A 58-year-old smoker has weight loss and hyponatraemia: serum sodium 118 mEq/L, plasma osmolality 252 mOsm/kg, urine osmolality 420 mOsm/kg, urine sodium 55 mEq/L. He is clinically euvolaemic and on no diuretics. Chest imaging shows a hilar mass. What is the first-line management of the electrolyte abnormality?
- A Isotonic saline infusion at maintenance rate
- B Oral tolvaptan 15 mg daily
- C Fluid restriction to 800 mL/day ✓
- D Demeclocycline 300 mg twice daily
Correct answer: C. Fluid restriction to 800 mL/day
Explanation
The picture of euvolaemic hyponatraemia with concentrated urine and high urine sodium in a patient with a lung mass indicates paraneoplastic SIADH, classically from small cell carcinoma. Mild to moderate asymptomatic SIADH is managed first with free water restriction, which reverses the positive water balance driving the hyponatraemia. Isotonic saline can worsen it because the retained water exceeds excreted solute. Vaptans are second line when restriction fails.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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