A 58-year-old man with small-cell lung cancer presents with severe symptomatic hyponatremia (serum sodium 118 mEq/L, osmolality 245 mOsm/kg). He is confused and has had one brief seizure. Urine osmolality is 580 mOsm/kg and urine sodium is 95 mEq/L. What is the drug of choice for acute symptomatic hyponatremia in this patient?

## Clinical Context This patient has **acute symptomatic hyponatremia** secondary to SIADH (syndrome of inappropriate antidiuretic hormone secretion), evidenced by: - Low serum sodium with low serum osmolality - Inappropriately concentrated urine (high osmolality) - High urine sodium (euvolemic state) - Neurological symptoms (confusion, seizure) ## Management Principle **Key Point:** Acute symptomatic hyponatremia (serum Na <120 mEq/L with neurological symptoms) is a medical emergency requiring rapid partial correction to stop seizures and prevent cerebral edema. **High-Yield:** The rate of correction differs by acuity: - **Acute hyponatremia** (< 48 hrs): Correct at 10–12 mEq/L in first hour, then 8 mEq/L per 24 hrs - **Chronic hyponatremia** (> 48 hrs): Correct slowly at 8 mEq/L per 24 hrs to avoid osmotic demyelination syndrome (ODS) ## Drug of Choice: Hypertonic Saline (3% NaCl) ### Why 3% Saline? 1. **Raises serum sodium rapidly** — essential in symptomatic acute hyponatremia 2. **Stops seizure activity** — by raising osmolality and reducing cerebral edema 3. **Safe in acute setting** — risk of ODS is negligible when correction is rapid in acute hyponatremia 4. **Administered IV** — 100–150 mL bolus over 20 minutes, then reassess; repeat if seizures persist ### Calculation $$\text{Na deficit} = 0.6 \times \text{body weight (kg)} \times (\text{desired Na} - \text{current Na})$$ For this patient: target Na ~125 mEq/L (10–12 mEq/L rise acutely). **Clinical Pearl:** After acute symptoms resolve, switch to fluid restriction or slower correction to avoid overcorrection and ODS. ## Why Other Agents Are Not First-Line | Agent | Role | Limitation in This Case | |-------|------|-------------------------| | **Desmopressin (DDAVP)** | Used in central DI, not SIADH | Worsens hyponatremia in SIADH; contraindicated here | | **Furosemide + hypotonic saline** | Used in euvolemic/hypervolemic SIADH when symptoms are mild–moderate | Too slow for acute symptomatic; seizure risk remains | | **Lithium carbonate** | Chronic SIADH management (blocks ADH action) | Onset too slow (days); no role in acute symptomatic hyponatremia | **Warning:** Do NOT use desmopressin or hypotonic fluids in acute symptomatic hyponatremia — they will worsen hyponatremia and increase seizure risk.