Subarachnoid hemorrhage (SAH) is bleeding into the space around the brain that is filled with cerebrospinal fluid (the subarachnoid space). A spontaneous SAH is often due to a ruptured brain aneurysm. 90% of patients survive to reach the hospital, but over half will be disabled or dead. This occurs despite the patient being in hospital and with optimal medical treatment. This risk of poor outcome or death shows the obvious need for better treatment for SAH.