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 
Primary headache
Tinnitus
Migraine
Dizziness
Tension
Sore throat
Cluster