Peter, aged 5 months, was taken to his GP with a two-day history of fever, vomiting and diarrhoea. During the preceding hours his mother had been concerned that he was sleeping excessively and was not feeding, which was most unlike him. He was on formula milk and weaning foods and normally had a good appetite. In general he was an irritable infant, often difficult to settle, and rarely slept for more than a couple of hours at a time. He regularly required laxatives for constipation. His mother had tried him on a number of different formula milks and treatments for colic.

He was a full term delivery with a birth weight of 3500 g. His GP had seen him for routine checks and immunisations. On examination he was quiet, pale and thin with a sunken fontanelle and tachycardia. His weight was on the 3rd centile and his length and head circumference were on the 50th centile. The GP thought that he was clinically dehydrated but noted that he was still having wet nappies. The GP also noted he was in double nappies, which his mother said was necessary because he frequently saturated his nappy and cot despite frequent changes. Urine was negative for glucose. He was referred to the paediatric unit for further evaluation and rehydration.

He was assessed in the observation ward of the accident and emergency department and noted to be dehydrated and peripherally cool. He was admitted to hospital for intravenous rehydration. He was given a bolus of normal saline (10 ml/kg). The results of initial investigations are shown in table 1.

The admitting paediatrician noted the hypernatraemic, hyperchloraemic metabolic acidosis and low–normal blood glucose. He was concerned that although gastrointestinal losses could account for the dehydration, metabolic acidosis and possibly the high …