How do uterine leiomyomas cause abnormal uterine bleeding in a 30‑50‑year‑old woman?

Mechanisms of Abnormal Uterine Bleeding in Leiomyomas

Uterine leiomyomas cause abnormal bleeding primarily through distortion of the endometrial surface area and vasculature, particularly when submucous or intramural fibroids encroach into the uterine cavity, leading to menometrorrhagia that can range from mild to massive hemorrhage. 1

Primary Mechanisms of Bleeding

Anatomic Distortion and Surface Area Expansion

• Submucous and intracavitary leiomyomas directly increase the endometrial surface area available for bleeding, creating a larger bleeding surface during menstruation 2
• Intramural myomas that encroach into the uterine cavity similarly expand the endometrial surface and disrupt normal endometrial architecture 2
• The spherical nodular growth pattern of leiomyomas distorts the normal myometrial architecture, affecting the endometrial-myometrial interface 1

Vascular Alterations

• Leiomyomas are well-vascularized tumors that alter normal uterine blood flow patterns, with variable vascular patterns around individual tumors 1
• The increased vascularity and disrupted vascular architecture impair normal hemostatic mechanisms in the overlying endometrium 1
• Growth factors including platelet-derived growth factor, heparin-binding epidermal growth factor, and basic fibroblast growth factor contribute to abnormal angiogenesis 1

Molecular and Cellular Mechanisms

• Dysregulation of growth factors, particularly TGF-β3 (elevated 5-fold in leiomyomas), promotes smooth muscle cell proliferation and extracellular matrix production that disrupts normal endometrial function 1
• Increased collagen deposition and fibrosis within leiomyomas alter the structural integrity of the endometrial-myometrial interface 1
• Sex steroid responsiveness (estrogen and progesterone) drives clonal expansion of myometrial cells, with hormonal fluctuations during the menstrual cycle exacerbating bleeding 1

Location-Specific Bleeding Patterns

High-Risk Locations

• Submucous leiomyomas cause the most severe bleeding due to direct endometrial surface involvement 1, 2
• Intracavitary fibroids produce menometrorrhagia through maximal endometrial distortion 1
• Intramural myomas encroaching into the cavity create intermediate bleeding severity 2

Lower-Risk Locations

• Subserosal leiomyomas rarely cause abnormal bleeding unless they achieve very large size 1
• Pedunculated fibroids typically do not cause menorrhagia 3

Clinical Implications

Size-Symptom Discordance

• Small leiomyomas may cause massive uterine bleeding while large tumors can remain asymptomatic, depending entirely on location relative to the endometrial cavity 1
• The number and location of fibroids matter more than absolute size for bleeding symptoms 1

Diagnostic Considerations

• Transvaginal ultrasound with Doppler has 90-99% sensitivity for detecting fibroids and can assess their relationship to the endometrial cavity 3, 4
• Saline infusion sonohysterography achieves 96-100% sensitivity and can distinguish submucous from intramural fibroids with 97% accuracy 1, 4
• The PALM-COEIN classification system categorizes leiomyomas as a structural cause of abnormal uterine bleeding 4

Common Pitfalls

• Do not assume all bleeding in women with fibroids is fibroid-related—always exclude endometrial hyperplasia, malignancy, and other causes through appropriate sampling 1, 4
• Recognize that approximately 70% of women have identifiable leiomyomas at menopause, but most remain asymptomatic 1
• Endometrial biopsy may miss focal lesions; hysteroscopy provides superior visualization when initial sampling is non-diagnostic 1