An unusual case of extreme hypernatraemia.

A 4-year-old severely disabled boy with a congenital myopathy developed profuse diarrhoea with hypernatraemia (plasma Na 157 mmol/l). The initial blood urea, serum creatinine and urine output were within normal limits. Despite corrective measures within a hospital setting, the patient's serum sodium peaked at 202 mmol/l. A high fractional excretion of sodium (FE Na) in the context of dehydration and normal renal function was suggestive of a high sodium load. Subsequent investigations revealed an unusual combination of valproate-induced Fanconi syndrome, nephrogenic diabetes insipidus and excess sodium load. The case illustrates why severe hypernatraemia in children is such a diagnostic challenge.