A 55-year-old man with small cell lung carcinoma develops hyponatraemia (Na 116 mEq/L) with confusion. Serum osmolality is 250 mOsm/kg, urine osmolality 420 mOsm/kg, urine sodium 65 mEq/L, and thyroid, adrenal and renal function are normal. After seizure risk is assessed, what is the appropriate initial treatment?

• A Isotonic saline infusion at maintenance rate
• B Fluid restriction of 800 mL/day with close monitoring ✓
• C Immediate 3% hypertonic saline bolus to full correction
• D Furosemide with high-sodium diet

Explanation

Euvolaemic hypoosmolar hyponatraemia with inappropriately concentrated urine defines SIADH, and fluid restriction is the accepted first line therapy for chronic asymptomatic or mildly symptomatic cases. Isotonic saline worsens SIADH because retained water concentrates the administered sodium. Hypertonic saline is reserved for severe neurological symptoms such as seizures or coma, and even then correction must stay under 10 mEq/L in 24 hours to avoid osmotic demyelination syndrome.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.