What caused this subarachnoid hemorrhage?

Subarachnoid hemorrhage is the least common cause of stroke, accounting for approximately 3% of cases. Primary subarachnoid hemorrhage is due to rupture of a vessel within the cerebrospinal fluid-containing subarachnoid space on the outer surface of the brain. Although 85% of primary subarachnoid hemorrhages are due to rupture of saccular aneurysms, with an additional 5% due to arteriovenous malformations, dural arteriovenous fistulas and mycotic aneurysm, no culpable lesion is identified in 10% of cases. Many processes affecting the brain parenchyma can cause secondary subarachnoid hemorrhage by extension of bleeding across the pia membrane, most notably spontaneous lobar intracerebral hemorrhages. Familiarity with the various causes of subarachnoid hemorrhage, their characteristic presentations, and use of a systematic approach to neuroimaging will readily identify the correct diagnosis in most cases.