, 2
- Pelvic pain is the most common symptom, manifesting in multiple forms 1, 2
- Infertility affects approximately 26-50% of patients with endometriosis 2, 3
- Systemic effects: The disease affects metabolism in liver and adipose tissue, causes systemic inflammation, and alters brain gene expression leading to mood disorders 5, 6
Diagnostic Challenges
A critical pitfall is the diagnostic delay averaging 5-12 years after symptom onset, with most women consulting 3 or more clinicians before diagnosis. 3, 7 While definitive diagnosis historically required surgical visualization with histological verification, current guidelines support clinical diagnosis based on symptoms, physical examination, and imaging with transvaginal ultrasound or pelvic MRI. 3, 4 Normal physical examination and imaging do not exclude the diagnosis. 3
Economic and Quality of Life Impact
The annual economic burden is estimated at $69.4 billion, comparable to type 2 diabetes, Crohn's disease, and rheumatoid arthritis. 2, 4 This reflects not just healthcare costs but the substantial impact on quality of life for affected women and their families. 4, 6