[Salt fever].

The importance of hypernatraemia-induced fever was highlighted in this journal by Pál Heim 100 years ago. Despite the longstanding experience, its possibility is still rarely considered. Here we present the differential diagnosis of neonatal hypernatraemia through the example of a four-day-old boy admitted for fever. Hypernatraemia at this age typically results from inadequate breast feeding, with weight loss exceeding 10% of the birth weight. The degree of weight loss is proportional to the degree of hypernatraemia. Hypernatraemia and not dehydration induces the fever, as cases of infants with salt poisoning have formerly shown. Hypernatraemia secondary to either dehydration or salt toxicity is associated with increased urine osmolality, but the fractional sodium excretion is elevated only in salt poisoning. However, the urine osmolality of the admitted newborn was low, <100 mOsm/kg, the specific gravity was 1005 g/L. The coexistence of hypernatraemia (156 mmol/L) and hyposmolar urine was suggestive of diabetes insipidus. Its most common, X-linked form was confirmed by the identification of a frameshift AVPR2 variant. The polyuria responded well to hypothiazide and indomethacin treatment, and no hypernatraemia reoccured by ensuring water intake in the next months. Orv Hetil. 2024; 165(29): 1107–1111.