A 41 year old woman was admitted with sudden onset of severe headache followed by collapse and decreased consciousness.

She had a history of frequent migraines but was not taking regular drugs. She was a non-smoker with no history of recreational drug use.

On admission her Glasgow coma scale was 7/15 (E1 V1 M5). Her pupils were unequal but reactive. Examination of her peripheral nervous system showed increased tone in all four limbs with brisk reflexes. Plantar reflexes were equivocal. Her blood pressure was 150/80 mm Hg, pulse was regular at 44 beats/min, and oxygen saturation was 99% on 10 l/min. Cardiovascular examination was normal. A respiratory examination was normal.

The patient was intubated. Computed tomography of the head excluded intracranial haemorrhage.

A lumbar puncture was performed, and cerebrospinal fluid was sent for analysis; it showed white blood cell count 0×10⁶/l (reference range 0-5 (all lymphocytes, no neutrophils)), red blood cell count 23×10⁶/l (0-10), protein 0.3 g/l (0.2-0.4), glucose 3.8 mmol/l (3.3-4.4 or about two thirds of the blood glucose concentration), and no xanthochromia. Magnetic resonance imaging (MRI) of the brain was performed one week later (fig 1⇓).

Transoesophageal echocardiography was performed three weeks later (fig 2⇓).