Subarachnoid hemorrhage refers to the rupture of blood vessels due to lesions at the base or on the surface of the brain, with blood directly flowing into the subarachnoid space, causing a clinical syndrome. This is different from cerebral hemorrhage, which refers to bleeding within the brain tissue itself, not into the subarachnoid space. So, what is the subarachnoid space? The human brain is covered by three layers of membranes: the pia mater, arachnoid, and dura mater. The subarachnoid space is the area between the pia mater and the arachnoid membrane, named as such. When there is a rupture in cerebral vascular malformations or cerebral aneurysms, blood flows directly into the subarachnoid space rather than causing bleeding in the brain tissue. Subarachnoid hemorrhage is considered a very serious medical condition with a very high mortality rate. Its main symptoms include severe headache, increased intracranial pressure, nausea, and projectile vomiting. If it is a second occurrence of subarachnoid hemorrhage, the mortality rate can reach up to 50%. A third occurrence of subarachnoid hemorrhage almost certainly results in death.