A 58-year-old man with small-cell lung cancer presents with confusion and lethargy. Serum sodium is 118 mEq/L, urine osmolality is 650 mOsm/kg, and urine sodium is 95 mEq/L. He is euvolemic on clinical examination. What is the drug of choice for this acute symptomatic hyponatremia?

Explanation

## Clinical Context This patient has **SIADH** (Syndrome of Inappropriate Antidiuretic Hormone secretion) secondary to small-cell lung cancer, presenting with acute symptomatic hyponatremia (Na⁺ = 118 mEq/L with neurological symptoms: confusion, lethargy). **Key Point:** Acute symptomatic hyponatremia is a medical emergency. Symptoms (seizures, altered mental status, cerebral edema) indicate severe hyponatremia requiring rapid but controlled correction. ## Why Hypertonic Saline (3%) is Correct 1. **Symptomatic hyponatremia requires hypertonic saline** — the only osmotically active fluid that raises serum sodium acutely and reduces cerebral edema. 2. **Target correction rate:** 4–6 mEq/L in the first 1–2 hours to stop seizures; then slower correction (8–10 mEq/L per 24 hours total) to avoid osmotic demyelination syndrome. 3. **Mechanism:** 3% NaCl (513 mEq/L) creates a positive osmotic gradient, drawing free water out of neurons and reducing intracranial pressure. 4. **Dosing formula:** $$\text{mEq Na}^+ \text{ needed} = 0.6 \times \text{body weight (kg)} \times (\text{desired Na}^+ - \text{current Na}^+)$$ For this patient: 0.6 × 70 × (125 − 118) ≈ 294 mEq → infuse 3% saline at 50 mL/hr initially, then reassess. **High-Yield:** The **only** drug that works acutely in symptomatic hyponatremia is hypertonic saline. All other agents (desmopressin, tolvaptan, NSAIDs, fluid restriction) are for chronic or asymptomatic cases. ## Comparison with Distractors | Agent | Indication | Why NOT here | |-------|-----------|---------------| | **Hypertonic saline (3%)** | Acute symptomatic hyponatremia (seizures, coma) | ✓ **CORRECT** | | **Desmopressin** | Diabetes insipidus, nocturnal enuresis | Worsens SIADH; contraindicated | | **Tolvaptan** | Chronic SIADH (euvolemic, asymptomatic) | Slow onset (hours); not for acute symptoms | | **Normal saline (0.9%)** | Hypovolemic hyponatremia | Hypotonic relative to urine (650 mOsm/kg); worsens hyponatremia in SIADH | **Clinical Pearl:** Desmopressin is a trap — it is a V2-receptor agonist that increases ADH activity, directly worsening SIADH. Never give desmopressin in SIADH-related hyponatremia. **Warning:** Overcorrection (>10 mEq/L per 24 hours) risks osmotic demyelination syndrome (central pontine myelinolysis), causing permanent neurological damage. Hypertonic saline must be infused slowly with frequent sodium checks (every 2–4 hours initially).