A 58-year-old man with small-cell lung cancer presents with severe hyponatremia (Na+ 118 mEq/L), elevated urine osmolality (450 mOsm/kg), and low serum osmolality. He is euvolemic on clinical examination. What is the drug of choice for acute symptomatic hyponatremia in this patient with SIADH?

Explanation

## SIADH in Paraneoplastic Syndrome **Key Point:** Acute symptomatic hyponatremia (Na+ <120 mEq/L with neurological signs: seizures, altered mental status, coma) is a medical emergency requiring rapid partial correction. ### Acute vs Chronic Management | Feature | Acute Symptomatic | Chronic Asymptomatic | |---------|-------------------|----------------------| | **Sodium level** | <120 mEq/L | 120–130 mEq/L | | **Symptoms** | Seizures, coma, altered sensorium | Asymptomatic or mild | | **First-line drug** | **3% hypertonic saline** | Fluid restriction, demeclocycline, vaptans | | **Rate of correction** | 4–6 mEq/L in first 1–2 hours | <10 mEq/L per 24 hours | | **Goal** | Stop seizures, prevent cerebral edema | Avoid osmotic demyelination | **High-Yield:** In acute symptomatic hyponatremia, hypertonic saline is the **only** drug that rapidly raises serum sodium and prevents seizure-related mortality. The mechanism is osmotic gradient creation, drawing water out of the brain. ### Why Hypertonic Saline in This Case This patient has: - Severe hyponatremia (118 mEq/L) — below seizure threshold - SIADH (elevated urine osmolality despite low serum osmolality) — paraneoplastic from small-cell lung cancer - Euvolemia — confirms SIADH (not hypovolemia) - **Acute presentation** — requires immediate sodium correction **Clinical Pearl:** SIADH is the most common paraneoplastic endocrine syndrome in small-cell lung cancer (10–15% of cases). Ectopic ADH secretion by tumor cells drives water reabsorption despite low serum osmolality. ### Dose and Monitoring 1. **3% NaCl bolus:** 100–150 mL IV over 10–20 minutes 2. **Repeat** if seizures persist; aim for Na+ rise of 4–6 mEq/L 3. **Monitor:** Serum sodium every 2–4 hours during acute phase 4. **Stop** once seizures resolve or Na+ reaches 120–125 mEq/L **Warning:** Overcorrection (>10 mEq/L per 24 hours) risks osmotic demyelination syndrome (ODS) — irreversible neurological damage. After acute stabilization, switch to fluid restriction or demeclocycline for chronic management. ### Why Other Drugs Are Not First-Line in Acute Setting - **Demeclocycline:** Slow onset (3–5 days); used for chronic SIADH - **Vaptans:** Slower than hypertonic saline; used for chronic symptomatic hyponatremia - **Desmopressin:** Worsens hyponatremia; contraindicated in SIADH [cite:Harrison 21e Ch 297]