Box 1.

Red flags

Sudden-onset (thunderclap) headache Clinical signs of raised intracranial pressure (for example, papilloedema) Neurological deficits (for example, confusion, reduced level of consciousness, seizure, and limb weakness) New visual symptoms (for example, double vision and deteriorating vision) Associated pregnancy risk (for example, severe vomiting and dehydration in first trimester, pre-eclampsia, or postnatal — all increase coagulability) Systemically unwell (for example, fever or meningism) Recent head or neck injury