The history

A 45-year-old woman was brought to the emergency department by the police. She had been stopped for driving erratically and noted to be confused; an alcohol breathalyser test was negative. In the emergency department, she was disoriented in time and place, unable to give a history, but denied any symptoms.

The patient's sister had noted a behavioural change about 12 months previously when the patient began using cannabis, but otherwise she had functioned normally until 4 weeks before this presentation. Then her memory had deteriorated, her behaviour changed further and she began to have difficulties with routine tasks at work (eg, she required help to use a photocopier). She had been able to continue in her job as a lecturer, but colleagues had noticed her poor performance.

Two weeks before this admission, a neighbour found her in a confused state on the floor, having vomited and been incontinent of urine. After this, she complained of leg weakness which resolved. She did not seek medical attention at the time.

She had no medical or psychiatric history, and took no regular medication. She lived alone and worked as a lecturer in nursing. She drank 14 units of alcohol per week and had not smoked for 20 years. She had smoked cannabis for about 12 months but there was no other known recreational drug use. She had no travel history and no family history of neurological illness.

Examination

She was afebrile with no meningism. General medical examination was normal.

Although alert, she was disoriented in time and place and was unable to name the monarch, prime minister or dates of the Second World War. She scored 26/100 on the Addenbrooke's Cognitive Examination with marked deficits in all domains. She had difficulty following two-step commands. Cranial nerve examination was normal. She had no weakness, but …