Polycystic ovary syndrome (PCOS) causes infertility primarily through which mechanism?

Explanation

## PCOS and Anovulatory Infertility: Pathophysiology **Key Point:** PCOS causes infertility primarily through **anovulation**, resulting from abnormal gonadotropin-releasing hormone (GnRH) pulsatility and elevated luteinizing hormone (LH) relative to follicle-stimulating hormone (FSH), which arrests follicular development at the antral stage. ### Pathophysiologic Cascade in PCOS ```mermaid flowchart TD A[Insulin resistance / Hyperinsulinemia]:::outcome --> B[Increased LH secretion]:::outcome A --> C[Increased androgen production]:::outcome B --> D[Abnormal GnRH pulsatility]:::outcome D --> E[Elevated LH:FSH ratio]:::outcome C --> F[Follicular arrest at antral stage]:::outcome E --> F F --> G[Anovulation]:::urgent G --> H[Infertility]:::urgent ``` ### Key Hormonal Abnormalities in PCOS | Parameter | Finding | Consequence | |-----------|---------|-------------| | LH:FSH ratio | Elevated (typically >3:1) | Preferential stimulation of theca cells → androgen excess | | Androgen levels | ↑ Free testosterone, ↑ androstenedione | Follicular atresia, anovulation | | Insulin | ↑ Fasting insulin (hyperinsulinemia) | Drives ovarian androgen synthesis | | Progesterone | Low or absent | No luteal phase (anovulation) | **High-Yield:** PCOS is the **most common cause of anovulatory infertility**, accounting for 70–80% of anovulatory infertility cases. **Clinical Pearl:** Ovarian ultrasound in PCOS shows **≥12 follicles of 2–9 mm diameter** in at least one ovary (Rotterdam criteria). The ovaries are typically enlarged but **structurally normal** — no fibroids, no tubal disease. **Mnemonic:** **HAIR-AN** — **H**yperinsulism, **A**canthosis nigricans, **I**nfertility, **R**esistance to insulin, **A**canthosis nigricans, **N**igricans (features of severe PCOS with metabolic syndrome). ### Why Anovulation Occurs 1. Elevated LH stimulates theca cells → excess androgens 2. Excess androgens inhibit aromatase in granulosa cells → reduced estrogen 3. Reduced FSH (relative to LH) → inadequate granulosa cell proliferation 4. Follicle arrests at antral stage → never reaches ovulation 5. No corpus luteum → no progesterone → anovulation