A previously well 35-year-old, right-handed, female disc jockey presented to the emergency department in a confused state. She was amnesic for recent events and disorientated in time and place. A third party history was obtained from a relative. The patient had become unwell 1 week earlier following a party when she had become dizzy and collapsed without losing consciousness. Two days later she phoned her brother as she was feeling increasingly unwell and described a constant, aching, parieto-occipital headache with altered vision—‘like looking through a kaleidoscope’. She subsequently became increasingly drowsy and difficult to rouse, sleeping for long periods of the day. Confusion and memory impairment developed on the day of admission. During the initial history taking the patient struggled to remember events from the previous few minutes to few days.

The only past medical history was of a weekly headache associated with photophobia, nausea and positive coloured visual phenomena which had started 8 months before her current presentation. The patient had self-diagnosed these headaches as migraine, and had occasionally taken over the counter paracetamol during a severe attack. Her only prescribed medication was the combined contraceptive pill. She smoked 20 cigarettes per day. Her alcohol consumption was unknown.

On examination, the patient was drowsy and disorientated in time and place, and her Glasgow Coma Score was 14/15 (E4, V4, M6). Cardiorespiratory and abdominal examination were normal, except for a blood pressure of 214/127 mm Hg. There was no fever. Finger counting was possible bilaterally in the left, but not right, hemifields. The fundi were normal. The remainder of the cranial nerves were normal. Tone, power and reflexes were normal in all limbs. Both plantars were withdrawn. In the right upper limb there was impairment of finger nose testing and dysdiadochokinesis.