Human BrainThe human brain has three major structural components: the large dome-shaped cerebrum (top), the smaller somewhat spherical cerebellum (lower right), and the brainstem (center). Prominent in the brainstem are the medulla oblongata (the egg-shaped enlargement at center) and the thalamus (between the medulla and the cerebrum). The cerebrum is responsible for intelligence and reasoning. The cerebellum helps to maintain balance and posture. The medulla is involved in maintaining involuntary functions such as respiration, and the thalamus acts as a relay center for electrical impulses traveling to and from the cerebral cortex. Lack of blood flow to any part of the brain results in a stroke, permanent damage that interferes with the functions of the affected part of the brain.London Scientific Films/Oxford Scientific Films
Stroke, brain damage caused by a lack of blood flow to part of the brain. In order to perform its many functions and direct activities throughout the body—from walking to seeing to reasoning—the brain requires a constant supply of energy, provided by the oxygen and nutrients that are delivered by the flowing blood. If blood flow is restricted or cut off at any point between the heart and the brain, portions of the brain relying on blood from the obstructed blood vessel become deprived of oxygen. Brain cells are extremely sensitive to such oxygen deprivation, and if they are deprived of oxygen and nutrients for more than several minutes, they, in effect, starve to death. A stroke results in permanent damage to the brain tissue—and in many cases, permanent disability for the patient. For example, a patient who has had a stroke may develop paralysis on one or both sides of the body; have difficulty with walking, eating, or other daily activities; or lose the ability to speak or understand speech.
ErythrocytesErythrocytes, or red blood cells, are the primary carriers of oxygen to the cells and tissues of the body. The brain requires a constant supply of oxygen carried by red blood cells. If the flow of blood is blocked for more than a few moments, brain cells will die.Dr. Tony Brain/Science Source/Photo Researchers, Inc.
Stroke is the third leading cause of death and a leading cause of serious, long-term disability in the United States. According to the American Heart Association, approximately 700,000 Americans suffer a stroke each year; about 25 percent of these strokes are fatal. Someone suffers a stroke every 45 seconds, and every 3.1 minutes someone dies of a stroke. Stroke is responsible for an estimated $40 billion in health-care costs and lost productivity each year.
II TYPES OF STROKE
Arterial PlaqueAtherosclerosis, or the narrowing of arteries due to the buildup of plaque along the inner lining, is the single most lethal condition in the United States. The plaques consist principally of fat and cholesterol deposits but also contain blood platelets, decomposing muscle cells, and other tissue. Since plaques usually reduce blood flow in major arteries, their presence represents a serious health risk, leading to heart disease, stroke, and the disruption of kidney and intestinal function. Poor circulation, also a result of plaque buildup, impairs movement of the limbs. Fragments of the plaques may break off and travel through the bloodstream to obstruct smaller vessels. The plaques unfortunately become larger and more numerous with age, especially in people with high levels of cholesterol in their diet and bloodstream.Martin M. Rotker/Science Source/Photo Researchers, Inc.
Think of a blood vessel as a flexible, cylindrical tube, like a straw. The flow of liquid through a straw can be impeded in two different ways: by an obstruction within the straw, or by compression or pinching from outside the straw. The flow of blood through a blood vessel can also be blocked in these two ways. The two main types of stroke, ischemic stroke and hemorrhagic stroke, correspond to these two mechanisms of flow interruption.
A Ischemic Strokes
Ischemic strokes, which account for about 80 percent of all strokes, are caused by an obstruction in an artery, generally one of the carotid arteries, the major arteries in the neck that carry oxygen-rich blood from the heart to the brain. The path to an ischemic stroke begins when atherosclerosis, in which fatty deposits build up on the inner wall of an artery, develops in one of the carotid arteries (see Arteriosclerosis). As the fatty deposit grows, it narrows the space through which blood can flow.
