A 41 year old man was brought to the emergency department late in the afternoon with a reduced conscious level. He was found unresponsive in his bedroom, having last been seen well that morning. On presentation, his temperature was 36.1°C, pulse rate 90/min in sinus rhythm, blood pressure 140/70 mm Hg, respiratory rate 15/min and oxygen saturation 96% on room air. He had a blood sugar level of 7 mmol/l. The patient had a Glasgow Coma Scale score of 5 (E1 V1 M3) without external signs of a head injury. His pupils were sluggishly reactive, at 4 mm on the right and 2 mm on the left. He had a history of chronic schizophrenia treated with thioridazine, a previous head injury and was a heavy smoker.

His biochemistry on presentation showed serum sodium concentration of 113 mmol/l, potassium concentration 3.4 mmol/l, chloride concentration 76 (100–110 mmol/l), …