Stroke

Overview

A stroke (syn. Cerebral Vascular Accident or "CVA") is a sudden, often focal, loss of brain function due to an interruption in the blood supply to all or part of the brain. Loss of brain function occurs when neurons loose their oxygen supply. This is often due to a disturbance in cerebral perfusion (blood flow to the brain). There are many different causes for the interruption of blood supply, and different parts of the brain can be affected. Because of this, strokes can have many different clinical presentations. Approximately 700,000 Americans per year experience a stroke. Stroke is a medical emergency and can cause permanent neurologic damage or even death if not promptly diagnosed and treated. It is the third leading cause of death and the leading cause of adult disability in the United States and industrialized European nations. On average, a stroke occurs every 45 seconds and someone dies from a stroke every 3 minutes.[1]

Risk factors include atherosclerosis, advanced age, hypertension (high blood pressure), diabetes mellitus, high cholesterol, cigarette smoking, atrial fibrillation, ethnic identity, migraine with aura, and some blood clotting disorders. Cigarette smoking is the most important modifiable risk factor of stroke. African Americans have twice the risk of a first stroke as whites.

The term "brain attack" has been advocated fo use in the United States for stroke, just as the term "heart attack" is used for myocardial infarction, where a cutoff of blood causes necrosis to the tissue of the heart. Many hospitals have multidisciplinary "stroke teams" specifically for swift treatment of stroke.

Strokes can be classified as ischemic or hemorrhagic.

• Ischemic strokes make up about 80% of all strokes and can be due to thrombosis, embolism, or generalized cerebral hypoperfusion. Lacunae, or small vessel ischemic strokes, are responsible for about 20% of all strokes and are common in diabetes mellitus.
• Hemorrhagic strokes can be due to intracerebral hemorrhage, subarachnoid hemorrhage subdural hemorrhageand epidural hemorrhage.

Etiology

Ischemic Stroke

Atherosclerosis is responsible for the majority of ischemic strokes. Atheroembolism can occur within the cerebral circulation or can originate outside the cerebral circulation. The etiology of atherosclerosis-related strokes is very similar to that of heart attacks. An atherosclerotic plaque in a cerebral artery can gradually develop an associated thrombus or rupture suddenly causing a rapid occlusion, or the thrombus can break off and lodge in a vessel even deeper in the brain. "Thrombotic stroke" usually refers to in-situ thrombus, "embolic stroke" to thrombosis from distant sites.

Thrombotic Stroke

Thrombotic and thromboembolic strokes can originate in either large or small blood vessels, and are usually due to abnormalities in the vessel (most commonly atherosclerosis). One of the most important etiologies is carotid artery disease. Lacunae are also a subset of thrombotic stroke.

Embolic Stroke

Embolism of thrombi from outside the cerebral circulation are responsible for a large and important subset of ischemic strokes. In these cases a thrombus (blood clot) travels from its origin and lodges in a cerebral artery. Carotid artery disease is responsible for a large subset of these. Most of the rest are of cardiac origin (Cardioembolic).

• Cardioembolic Stroke: the majority of embolic strokes originating in the heart are due to atrial fibrillation. In fact, about 16% of strokes are associated with atrial fibrillation, and the presence of atrial fibrillation increases stroke risk by about 5% per year. The relative stasis of blood in the left atrium leads to blood clot formation, and these clots can be expelled from the heart to enter the cerebral circulation.

Hemorrhagic stroke

A hemorrhagic stroke, or cerebral hemorrhage, is a form of stroke that occurs when a blood vessel in the brain ruptures or bleeds. Like ischemic strokes, hemorrhagic strokes interrupt the brain's blood supply because the bleeding vessel can no longer carry the blood to its target tissue. In addition, blood irritates brain tissue, and if the bleeding continues, it can cause increased intracranial pressure which physically impinges on brain tissue and restricts blood flow into the brain. In this respect, hemorrhagic strokes are more dangerous than their more common counterpart, ischemic strokes. There are two types of hemorrhagic stroke: intracerebral hemorrhage, and subarachnoid hemorrhage.

Intracerebral hemorrhage

Intracerebral hemorrhage (ICH) is bleeding directly into the brain tissue, forming a gradually enlarging hematoma (pool of blood). It generally occurs in small arteries or arterioles and is commonly due to hypertension, trauma, and vascular malformations. The hematoma enlarges until pressure from surrounding tissue limits its growth, or until it decompresses by emptying into the ventricular system, CSF or the pial surface. ICH has a mortality rate of 44 percent after 30 days, higher than ischemic stroke or even the very deadly subarachnoid hemorrhage.

Subarachnoid hemorrhage

Subarachnoid hemorrhage (SAH) is bleeding into the cerebrospinal fluid (CSF) surrounding the brain. The two most common causes of SAH are rupture of aneurysms from the base of the brain and bleeding from vascular malformations near the pial surface. Bleeding into the CSF from a ruptured aneurysm occurs very quickly, causing rapidly increased intracranial pressure. The bleeding usually only lasts a few seconds but rebleeding is common. Death or deep coma ensues if the bleeding continues. Hemorrhage from other sources is less abrupt and may continue for a longer period of time. SAH has a 40% mortality over 30 day period.

Systemic hypoperfusion (Watershed stroke)

Systemic hypoperfusion is the reduction of blood flow to all parts of the body. It is most commonly due to various types of shock. Hypoxemia (low blood oxygen content) may precipitate the hypoperfusion. Because the reduction in blood flow is global, all parts of the brain may be affected, especially "watershed" areas --- border zone regions supplied by the major cerebral arteries. Blood flow to these areas does not necessarily stop, but instead it may lessen to the point where brain damage can occur.