Aims To describe incidence, presentation and short term outcomes of severe neonatal hypernatraemia (SNH) (≥160 mmol/l).

Methods Prospective, population based surveillance study over 13 months using the BPSU. Cases were >33 weeks gestation at birth, fed by any means and <28 days of age at presentation.

Results 62 cases of SNH were reported (66 per 100 000 livebirths; 95%CI 5.1 to 8.5). 61/62 mothers intended to achieve exclusive breastfeeding. 42 had no supplementary formula prior to admission. Infants presented at median day 6 (range 2–17) with median weight loss of 19.5% (range 8.9–30.9). 13 had jaundice and 57 weight loss as a presenting feature. 58 presented with weight loss ≥15%. Serum sodium fell by median 12.9 mmol/l per 24 h (range 0–30). No baby died, had convulsions, coma or was treated with dialysis or a central line. At discharge, babies had gained 11% of birthweight after a medium admission of 4 (range 2–14) days. 10/62 were exclusively breastfed on discharge from hospital.

Conclusions Neonatal hypernatraemia at this level is strongly associated with weight loss. It occurs almost exclusively after attempts to initiate breastfeeding, occurs rarely and is not associated with serious short term morbidities. The reported incidence is higher than that in the Netherlands.1