Hyponatraemia and hypernatraemia — formative SAQs

SAQ 1 (10)

A 3-year-old boy is admitted with pneumonia. On day two of his admission he has a generalised seizure. He has been receiving 0.45 percent saline with dextrose as maintenance fluid. His serum sodium is 116 mmol/L, serum osmolality 258 mOsm per kg, urine osmolality 340 mOsm per kg, and urine sodium 52 mmol/L. He is post-ictal but arousable. ^[3][7]

1. Classify this child's hyponatraemia and explain the mechanism by which it developed. (3) ^[1][8]
2. Give the emergency management in the first hour, naming each step, drug, dose, route, and the specific goal. (5) ^[5][11]
3. Outline the definitive management over the next 24 hours, including the correction-rate ceiling and the safety pitfalls, and state how you would prevent recurrence. (2) ^[6][7]

Model answer

Classification and mechanism. This is hypotonic, euvolaemic hyponatraemia (low serum osmolality of 258) with the SIAD pattern: an inappropriately concentrated urine (340 mOsm per kg, above 100) and a high urine sodium (52, above 40). The mechanism is hospital-acquired hyponatraemia: the pneumonia drives non-osmotic arginine vasopressin release, and the hypotonic maintenance fluid (0.45 percent saline) provides free water that the ADH-activated kidney retains, diluting the serum sodium. The acute fall has not given the brain time to adapt, which is why he has seized from cerebral oedema. ^[1][8]

Emergency management (first hour). This is severe symptomatic hyponatraemia with cerebral oedema, a neurological emergency. First, secure the airway, breathing, and circulation and attach cardiac and neurological monitoring. Second, give intravenous 3 percent hypertonic saline at 2 mL per kg over 10 minutes, repeated up to two or three times if seizures continue, aiming for a 4 to 6 mmol per litre rise or seizure control, whichever comes first. The goal is not normalisation but a measured rise sufficient to pull water out of the swollen brain and relieve the cerebral oedema. A 2 mL per kg bolus raises the sodium by roughly 2 to 4 mmol per litre. Stop the hypotonic maintenance fluid immediately. Check the serum sodium after the first bolus. ^[5][11]

Definitive management and pitfalls. Once the brain is safe, switch to slow correction at no more than 8 mmol per litre in 24 hours to avoid osmotic demyelination syndrome, with the sodium checked every 2 to 4 hours. Change the maintenance fluid to isotonic (0.9 percent saline with dextrose), and fluid-restrict if the SIAD persists. Two pitfalls: (1) over-correcting beyond 8 mmol per litre in 24 hours risks osmotic demyelination, and if it happens the rescue is to lower the sodium with desmopressin and free water; (2) fear of osmotic demyelination must never delay 3 percent saline in a seizing child, because cerebral herniation kills faster than ODS. Recurrence is prevented by using isotonic maintenance fluid as the default, as proven by the McNab trial. ^[6][7]