<unk>  accounts for ~2.2% of all ED visits
The majority of these<unk> ve a benign cause, but serious causes can be devastating, and a thorough H&P with an eye toward "red flag" symptoms is important in ED evaluation.
Features
Sudden onset or accelerating pattern
Maximum intensity of pain at onset (i.e. "thunderclap")
Worse with valsalva
Worse in the morning or at night
No similar<unk>  in past
Age >50 yr or <5 yr
Occipitonuchal<unk> 
Visual disturbances
Exertional or postcoital
Family or personal history of SAH, cerebral aneurysm, or AVM
Focal neurologic signs
Diastolic BP >120
Papilledema
Jaw claudication
Clinical Context<unk>  in setting of:
Infection
Cancer
Immunosuppression
Seizure
Syncope
Trauma
Altered mental status
Systemic illness (fever, stiff neck, rash)
Nausea/vomiting
Patient on anticoagulation, steroids, NSAIDs, antiplatelet
Headache
Migraine headache
Tinnitus
Tension headache
Dizziness
Cluster headache
Sore throat
Meningitis/encephalitis