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What is a Stroke?

The major areas of the brain have one or more specific functions.
Visit our Health Encyclopedia to learn more about stroke.

 

What is a stroke?

A stroke is an interruption of the blood supply to any part of the brain. A stroke is sometimes called a "brain attack."  

There are two major types of stroke: ischemic stroke and hemorrhagic stroke.

ISCHEMIC STROKE

Ischemic stroke occurs when a blood vessel that supplies blood to the brain is blocked by a blood clot. This may happen in two ways:

  • A clot may form in an artery that is already very narrow. This is called a thrombus. If it completely blocks the artery, it is called a thrombotic stroke.
  • A clot may break off from somewhere in the body and travel up to the brain to block a smaller artery. This is called an embolism. It causes an embolic stroke.

Ischemic strokes may result from clogged arteries, a condition called atherosclerosis. This may affect the arteries within the brain or the arteries in the neck that carry blood to the brain. Fat, cholesterol, and other substances collect on the wall of the arteries, forming a sticky substance called plaque. Over time, the plaque builds up. This often makes it hard for blood to flow properly, which can cause the blood to clot.

Ischemic strokes may also be caused by blood clots that form in the heart. These clots travel through the blood and can get stuck in the small arteries of the brain. This is known as a cerebral embolism.

Certain drugs and medical conditions can make your blood more likely to clot and raise your risk for ischemic stroke. A common cause of ischemic stroke in people under age 40 is carotid dissection, or a tear in the lining of the carotid artery. The tear lets blood flow between the layers of the carotid artery. This causes narrowing of the carotid artery that is not due to plaque buildup.

HEMORRHAGIC STROKE

Hemorrhagic stroke occurs when a blood vessel in part of the brain becomes weak and bursts open, causing blood to leak into the brain. Some people have defects in the blood vessels of the brain that make this more likely. The flow of blood that occurs after the blood vessel ruptures damages brain cells.

What are the stroke warning signs?

  • Sudden numbness or weakness of the face, arm or leg, especially on one side of the body.
  • Sudden confusion, trouble speaking or understanding.
  • Sudden trouble seeing in one or both eyes.
  • Sudden trouble walking, dizziness, loss of balance or coordination.
  • Sudden, severe headache with no known cause.


What causes a stroke?

Stroke is a cardiovascular disease that affects the blood vessels supplying blood to the brain. A stroke occurs when a blood vessel bringing oxygen and nutrients to the brain bursts or is clogged by a blood clot or some other particle. Because of this rupture or blockage, part of the brain doesn't get the blood flow it needs. Deprived of oxygen, nerve cells in the affected area of the brain can't function and die within minutes. And when nerve cells can't function, the part of the body controlled by these cells can't function either. The devastating effects of stroke are often permanent because dead brain cells aren't replaced.

There are four main types of stroke: two caused by blood clots or other particles, and two by bleeding or hemorrhage. Cerebral thrombosis and cerebral embolism are by far the most common, accounting for about 70-80 percent of all strokes. They're caused by clots or particles that plug an artery. Cerebral and subarachnoid hemorrhages are caused by ruptured blood vessels. They have a much higher fatality rate than strokes caused by clots.

What is a cerebral thrombosis?

Cerebral thrombosis is the most common type of stroke. It occurs when a blood clot (thrombus) forms and blocks blood flow in an artery bringing blood to part of the brain. Blood clots usually form in arteries damaged by atherosclerosis.

One identifying feature of cerebral thrombotic strokes is that they often occur at night or first thing in the morning, when blood pressure is low. Another is that very often they're preceded by a transient ischemic attack, also called a T.I.A. or "mini-stroke."

What is a cerebral embolism?

Cerebral embolism occurs when a wandering clot (an embolus) or some other particle forms in a blood vessel away from the brain, usually in the heart. The clot is carried by the bloodstream until it lodges in an artery leading to or in the brain, blocking the flow of blood.

The most common cause of these emboli is blood clots that form during atrial fibrillation , a disorder found in some 2-3 million Americans. In atrial fibrillation the two small upper chambers of the heart, the atria, quiver instead of beating effectively. Blood isn't pumped completely out of them when the heart beats, so it pools and clots. Stroke occurs when the blood clot enters the circulation and becomes lodged in a narrowed artery of the brain.

What is a hemorrhage?

A subarachnoid hemorrhage occurs when a blood vessel on the brain's surface ruptures and bleeds into the space between the brain and the skull (but not into the brain itself).

Another type of stroke occurs when a defective artery in the brain bursts, flooding the surrounding tissue with blood. This is a cerebral hemorrhage.

Hemorrhage (or bleeding) from an artery in the brain can be caused by a head injury or a burst aneurysm . Aneurysms are blood-filled pouches that balloon out from weak spots in the artery wall. They're often caused or made worse by high blood pressure. Aneurysms aren't always dangerous, but if one bursts in the brain, a hemorrhagic stroke results.

When a cerebral or subarachnoid hemorrhage occurs, the loss of a constant blood supply means some brain cells no longer can work. Another problem is that accumulated blood from the burst artery may put pressure on surrounding brain tissue and interfere with how the brain functions. Severe or mild symptoms can result, depending on the amount of pressure.

The amount of bleeding determines the severity of cerebral hemorrhages. In many cases, people with cerebral hemorrhages die of increased pressure on their brains. Those who live, however, tend to recover much more than those who've had strokes caused by a clot. The reason is that when a blood vessel is blocked, part of the brain dies - and the brain doesn't regenerate. But when a blood vessel in the brain bursts, pressure from the blood compresses part of the brain. If the person survives, gradually the pressure goes away and the brain may regain some of its former function.

