Subarachnoid hemorrhage occurs when blood leaks into the subarachnoid space surrounding the brain. The most common causes are aneurysmal rupture or trauma. Clinical features include a sudden, severe "thunderclap" headache and may include neck stiffness, nausea, vomiting, and loss of consciousness. Diagnosis is made through lumbar puncture and brain imaging. Untreated, SAH has high mortality with 50% dying within 30 days. Survivors often have neurological deficits. Treatment focuses on securing the ruptured aneurysm to prevent rebleeding.