Introduction The relative frequencies of the causes of hypernatraemia in children after the neonatal period are unknown. Salt poisoning and osmoregulatory dysfunction are extremely rare and potentially fatal. In this retrospective 10-year study, the incidence, causes and differential biochemistry of hypernatraemia in children is examined.

Methods Children with hypernatraemia (sodium ≥150 mmol/litre) aged >2 weeks to 17 years were identified from laboratory data of two paediatric departments serving the Lothian region of Scotland. A review of patient notes established time of onset and cause. Denominator data were available from the Scottish Health Service.

Results On admission to hospital, 1 in 2288 children (1:1535 admitted as an emergency) had hypernatraemia. This is 1 in 30 563 Lothian children <17 years. Overall 0.04% hospital stays had an episode of hypernatraemia. In 45 children admitted with 64 separate episodes (11 from a case of salt poisoning), the commonest cause was dehydration secondary to either gastroenteritis or systemic infection; 31% had an underlying chronic neurological disorder. A total of 177 further cases developed hypernatraemia after admission. The commonest causes were dehydration secondary to severe systemic infection and postoperative cardiac surgery. Urine sodium:creatinine ratio and fractional excretion of sodium were both much higher in the salt poisoning case than in a child with osmoregulatory dysfunction or children with simple dehydration.

Conclusions Hypernatraemia after 2 weeks of age is uncommon, and on admission is usually associated with dehydration. Salt poisoning and osmoregulatory dysfunction are rare but should be considered in cases of repeated hypernatraemia without obvious cause. Routine measurement of urea, creatinine and electrolytes on paired urine and plasma on admission will differentiate these rare causes.