A 60-year-old man has fatigue and confusion. Serum sodium is 118 mEq/L, plasma osmolality 252 mOsm/kg, urine osmolality 380 mOsm/kg, urine sodium 65 mEq/L, and he appears clinically euvolaemic. Chest imaging suggests a hilar mass. What is the first-line treatment for the hyponatraemia?
- A Fluid restriction to approximately 800 mL per day ✓
- B Infusion of 3% hypertonic saline at 100 mL per hour until sodium normalises
- C Tolvaptan combined with aggressive normal saline infusion
- D High-dose loop diuretic with free water replacement
Correct answer: A. Fluid restriction to approximately 800 mL per day
Explanation
Euvolaemic hyponatraemia with concentrated urine, high urine sodium, and low serum uric acid in a smoker indicates the syndrome of inappropriate antidiuresis, classically from small cell lung carcinoma. First-line therapy is restriction of free water intake to roughly 800 mL per day. Hypertonic saline is reserved for seizures or severe encephalopathic symptoms, and correction must stay under 8 to 10 mEq/L in 24 hours to avoid osmotic demyelination.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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