A 58-year-old man with a 40-pack-year smoking history presents with progressive muscle weakness, autonomic dysfunction, and dry mouth. Investigations reveal small-cell lung cancer (SCLC). Which of the following is NOT a recognized paraneoplastic syndrome associated with SCLC?

Question

Accepted Answer

A. Acute coronary syndrome as a direct paraneoplastic manifestation. ## 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)

Answer

B. Syndrome of inappropriate antidiuretic hormone (SIADH) secretion

Answer

C. Lambert-Eaton myasthenic syndrome (LEMS) due to anti-voltage-gated calcium channel antibodies

Answer

D. Paraneoplastic cerebellar degeneration due to anti-Yo antibodies

A 58-year-old man with a 40-pack-year smoking history presents with progressive muscle weakness, autonomic dysfunction, and dry mouth. Investigations reveal small-cell lung cancer (SCLC). Which of the following is NOT a recognized paraneoplastic syndrome associated with SCLC?

Explanation

Paraneoplastic Syndromes in Small-Cell Lung Cancer

Common SCLC-Associated Paraneoplastic Syndromes

Why Acute Coronary Syndrome is NOT a Paraneoplastic Syndrome

Mnemonic: SCLC Paraneoplastic Syndromes

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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