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?

Question

Accepted Answer

C. Hypertonic saline (3% NaCl). ## 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]

Answer

A. Demeclocycline

Answer

B. Desmopressin

Answer

D. Vaptans (tolvaptan)

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

Acute vs Chronic Management

Why Hypertonic Saline in This Case

Dose and Monitoring

Why Other Drugs Are Not First-Line in Acute Setting

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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