What is the difference between fibroids (uterine leiomyomas) and endometriosis in women of reproductive age?

Fibroids and Endometriosis: Key Differences

Fibroids (uterine leiomyomas) are benign smooth muscle tumors that grow within the uterine wall itself, while endometriosis is a condition where endometrial-like tissue grows outside the uterus in abnormal locations throughout the pelvis. These are fundamentally different diseases with distinct origins, locations, and mechanisms, though they can coexist in the same patient 1.

Fundamental Pathophysiology

Origin and Tissue Type:

• Fibroids are monoclonal tumors arising from the myometrium (uterine muscle layer), composed of smooth muscle cells and excessive extracellular matrix 2
• Endometriosis consists of endometrial-like glandular and stromal tissue that implants outside the uterus, commonly on the ovaries, peritoneum, and pelvic structures 3

Location:

• Fibroids remain within or attached to the uterus, classified as submucosal (protruding into the uterine cavity), intramural (within the uterine wall), or subserosal (on the outer uterine surface) 4
• Endometriosis lesions develop outside the uterus entirely, with no connection to the uterine wall 3

Clinical Presentation Differences

Symptoms:

• Fibroids primarily cause heavy menstrual bleeding (menorrhagia), bulk symptoms from uterine enlargement (pelvic pressure, urinary frequency), and may be completely asymptomatic in many women 5
• Endometriosis characteristically causes chronic pelvic pain, severe dysmenorrhea (painful periods), dyspareunia (painful intercourse), and dyschezia (painful bowel movements) 3

Prevalence:

• Fibroids affect approximately 75% of women of reproductive age, though only 30% experience symptoms 5
• Endometriosis affects at least 10% of reproductive-aged women and is present in at least one-third of women with infertility 3

Impact on Fertility

Mechanism of Infertility:

• Fibroids affect fertility primarily through mechanical distortion of the uterine cavity (especially submucosal types), altered endometrial receptivity, and changes in uterine contractility—with submucosal fibroids showing pregnancy rates of only 10% and implantation rates of 4.3% in assisted reproduction 6
• Endometriosis causes infertility through mechanisms that remain unclear but likely involve inflammatory changes, altered peritoneal environment, and adhesion formation 3

Fertility Outcomes:

• Subserosal fibroids do not significantly impact fertility (pregnancy rates 34.1%), while intramural fibroids reduce pregnancy rates to 16.4% and submucosal fibroids to 10% 6
• The specific impact of endometriosis on fertility varies by stage and location, but the association is well-established even when the mechanism is not fully understood 3

Diagnostic Approaches

Imaging:

• Fibroids are readily diagnosed with transvaginal ultrasound (90-99% sensitivity), which shows well-defined masses within or on the uterus 4
• Endometriosis has limited imaging detection—small implants are not well visualized, and laparoscopy remains the gold standard for diagnosis and staging 3

Definitive Diagnosis:

• Fibroids can be definitively diagnosed by imaging characteristics alone in most cases 4
• Endometriosis requires surgical visualization (laparoscopy) for definitive diagnosis and staging 3

Treatment Implications

Surgical Options:

• Fibroids can be removed while preserving the uterus through myomectomy (hysteroscopic, laparoscopic, or open approach depending on location and size) 3
• Endometriosis treatment involves laparoscopic excision or ablation of lesions, which requires identifying scattered implants throughout the pelvis 3

Medical Management:

• Both conditions may respond to hormonal therapies (GnRH agonists, progestins), though mechanisms differ—fibroids shrink with estrogen suppression, while endometriosis lesions become less active 7
• Selective progesterone receptor modulators (like ulipristal acetate) specifically target fibroid growth 8

Comorbidity Considerations

Coexistence:

• Considerable comorbidity exists between fibroids and endometriosis, requiring careful evaluation to identify both conditions when present 1
• The presence of one condition does not exclude the other, and symptoms can overlap, particularly abnormal bleeding and pelvic pain 1

Diagnostic Pitfall:

• When evaluating abnormal uterine bleeding or infertility, clinicians must consider both conditions may be present simultaneously and require different diagnostic and therapeutic approaches 1