In hyponatraemia that has developed over more than 48 hours, or of unknown duration, the brain has adapted, and correcting too fast risks osmotic demyelination. The usual limit is a rise of no more than 8 to 10 mmol/L in the first 24 hours, lower in high-risk patients — those with alcohol use disorder, malnutrition, hypokalaemia or liver disease, or a starting sodium of 120 or less.
The danger often comes after the cause is removed: stopping a thiazide, giving saline to a volume-depleted patient or treating hypocortisolism can trigger a brisk water diuresis. Check sodium every few hours in the early phase, and watch urine output. If the limit is exceeded, re-lowering with dextrose and desmopressin is an accepted rescue under specialist guidance.
- Aim for a sodium rise of no more than 8–10 mmol/L in the first 24 hours in chronic hyponatraemia.
- Use a lower limit in alcohol use disorder, malnutrition, hypokalaemia or a starting sodium of 120 or less.
- Check sodium every 4–6 hours early on, and more often if urine output rises sharply.
- Expect overcorrection after stopping a thiazide or correcting hypovolaemia.
Why it matters
Overcorrection usually happens after the cause is fixed, when attention has moved on.
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