Hyponatraemia — a serum sodium below 135 mmol/L — is found in a large proportion of hospital inpatients. Most cases are mild, but severe or rapidly developing hyponatraemia can cause cerebral oedema, seizures and death.
Step one: is it dangerous now?
Severe symptoms such as seizures, reduced consciousness or vomiting need immediate treatment, usually with a small bolus of hypertonic (3%) saline, repeated if necessary. The aim is a modest early rise to relieve symptoms.
Step two: find the cause
Key investigations:
- Serum osmolality – to exclude pseudohyponatraemia and hyperglycaemia
- Urine osmolality – is ADH active?
- Urine sodium – helps separate renal from non-renal losses
- Clinical volume status – hypovolaemic, euvolaemic or hypervolaemic
- Thyroid function and morning cortisol
Step three: correct safely
Raising the sodium too quickly risks osmotic demyelination, a devastating neurological complication. Most guidelines advise limiting correction to around 8–10 mmol/L in the first 24 hours, and lower in high-risk patients such as those with alcohol use disorder, malnutrition or hypokalaemia.
“In chronic hyponatraemia, the speed of correction matters as much as the direction.”
Check sodium frequently during treatment and watch for a sudden water diuresis once the cause is removed.
This article is for education only and does not replace clinical judgement or local guidelines.
