Hemorrhage, subarachnoid: Bleeding within the head into the space between two membranes that surround the brain. The bleeding is beneath the arachnoid membrane and just above the pia mater. (The arachnoid is the middle of three membranes around the brain while the pia mater is the innermost one.)
Subarachnoid hemorrhages are typically acute (sudden). They may follow a head injury or rupture of a blood vessel in the central nervous system (brain and spinal cord) as, for example, because of an aneurysm in the vessel.
The first-degree relatives of patients with subarachnoid hemorrhage are more likely to develop the condition than those who do not have a family history of it. Smoking, alcohol abuse, and high blood pressure are risk factors for subarachnoid hemorrhage.
Picture of the brain and potentially brain injury areas