What layers is a subarachnoid hemorrhage (SAH) between?

A subarachnoid hemorrhage occurs between the arachnoid mater and the pia mater, which are two of the three meningeal layers that cover the brain and spinal cord. Specifically, bleeding occurs into the subarachnoid space, which is filled with cerebrospinal fluid (CSF). This space lies directly beneath the arachnoid membrane and above the pia mater, which is tightly attached to the surface of the brain. Subarachnoid hemorrhages are most commonly caused by the rupture of an aneurysm in the cerebral vasculature, allowing blood to escape into this CSF-filled space. The presence of blood in this area can cause severe headaches often described as "the worst headache of my life," neck stiffness, photophobia, and altered consciousness. This condition is a medical emergency requiring immediate attention as blood in the subarachnoid space can lead to increased intracranial pressure, vasospasm of cerebral blood vessels, and potential brain damage if not promptly treated.