Aims A 3 month old previously well female infant presented to a district general hospital paediatric assessment unit off-colour, with reduced feeds and two episodes of vomiting. Initial blood gas showed a metabolic acidosis with severe hypernatraemia of 164 mmol/L, disproportionate to the clinical history given. Further analysis of the clinical case aims to explore possible causes of her hypernatraemia, exacerbated by an acute episode of illness. Methods A full septic screen was undertaken. The clinical history was disproportionate to the degree of hypernatraemia, but remained consistent from the parents despite questioning from different clinicians. There were no social concerns and no concerns when parents were observed making up her formula feeds. Initial management included administration of broad spectrum intravenous antibiotics and one fluid bolus of normal saline, followed by intravenous fluids.

Results Urinary sodium was >20 mmol/L and serum potassium was 4.8 mmol/L, making diagnoses’ of diabetes insipidus and hyperaldosteronism unlikely. Water deficit calculations were performed and her 24 hour fluid requirements calculated to add in free water to her additional fluids, to correct her sodium slowly. Her sodium peaked at 174 mmol/L, but with careful fluid balance review and fluid management, her sodium reached normal levels following her recovery from acute illness and she was discharged 5 days later. Her father expressed concern that her formula milk was made using with softened water at home and brought in water samples for testing. Since discharge from hospital, the family had made her formula using bottled water. Unofficial testing of home water samples revealed mains water sodium concentrations of 4 mmol/L=92 mg/L and softened water (used to make up the infant’s feeds) 14 mmol/L=322 mg/L. Anglian Water regulations state maximum concentration of sodium in drinking water should be 200 mg/L.

Conclusion There are no reported cases in the literature of hypernatraemia resulting from high sodium in softened water, though several regarding deliberate salt poisoning in the context of child maltreatment. We conclude that this was a diagnosis of accidental salt poisoning that should highlight dangers to parents of using softened water to make up infant formula.