A 61-year-old man with small cell lung carcinoma is found to have serum sodium of 121 mEq/L. He has mild lethargy but no seizures. Urine osmolality is 480 mOsm/kg, urine sodium is 65 mEq/L, and he is clinically euvolaemic. The most appropriate initial management is:
- A Fluid restriction to 800 mL/day ✓
- B Infusion of 3% hypertonic saline
- C Infusion of isotonic saline at 150 mL/hour
- D Demeclocycline therapy
Correct answer: A. Fluid restriction to 800 mL/day
Explanation
This is SIADH: euvolaemic hyponatraemia with inappropriately concentrated urine and urinary sodium wasting. In a mildly symptomatic patient, fluid restriction is the established first-line measure and often suffices. Hypertonic 3% saline is reserved for seizures or profound encephalopathy because of the risk of osmotic demyelination with rapid correction. Isotonic saline worsens SIADH since the kidney retains the free water and excretes the sodium. Demeclocycline is a second-line agent limited by nephrotoxicity.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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