## Paraneoplastic Syndromes in Small-Cell Lung Cancer ### Common SCLC-Associated Paraneoplastic Syndromes | Syndrome | Antibody/Mechanism | Frequency | Clinical Features | |----------|-------------------|-----------|------------------| | LEMS | Anti-VGCC (P/Q-type) | 3–5% of SCLC | Proximal weakness, autonomic dysfunction, areflexia | | SIADH | Ectopic ADH secretion | 10–15% of SCLC | Hyponatremia, confusion, seizures | | PCD | Anti-Yo, anti-Hu | 1–3% of SCLC | Cerebellar ataxia, dysarthria, nystagmus | | Encephalomyelitis | Anti-Hu, anti-CRMP5 | 1–2% of SCLC | Cognitive decline, seizures, sensory neuropathy | ### Why Acute Coronary Syndrome is NOT a Paraneoplastic Syndrome **Key Point:** Acute coronary syndrome (ACS) is a vascular event caused by atherosclerotic plaque rupture or thrombosis — it is NOT mediated by tumor-secreted substances, autoantibodies, or immune cross-reactivity with tumor antigens. **High-Yield:** Paraneoplastic syndromes are defined as clinical manifestations caused by: 1. Ectopic hormone or cytokine secretion (e.g., SIADH, ACTH) 2. Immune-mediated cross-reactivity (e.g., anti-Yo, anti-Hu antibodies) 3. Tumor metabolite effects (e.g., hypercalcemia from PTHrP) ACS may occur *coincidentally* in cancer patients due to shared risk factors (smoking, age) or chemotherapy cardiotoxicity, but it is NOT a paraneoplastic phenomenon. **Clinical Pearl:** LEMS is the most common paraneoplastic syndrome in SCLC (found in ~85% of LEMS cases), whereas SIADH is the most frequent endocrine paraneoplastic syndrome in SCLC overall. ### Mnemonic: SCLC Paraneoplastic Syndromes **LASH** — LEMS, Antibodies (anti-Hu, anti-Yo), SIADH, Hypercoagulability (thrombosis, not ACS)
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