A subarachnoid hemorrhage is bleeding in the space between your brain and the surrounding membrane (subarachnoid space). The primary symptom is a sudden, severe headache. The headache is sometimes associated with nausea, vomiting and a brief loss of consciousness.
Bleeding usually results from the rupture of an abnormal bulge in a blood vessel (aneurysm) in your brain. Sometimes bleeding is caused by trauma, an abnormal tangle of blood vessels in your brain (arteriovenous malformation), or other blood vessel or health problems.
Untreated, a subarachnoid hemorrhage can lead to permanent brain damage or death.