Subarachnoid hemorrhage is bleeding between the brain's outer membrane and surface caused by an aneurysm or arteriovenous malformation that causes the worst type of headache and can be diagnosed using a CT scan, lumbar puncture, or angiography. Treatment involves intensive care, stabilizing vital signs, and either clipping or coiling an aneurysm, but there are often side effects like neurogenic damage, vasospasm, or stroke, and only about 50% of patients survive the first month, with just one-third returning to normal life.