Subarachnoid hemorrhage (SAH) is bleeding directly into the cerebrospinal fluid (CSF) within the subarachnoid space that surrounds the brain. It usually presents as a severe sudden-onset headache, often described as the worst headache of the patient's life, with radiation of pain down the neck and back. Onset of headache may be accompanied by a brief loss of consciousness, nausea, vomiting, and meningismus. SAH occurs in about 10 per 100 000 people per year. Incidence increases with age and peaks in the sixth decade of life. Risk factors include female sex, smoking, alcohol use, drug abuse, hypertension, oral contraceptive use, known cerebral vascular malformation, collagen vascular disease, and family history of SAH. In the United States, African Americans are at higher risk.

Cerebral aneurysms > 5-7 mm are most likely to rupture. Approximately 1%-3% of patients visiting the emergency department for a headache are subsequently found to have a SAH. Trauma is the most common cause of SAH; however, nontraumatic SAH is found to result from a ruptured saccular aneurysm in 80% of cases. Patients with SAH require close monitoring and often require intensive care unit admission.

Approximately 10%-15% of patients die before reaching medical care. Even with appropriate care, the mortality rate from SAH is approximately 25%-50%. About one-half of patients who survive have significant morbidity.

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