A 58-year-old man with small-cell lung cancer presents with severe hyponatremia (Na+ 118 mEq/L), elevated urine osmolality (650 mOsm/kg), and low serum osmolality. He is diagnosed with syndrome of inappropriate antidiuretic hormone (SIADH). What is the drug of choice for acute symptomatic hyponatremia in this patient?

Explanation

## Management of Paraneoplastic SIADH-Induced Hyponatremia ### Clinical Context SIADH is one of the most common paraneoplastic syndromes, occurring in 10–15% of small-cell lung cancer (SCLC) patients. When hyponatremia is severe (Na+ <120 mEq/L) and symptomatic (seizures, altered mental status, coma), acute correction is life-saving. ### Drug of Choice: Hypertonic Saline (3%) + Furosemide **Key Point:** Hypertonic saline (3% NaCl) is the first-line agent for acute symptomatic hyponatremia, especially when serum osmolality is low and urine osmolality is inappropriately high (hallmark of SIADH). **Mechanism:** - 3% saline provides hypertonic fluid that raises serum Na+ and osmolality - Furosemide (loop diuretic) blocks the diluting segment of the distal tubule, preventing free water reabsorption and enhancing urine output - Together, they increase serum [Na+] while removing free water **Clinical Pearl:** The rate of correction is critical—raise Na+ by **8–10 mEq/L in the first hour**, then **10–12 mEq/L in the first 24 hours** to avoid osmotic demyelination syndrome (ODS). Overcorrection (>12 mEq/L/24 hrs) causes central pontine myelinolysis. ### Dosing - 3% saline: 1–2 mL/kg IV bolus over 10–20 minutes, repeated if seizures persist - Furosemide: 40–80 mg IV concurrent with saline infusion - Monitor serum Na+ every 2–4 hours during acute phase ### Long-Term Management of SIADH | Approach | Agent | Indication | |----------|-------|------------| | **First-line (chronic)** | Fluid restriction (0.5–1 L/day) | Mild-to-moderate asymptomatic hyponatremia | | **Second-line** | Vaptans (tolvaptan, conivaptan) | Refractory SIADH, euvolemic hyponatremia | | **Alternative** | Demeclocycline | Chronic SIADH, slower onset (5–7 days) | | **Adjunct** | Hypertonic saline + loop diuretic | Acute symptomatic hyponatremia (this case) | **High-Yield:** In SIADH, desmopressin is **contraindicated**—it worsens hyponatremia by increasing ADH effect. Thiazides also worsen SIADH-induced hyponatremia (they cause hyponatremia as a side effect). ### Why Hypertonic Saline Works in SIADH In SIADH, the kidneys are already maximally concentrating urine (osmolality >600 mOsm/kg). The problem is free water retention due to excess ADH. Hypertonic saline overcomes this by providing a hypertonic load that the kidneys cannot dilute further, forcing Na+ excretion while the loop diuretic prevents free water reabsorption.