Intense or abrupt onset

Change in headache characteristics

Atypical aura (>1 h, or with motor weakness)

Visual disturbance/scotoma

Prior or coexisting infection

Onset during exertion

History of HIV, syphilis, or cancer

Environmental exposure

No relief from pain medication

Toxic appearance

Fever

Altered mental status

Papilledema

Any localizing or lateralizing signs

Neurologic deficit (weakness, sensory loss, dysphagia)

Meningismus