A 32 year old woman was admitted with a two day history of extreme fatigue, severe headache, vomiting, postural dizziness, and blurring of peripheral vision on her left lateral gaze. Two days earlier she had had an emergency caesarean section for an anterior uterine rupture at 29 weeks’ gestation. During the operation she lost 1.5 litres of blood and became hypotensive with a blood pressure of 87/54 mm Hg. She could not lactate post partum.

On examination, she was confused and slow to respond. Her blood pressure was 98/60 mm Hg, pulse rate 80 beats per minute, and respiratory rate 14 breaths per minute. Neurological examination showed a left VIth cranial nerve palsy. Her visual fields were full to confrontation, and dilated fundoscopy was unremarkable. Initial investigations showed a serum sodium of 116 mmol/l (138 mmol/l immediately postoperatively) and potassium of 3.7 mmol/l. Her serum osmolality was 240 mOsm/kg (normal 275-95) with a urine osmolality of 535 mOsm/kg and her urine sodium concentration was 91 mmol/l.