What is it?
Cerebral vasospasm generally occurs due to a ruptured brain aneurysm, or (very rarely) hemorrhage from another blood vessel abnormality such as an arteriovenous malformation (AVM). The common factor here is the abnormal presence of a substantial amount of blood on the outer ("subarachnoid" or "adventitial") surface of the blood vessel. This can particularly affect arteries at the base of the brain, i.e., around the Circle of Willis. Tthe amount of blood in the subarachnoid space may be less compared with patients experiencing aneurysmal rupture. Nonetheless, vasospasm may still occur, and its occurrence may negatively influence "outcome" in patients with significant traumatic SAH.
Vasospasm is generally thought to occur only in arteries and not in smaller arterioles or capillaries or veins. The reason for this is at least partly related to physical differences in the wall structure between these types of vessels; arteries have thicker walls (especially due to a thicker smooth muscle layer) and can clamp down (or contract) harder than, say, a vein or capillary. There are also molecular differences between these vessels that may partly explain why vasospasm occurs selectively in arteries. Vasospasm is certainly known to occur in the large arteries comprising the Circle of Willis and the main branches arising from this vascular ring; it even occurs in small "pial" arteries that course over the surface of the brain.