Atherosclerosis does not actually cause ischemic strokes, but it sets up the conditions that make them likely to occur. The actual obstruction that cuts off blood flow in an ischemic stroke is a blood clot. Often the obstruction develops by a process known as thrombosis, the formation of a clot inside a blood vessel. A clot is likely to form at the site of an atherosclerotic deposit because the deposit causes blood to flow in a turbulent, disorderly fashion. This turbulence can cause blood to clot just as it does in response to a wound. When the blood clot, or thrombus, develops at the site of an atherosclerotic deposit and cuts off blood flow to part of the brain, a stroke results.
An ischemic stroke can also be caused by a traveling clot, or embolus (see Embolism). In this case, the clot develops at some other location in the circulation, usually in one of the heart’s chambers. The clot then travels through the bloodstream until it encounters a vessel too small to let it pass through—often a vessel narrowed by atherosclerosis.
A transient ischemic attack (TIA) sometimes precedes an ischemic stroke. In a TIA, also known as a ministroke, strokelike symptoms develop but disappear within five minutes to 24 hours. TIAs can occur when a clot develops at the site of an atherosclerotic deposit but dissolves right away, or an embolism lodges in a narrowed vessel but is soon dislodged on its own. A TIA can also be caused by atherosclerosis alone when the narrowing of blood vessels by atherosclerosis restricts blood flow to part of the brain enough to cause strokelike symptoms. Regardless of the cause, the oxygen deprivation is not severe enough to kill brain cells, and the cells are able to bounce back from their injury. About 10 percent of ischemic strokes are preceded by TIAs.
B Hemorrhagic Strokes
Hemorrhagic strokes account for the remaining 20 percent of all strokes. They occur when weakened blood vessels within the brain rupture and bleed into the surrounding tissue. The escaped blood can compress or pinch nearby blood vessels, cutting off blood flow and depriving the surrounding tissue of oxygen. Though hemorrhagic strokes occur less frequently than ischemic strokes, they tend to affect larger areas of the brain. Symptoms of a hemorrhagic stroke may be more sudden and more severe, and these strokes carry a greater risk of death than ischemic strokes.
Hemorrhagic strokes can result from an aneurysm, which develops when the wall of a blood vessel weakens and thins, ballooning outward. If left untreated, the aneurysm continues to expand and weaken, increasing the likelihood of rupture. Hemorrhagic strokes can also result from an arteriovenous malformation (AVM), a cluster of enlarged, structurally weak blood vessels that forms during fetal development or possibly at birth. These malformed blood vessels are susceptible to rupture from the normal forces exerted by the flowing blood.
III SYMPTOMS AND CONSEQUENCES OF STROKE
Nerve CellNerve cells transmit information from one part of the body to another. Each nerve cell has branching dendrites to connect to other dendrites, and a long axon to transmit or collect impulses. A stroke, in which nerve cells in the brain die for lack of oxygen, can result in permanent disability for the patient because the pathways that transmit information in the brain are interrupted.Francois Paquet-Durand/Photo Researchers, Inc.
A key feature of stroke symptoms is that they are unexpected and develop suddenly, though they may worsen over the next several hours or days. The symptoms often primarily affect only one side of the body because blood flow is cut off to only part of the brain during a stroke. One of the most common symptoms is a sudden weakness or numbness of one side of the face or of one arm or leg. Some stroke sufferers experience a sudden dimness or loss of vision, particularly in only one eye. They may also suddenly become unable to speak or have trouble understanding speech. Sudden, severe headaches with no known cause and sudden, unexplained dizziness, unsteadiness, or falls, can also be warning signs of a stroke. Anyone who experiences one or more of these symptoms should seek medical attention immediately.
