A patient admitted with heat stroke has serum sodium of 162 mEq/L and is alert. At what maximum rate should the sodium be lowered to avoid iatrogenic cerebral injury?
- A Not more than 4 mEq/L in 24 hours
- B Not more than 18 mEq/L in 24 hours
- C Not more than 10 mEq/L in 24 hours ✓
- D Correction rate does not matter in hypernatraemia
Correct answer: C. Not more than 10 mEq/L in 24 hours
Explanation
In chronic hypernatraemia brain cells generate idiogenic osmoles over one to two days to defend cell volume. Correcting faster than roughly 0.5 mEq/L per hour, or 10 mEq/L in 24 hours, makes plasma hypo-osmolar relative to brain tissue, driving water into neurones and causing seizures or cerebral oedema. The 4 mEq/L limit applies to chronic hyponatraemia, not its opposite. Asymptomatic hypernatraemia should be corrected gradually with free water deficit replaced over 48 to 72 hours.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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