Defined as<unk>  into the<unk>  space (between the arachnoid membrane and the pia mater). This may occur spontaneously, usually from a ruptured cerebral aneurysm, or may result from head injury.
The prevalence of<unk>  in patients presenting with true thunderclap headache is estimated at ~10%. 
Genetics (polycystic kidney disease, Ehler-Danlos, family history)
Hypertension
Atherosclerosis
Cigarette smoking
Alcohol
Age >50
Cocaine use
Estrogen deficiency
Ruptured aneurysm (85%)
Nonaneurysmal (15%)
Perimesencephalic<unk>  (10%) - lower risk of complications
Other: tumor, coagulopathy, dissection, vasculitis, SCD, venous sinus thrombosis
Perimesencephalic<unk>  (10%) - lower risk of complications
Other: tumor, coagulopathy, dissection, vasculitis, SCD, venous sinus thrombosis
Differentiate from aneurysmal rupture
Supportive care with prevention of hypertension, elevated ICP, and vasospasm with PO nimodipine
Patients with normal neurologic exam NOT on anticoagulation may not need a repeat head CT
Recommend 6 hour observation
Recommend 6 hour observation
Subarachnoid hemorrhage
Intra-axial
Subdural hemorrhage
Hemorrhagic stroke 
Ischemic stroke
Traumatic intracerebral hemorrhage
Hemorrhagic stroke
Extra-axial