Functions of the Cerebral CortexThe effects of a stroke depend on which area of the brain is deprived of oxygen. Many motor and sensory functions have been “mapped” to specific areas of the cerebral cortex, some of which are indicated here. In general, these areas exist in both hemispheres of the cerebrum, each serving the opposite side of the body. Less well defined are the areas of association, located mainly in the frontal cortex, operative in functions of thought and emotion and responsible for linking input from different senses. The areas of language are an exception: both Wernicke’s area, concerned with the comprehension of spoken language, and Broca’s area, governing the production of speech, have been pinpointed on the cortex.© Microsoft Corporation. All Rights Reserved.
The majority of stroke patients are left with some form of permanent disability that interferes with normal daily activities, such as walking, speech, vision, understanding, reasoning, and memory. The specific effects of a stroke vary greatly depending on what part of the brain was deprived of oxygen. For example, if blood flow is cut off from an area of the brain that controls speech, the stroke will result in a speech disability such as slurring or aphasia—the inability to express oneself through speech or writing, or the inability to understand speech.
If the stroke damages the area of the brain that controls motor skills, a patient may have trouble walking or moving an arm. Various forms of paralysis—especially hemiplegia, or paralysis affecting one side of the body—are common after a stroke. In addition to its physical consequences, a stroke can also have psychological effects. People who have had a stroke often become depressed or feel angry and frustrated at their inability to perform tasks that, before the stroke, were easy or automatic.
Magnetic Resonance Imaging ScanThis magnetic resonance imaging (MRI) scan of a normal adult head shows the brain, airways, and soft tissues of the face. The large cerebral cortex, appearing in yellow and green, forms the bulk of the brain tissue; the circular cerebellum, center left,in red, and the elongated brainstem, center, in red, are also prominently shown. MRI scans and other types of brain imaging help physicians diagnose strokes.CNRI/Science Source/Photo Researchers, Inc.
The symptoms typical of a stroke can also be caused by other conditions, including brain tumors, various infections, and overdoses of certain drugs. A patient that has strokelike symptoms may undergo a battery of imaging techniques to rule out other medical problems and confirm that a stroke has occurred. The key imaging technique used in diagnosing strokes is computed tomography (also known as CT or CAT scanning), which employs X rays to obtain images of the internal structures of the body. A CT scan can tell a doctor whether or not a stroke is occurring, whether the stroke is ischemic or hemorrhagic, and in most instances, the extent of brain damage caused by the stroke.
Several other imaging techniques are used along with computed tomography to gather more information about specific types of stroke. Magnetic resonance imaging (MRI) uses a magnetic field to generate images of the human body and produces high-resolution images that are particularly useful in diagnosing brain vessel abnormalities that may be involved in a hemorrhagic stroke. In X-ray angiography, a dye injected into the bloodstream is viewed using X rays to provide detailed images of blood vessels, enabling doctors to identify the source and location of an obstruction or gather anatomical information about aneurysms or AVMs. Two other imaging techniques, single photon emission computed tomography (SPECT) and positron emission tomography (PET), involve injecting a radioactive substance into the bloodstream. As the substance travels through the circulatory system, it constantly emits radiation, which is collected by a radiation detector. The images produced enable doctors to see regions of the brain with abnormally low blood flow, indicating brain tissue that has been injured or damaged by a stroke.
V TREATMENT AND RECOVERY
Occupational TherapyRehabilitation after a stroke may help some patients regain lost functions or learn new ways of accomplishing everyday tasks. Occupational therapy, pictured here, helps patients relearn daily activities such as dressing, cooking, and eating.Will and Deni McIntyre/Photo Researchers, Inc.
Once doctors have established that a patient is having a stroke, the treatment focuses on removing the obstruction, restoring blood flow to the deprived region of the brain, and preventing the development of complications. In the past, doctors could do little to treat stroke patients until the stroke had run its course. However, a promising advance occurred in June 1996, when the Food and Drug Administration (FDA) approved the drug tissue plasminogen activator (t-PA) for use in treating ischemic strokes. The drug is a thrombolytic agent—or clot buster—that can break up blood clots and thereby restore flow through the obstructed blood vessel when administered within the first three hours of a stroke. This small window of effectiveness makes it more critical than ever for patients to seek immediate medical attention when strokelike symptoms develop.
