Pathophysiology of Cerebrovascular Accident

A cerebrovascular accident (CVA) or stroke is caused by an acute lack of blood supply to part of the brain. There are two major types of stroke: ischemic, in which a blockage (usually a blood clot) reduces or halts blood flow, and hemorrhagic, in which bleeding in the brain triggers a response that restricts blood supply elsewhere. The pathophysiology of cerebrovascular accident is different for these two types of stroke.

Pathophysiology of Ischemic Stroke

An ischemic stroke may be caused by a blood clot that occurs in the affected artery (thrombosis), a blood clot that traveled from another part of the body (embolism), or a blockage due to damage to the arterial wall (lacunar infarct). Blockage of a single artery can often be compensated for by other arteries in the blood vessel network, call collaterals. Artherosclerosis (hardening of the arteries), other damage to arteries, and natural variations in the collateral network can prevent the collateral system from compensating fully. The result is a loss of perfusion, or blood supply, to an area of the brain (ischemia).

Arteriovenous malformations (AVMs), abnormal tangles of blood vessels in which arteries flow directly into veins, are another possible cause of ischemic stroke. AVMs can press directly on brain tissue, blocking blood flow.

Pathophysiology of Hemorrhagic Stroke

The main type of hemorrhage that can lead to stroke is subarachnoid hemorrhage. In this type of bleed, which usually results from head trauma or a ruptured aneurysm, there is uncontrolled bleeding between the innermost two of the three meninges (membranes lining the brain), the pia mater and the arachnoid mater.

The blood that pools or collects in a subarachnoid bleed is called a hematoma. Ischemia (lack of blood flow) from the hematoma is a secondary problem. It is caused by constriction of the arteries (vasospasm) as a protective response to reduce bleeding. About 25% of patients with subarachnoid hemorrhages will experience stroke symptoms resulting from secondary ischemia.

Another common type of brain hemorrhage is an intracerebral hemorrhage, in which an artery inside the brain tissue fails due to high blood pressure and begins to bleed. It usually results from hypertension, but other causes of burst intracerebral arteries include various types of aneurysm, brain tumors, and blood clotting disorders. Symptoms of intracerebral hemorrhages can mimic symptoms of ischemic stroke, especially for smaller bleeds, but the two problems have different etiologies and treatments.

References

The Merck Manual of Diagnosis and Therapy: Entries on Ischemic Stroke, Subarachnoid Hemorrhage, and Intracerebral Hemorrhage.