When a blood vessel just outside the brain bursts, the space surrounding the brain (the subarachnoid space) fills with blood. This condition is called subarachnoid hemorrhage, and is usually due to an aneurysm. This bleeding may irritate nearby blood vessels that can become very narrow, a condition known as cerebral vasospasm. This vasospasm can starve the brain of oxygen, and therefore damage parts of the brain similar to a stroke. Cerebral vasospasm is a delayed event after an aneurysmal subarachnoid hemorrhage, with a usual peak from days 7 to 9 after a bleed. It usually affects the large arteries near the ruptured aneurysm.