What Is a Stroke?
A stroke is sometimes called a brain attack. It occurs when an artery that supplies blood to part of the brain becomes blocked or ruptures.
As a result, blood flow is interrupted to a portion of the brain, and neurons (nerve cells) in the affected area are starved of the oxygen and nutrients they need to function properly. In as little as a few minutes, these neurons can suffer damage. After a few hours of interrupted blood flow, neurons can’t survive and some brain function is lost.
Nerve cell damage because of a stroke is usually permanent, producing impairments such as difficulty walking, speaking, and thinking. But despite the death of neurons, people who have had a stroke usually show improvement in function over time, because other neurons gradually take over the functions of those that were lost.
Types of strokes
There are two basic types of strokes: ischemic and hemorrhagic. In an ischemic stroke, blockage of blood flow to the brain causes damage to neurons. In a hemorrhagic stroke, the damage is because of bleeding into the brain.
Hypertension is an important risk factor for both ischemic and hemorrhagic strokes. Prompt and accurate diagnosis of the stroke type is essential for determining the best treatment.
1. Ischemic strokes
In the United States, about 87 percent of all strokes are ischemic and result from a blockage in a blood vessel providing blood to the brain. The blockage stops blood flow to the part of the brain that the blood vessel supplied.
Neurons become damaged within minutes and start to die—not only because of a lack of oxygen and nutrients but also because of a powerful chain of chemical reactions known as the ischemic (or glutamate) cascade. This chain reaction leads to a buildup of toxins that further contributes to the destruction of neurons.
Where the blockage occurs and how long it lasts determine whether the brain suffers temporary impairment, irreversible damage to a few highly vulnerable neurons, or extensive neurological damage. The two types of ischemic strokes are thrombotic and embolic.
• Thrombotic stroke. The most common type of ischemic stroke is a thrombotic stroke. It occurs when a blood clot (thrombus) forms along the wall of one of the major arteries supplying the brain and completely blocks blood flow. The affected artery may be one of the carotid or vertebral arteries or a smaller artery within the brain.
Blood clots most often develop in arteries that are already narrowed by fatty deposits called plaques; these plaques also cause coronary heart disease. The hard, rough, uneven surfaces of these plaques are ideal sites for the formation and growth of blood clots.
• Embolic stroke. Another type of ischemic stroke is an embolic stroke. It most often occurs when a portion of a blood clot or a piece of atherosclerotic plaque (embolus) breaks off and travels through the bloodstream until it lodges in a smaller artery supplying the brain, blocking blood flow. Most of these emboli originate in the heart or in large arteries such as the carotid arteries.
One of the most common causes of emboli is atrial fibrillation, an abnormal heart rhythm in which the atria (upper chambers of the heart) quiver chaotically instead of contracting in a rhythmic pattern. As a result, the atria do not empty completely of blood and the blood that remains behind can form clots.
These clots can then escape from the heart and travel along the increasingly narrow branches of blood vessels, ultimately lodging in an artery (usually in the brain). One-third of people with untreated atrial fibrillation suffer a stroke.
Other heart diseases that can increase the risk of an embolic stroke include heart attacks, heart failure (impaired ability of the heart to pump blood), valvular heart disease (damage to the heart’s valves) and plaque in the aorta (the body’s main artery).
2. Hemorrhagic strokes
Hemorrhagic strokes account for approximately 13 percent of all strokes in the United States. A hemorrhagic stroke occurs when an artery in the brain tears or ruptures and blood leaks into the surrounding tissue.
The bleeding can enter tissue deep within the brain (intracerebral hemorrhage) or it can flow into the space between the brain and skull (subarachnoid hemorrhage).
Damage from a hemorrhagic stroke occurs in two ways. First, blood supply is cut off to the parts of the brain beyond the rupture or tear site. Second—and posing the greatest danger—the escaped blood forms a mass that exerts pressure on the brain. Blood continues to leak from a torn or ruptured artery until the blood clots or pressure inside the skull equals that in the damaged artery.
An intracerebral hemorrhage is often caused by a tear in a small blood vessel in the brain; a subarachnoid hemorrhage is usually the result of a ruptured aneurysm in the brain.
An aneurysm is a blood-filled pouch that balloons out from a weak spot in a blood vessel wall. While some aneurysms are congenital (present at birth), they may be made worse or even caused by hypertension. An intracerebral hemorrhagic stroke can also be the result of the rupture of a congenital blood vessel defect called an arteriovenous malformation (AVM).