A 55-year-old man has serum sodium of 118 mEq/L, low plasma osmolality, urine osmolality 520 mOsm/kg, urine sodium 68 mEq/L. He has no oedema, euvolaemic. Serum urate is low. He is a known small cell lung cancer patient. Which is the correct management?
- A 3% hypertonic saline infusion rapidly to correct sodium by 12 mEq/L/day
- B Fluid restriction as the cornerstone; tolvaptan for refractory SIADH ✓
- C 0.9% normal saline infusion at 1 mL/kg/hour
- D Furosemide alone
Explanation
This is SIADH (syndrome of inappropriate antidiuretic hormone secretion) secondary to small cell lung cancer — characterised by euvolaemic hyponatraemia, low plasma osmolality, inappropriately high urine osmolality (>100 mOsm/kg), elevated urine sodium (>40 mEq/L), and low serum urate. First-line management is fluid restriction (500–1000 mL/day). Tolvaptan (vasopressin V2-receptor antagonist) is used for persistent/severe SIADH refractory to fluid restriction. Sodium correction should not exceed 10–12 mEq/L/day to avoid osmotic demyelination syndrome. Isotonic saline worsens SIADH.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.