What is tPA?

Tissue plasminogen (plaz-MIN'o-jin) activator (tPA) is a thrombolytic (thrombo-LIT'ik) agent (clot-busting drug). It's now approved for use in certain patients having a heart attack or stroke. The drug can dissolve blood clots, which cause most heart attacks and strokes.

How does tPA help people having a heart attack?

Studies have shown that tPA and other thrombolytic (clot-dissolving) agents can reduce the amount of damage to the heart muscle and save lives. However, to be effective, they must be given within a few hours after symptoms begin.

How does tPA help people having a stroke?

tPA has been shown to be effective in treating ischemic (is-KEM'ik) stroke. This kind of stroke is usually caused by blood clots that block blood flow to the brain.

In 1996 the U.S. Food and Drug Administration (FDA) approved the use of tPA to treat stroke in the first three hours following the onset of symptoms. This makes it very important for people who think they're having a stroke to seek help immediately. If given promptly, tPA can significantly reduce the effects of stroke and reduce permanent disability.

What imaging tests are done of the brain?

  • Computerized axial tomographic scan (C.T. or CAT scan) - Uses X-rays to generate an image of the brain. Doctors use CT to determine whether a stroke has occurred and to identify the type of stroke: ischemic (result of blockage) or hemorrhagic (result of bleeding). CT scanning takes from 20 minutes to an hour to complete.
  • Magnetic resonance imaging scanning (M.R.I.) - The patient is placed in a magnetic field, causing the behavior of brain cells to change. Once the patient is in a magnetic field, the head is subjected to bursts of energy of a known frequency. The response of the brain cells to these bursts of energy is detected as signals that ultimately generate an image of the brain. MRI can give very accurate images of the brain and is used to determine the presence, location and size of aneurysms and arteriovenous malformations, which are potential sources for hemorrhagic stroke.
  • Radionuclide angiography - Radioactive compounds are injected into a vein in the arm; the bloodstream then carries them toward the head. As the radioactive compound circulates in the bloodstream, it constantly emits bursts of radiation. Once the radioactive compound reaches the brain, these bursts of radiation are detected and used to form an image of the brain. This imaging procedure can show areas where the brain has been deprived of blood flow and is damaged.

What tests show the brain's electrical activity?

  • Electroencephalogram (E.E.G.) - Small metal disks (electrodes) are placed at strategic locations on a person's scalp. The electrodes can detect the electrical activity in the form of impulses that are then transcribed to paper. By observing such impulse characteristics as intensity (the size of the impulse), duration (the width of the impulse), frequency (how often impulses occur during a given time) and location (what region of the brain produces these impulses), an EEG can provide valuable information about underlying problems in the brain.
  • Evoked response test - A diagnostic procedure that provides a measurement of the brain's ability to process and react to different sensory stimuli. A doctor evokes a visual response by flashing a light or checkerboard pattern in front of a patient. For auditory evoked responses, a doctor produces a sound in one of the patient's ears. For bodily evoked responses, one of the nerves in an arm or leg is electrically stimulated. The responses from these sensory stimuli can indicate abnormal areas of the brain.

What tests show blood flow?

  • Doppler ultrasound test - Uses high-frequency sound waves to detect blockages in the carotid artery . A Doppler probe or instrument capable of generating ultrasound waves is placed on the neck very near to the carotid artery. Ultrasound waves from the probe travel through the neck and bounce off the moving blood cells. The reflected sound wave, now returning to the probe at a different frequency, is then detected by the same probe. The change in frequency of the sound waves relates to the speed of the blood cells and thus the blood flow.
  • Carotid phonoangiography - A sensitive microphone is placed on the neck, very close to the carotid artery, to record sounds. Ordinarily, in a normal artery, blood flows in a smooth and controlled manner. However, the presence of blockages, such as those caused by atherosclerosis, causes the blood flow to become turbulent. This turbulent blood flow can create a sound, called a bruit , that can be detected and registered by the microphone. A bruit indicates a blockage in the carotid artery and is cause for more tests.
  • Digital subtraction angiography (D.S.A.) - Gives an image of the brain's major blood vessels. A thin plastic tube (a catheter) is inserted into a major artery of the leg and navigated through the body's major vessels until it reaches the brain's blood vessels. A contrast dye is injected through the catheter and allowed to circulate in the bloodstream. At that point, an X-ray machine quickly takes a series of pictures of the head and neck. The images track the movement of the contrast dye as it moves through the brain's blood vessels. This imaging technique lets the doctor identify and localize the source of stroke.

What is atrial fibrillation?

Atrial fibrillation is a disorder found in about 2 million Americans. In it the two small upper chambers of the heart, the atria, quiver instead of beating effectively. Blood isn't pumped completely out of them when the heart beats, so it may pool and clot. If a piece of the blood clot in the atria becomes lodged in an artery in the brain, a stroke results. About 15 percent of strokes occur in people with atrial fibrillation.

How is atrial fibrillation treated?

Aspirin and warfarin are two medications currently used. They interfere with blood clotting, thus helping to reduce stroke risk in people with atrial fibrillation. A nationwide study reported in December 1996 found that less than 40 percent of patients with atrial fibrillation were taking warfarin. Anticoagulants are also given to people who have atrial fibrillation.

 

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