After a patient is stabilized, recovery and rehabilitation begins. Along with the stroke survivor, family, friends, support groups, and a variety of health-care professionals are involved in this long and often difficult process. In some cases an undamaged portion of a patient’s brain is able to take over the functions that were previously performed by the damaged portion of the brain, and the patient regains the functions that were lost due to the stroke. Rehabilitation can also help restore lost functions. Physical therapy helps patients regain the use of their limbs and the ability to walk, and helps prevent muscle stiffening in patients who are paralyzed. Speech therapy can help patients regain the ability to speak. Occupational therapy improves patients’ hand-eye coordination and helps them relearn basic activities, such as dressing, cooking, and writing.
VI RISK FACTORS AND PREVENTION
Blood Pressure CheckA patient’s blood pressure is checked through the use of a sphygmomanometer. High blood pressure, or hypertension, is one of the primary risk factors for stroke. However, many patients can control their blood pressure through diet or medication and reduce their risk of suffering a stroke.John Greim/Medichrome/The Stock Shop
Scientists have identified a number of factors that increase a person’s risk for stroke. The probability of stroke increases as people get older, with those over age 65 at greatest risk. The incidence of stroke in males and females is about equal, although females are more likely to die from stroke than males. As is the case with many diseases and medical conditions, people with a family history of stroke are more likely to have a stroke themselves. Finally, a person who has already had a stroke or TIA is at greater risk of having another stroke in the future.
Fortunately, a healthy lifestyle can help minimize some risk factors for stroke, such as high blood pressure, smoking, and high blood cholesterol. High blood pressure places abnormally high stress on the walls of blood vessels which, over time, weakens and damages the vessels. Certain chemicals found in cigarette smoke can affect the properties of the blood, making it more prone to forming clots. In addition, the nicotine in cigarette smoke damages blood vessel walls and makes them more susceptible to atherosclerosis. And when excess cholesterol is present in the bloodstream, it accumulates along the walls of blood vessels, accelerating the progression of atherosclerosis and thereby increasing the risk of a stroke. Quitting smoking, controlling high blood pressure through diet or medication, eating a low-fat, healthy diet, and becoming physically active can greatly decrease a person’s risk of having a stroke.
For people who have blood vessel abnormalities that can lead to a stroke, or who have already had a stroke or TIA, a variety of steps can be taken to prevent a first or second stroke from occurring. To prevent the blood vessel obstructions that cause ischemic strokes, physicians try to maximize blood flow through a narrowed vessel and minimize the risk of clotting. One common treatment is carotid endarterectomy, a surgical procedure that removes an atherosclerotic deposit from a diseased carotid artery. Sometimes anticoagulant drugs such as aspirin and warfarin are prescribed to prevent the formation of clots in patients at risk for thrombus or embolism formation.
To prevent rupture of the weakened blood vessels that cause hemorrhagic stroke, physicians try to provide an obstruction to protect a weakened vessel from further damage and redirect blood flow through normal vessels feeding the same regions of the brain. An aneurysm can be treated surgically by placing a metal clip at its base to seal off the weakened vessel. An AVM, on the other hand, is cut away from the normal brain tissue and removed. An alternative to surgery, which attempts to treat the diseased blood vessel from the outside, is endovascular therapy, which addresses the problem from within the vessel. In a typical endovascular procedure, a catheter—a long, thin, flexible tube—is inserted into a major artery, usually in the thigh, and guided through the blood vessels to the location of the aneurysm or AVM. Tiny platinum coils are then deposited from the end of the catheter into the diseased vessel, providing a physical barrier to prevent the forces exerted by blood flow from further damaging the weakened vessel. These treatments can be beneficial even when performed after a hemorrhagic stroke because they can prevent an aneurysm or AVM from rupturing a second time and causing another stroke.
George James Hademenos