The 9-month-old with a seizure and a sodium of 119
A 9-month-old boy, previously well and exclusively breastfed until 6 months then weaned, is brought after a generalised tonic-clonic seizure lasting 2 minutes that stopped spontaneously. He has had 3 days of vomiting and watery diarrhoea and has been refusing feeds. On examination he is drowsy but rousable, weight 8.0 kg (10% below his weight 2 weeks ago), heart rate 150/min, capillary refill 3 seconds, mucous membranes dry, and no oedema. Serum sodium is 119 mmol/L, potassium 3.4 mmol/L, chloride 92 mmol/L, bicarbonate 16 mmol/L, urea 9.8 mmol/L, creatinine 38 µmol/L, glucose 4.6 mmol/L. Urine sodium is 12 mmol/L and urine osmolality is 620 mOsm/kg.
1. Classify the hyponatraemia and explain why the urine findings matter.
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This is hypovolaemic (depletional) hyponatraemia with a raised anion gap metabolic acidosis from gastrointestinal losses.
- The history of vomiting and diarrhoea with 10% weight loss and delayed capillary refill points to extracellular fluid depletion. Sodium loss has exceeded water loss, and the child has also been taking hypotonic fluids (breast milk/water) with impaired free-water excretion because of volume-mediated ADH release.
- Urine sodium of 12 mmol/L (appropriately low) and urine osmolality of 620 mOsm/kg (appropriately concentrated) show that the kidney is conserving sodium and water in response to hypovolaemia. This is the expected renal response and confirms the losses are extrarenal.
- This contrasts with SIADH (urine sodium typically >30–40 mmol/L with euvolaemia) or salt-wasting renal disease. It also makes cerebral salt wasting unlikely in this context.
Pseudohyponatraemia (hyperlipidaemia/hyperproteinaemia) and hyperglycaemia are excluded by the normal glucose and the congruent clinical picture.
2. Outline your fluid and electrolyte plan, including the rate of sodium correction and the reason for that limit.
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Resuscitation first: he is shocked (tachycardia, delayed refill, 10% deficit). Give 10–20 mL/kg of 0.9% sodium chloride as a bolus and reassess; repeat if still shocked. Do not use hypotonic fluid for resuscitation.
Deficit and maintenance:
- Deficit ≈ 10% × 8 kg = 800 mL.
- Replace the deficit over 48 hours (not 24) because of chronic hyponatraemia, plus maintenance.
- Use an isotonic fluid (0.9% saline with added potassium) for the deficit; avoid 0.18% saline with 4% dextrose, which is a well-recognised cause of hospital-acquired hyponatraemic encephalopathy in children.
- Add potassium (20–40 mmol/L) once urine output is established; his K is 3.4.
Rate of sodium correction: aim for a rise of no more than 8–10 mmol/L in 24 hours. Rapid correction of chronic hyponatraemia risks osmotic demyelination; the brain has adapted by extruding osmolytes, and a sudden rise shrinks cells.
Monitoring: check sodium every 4–6 hours initially. If the sodium rises too quickly, slow the rate and consider fluid with a lower sodium content. If he develops signs of cerebral oedema (worsening drowsiness, seizures, bradycardia, hypertension), that is a medical emergency requiring hypertonic (3%) saline - but this is treatment of symptomatic hyponatraemia, not a licence for rapid routine correction.
3. The seizure recurs 6 hours later and the sodium is now 121 mmol/L. How does this change your approach?
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A recurrent seizure with a sodium that has risen from 119 to 121 (only 2 mmol/L in 6 hours) suggests the seizure is not simply due to the rate of correction - the rise is within safe limits. Consider:
- Ongoing hyponatraemia-related cerebral irritation - 121 mmol/L is still low, and symptomatic hyponatraemia with seizures warrants hypertonic 3% saline given as a small bolus (typically 2–3 mL/kg) to raise the sodium by 4–6 mmol/L acutely, which relieves cerebral oedema. This is the exception where a rapid initial rise is appropriate - but the total 24-hour rise should still not exceed ~8–10 mmol/L.
- Alternative causes of the seizure - hypoglycaemia (recheck glucose), hypocalcaemia/hypomagnesaemia, intracranial infection, or an underlying epilepsy. Examine for meningism and check calcium, magnesium and glucose.
- Reassess volume status - if he has become euvolaemic or overloaded, ADH may still be high and the sodium will not rise; ensure the fluid prescription matches the current deficit.
Give anticonvulsant therapy for the seizure itself (benzodiazepine per protocol), protect the airway, and involve senior/ICU support. Document the sodium trend carefully, and reconsider whether the deficit has been over- or under-estimated.