Ischemic Stroke
An ischemic stroke is the death of brain tissue (cerebral infarction) resulting from an inadequate supply of blood and oxygen to the brain.
Causes
Ischemic strokes result from blockage of the arteries that supply the brain, most commonly in the branches of the internal carotid arteries. The blockage usually results when a piece of a blood clot (thrombus) or of a fatty deposit (atheroma) due to atherosclerosis breaks off (becoming an embolus), travels through the bloodstream, and lodges in an artery that supplies the brain.
Blood clots may form when a fatty deposit in the wall of an artery ruptures. The rupture of such a fatty deposit may also form when a large fatty deposit (see Section 3, Chapter 32) slows blood flow, reducing it to a trickle, like the flow of water through a clogged pipe. Blood that flows slowly is more likely to clot. Thus, the risk of a clot forming in and blocking a narrowed artery is high.
Blood clots may also form in other areas, such as in the heart or on a heart valve. Strokes due to such blood clots are most common among people who have recently had heart surgery and people who have a heart valve disorder or an abnormal heart rhythm (arrhythmia), especially atrial fibrillation. Also, in certain disorders such as an excess of red blood cells (polycythemia), the risk of blood clots is increased because the blood is thickened.
Rarely, a condition that resembles a stroke results when small pieces of fat from the marrow of a broken long bone, such as one in an arm or a leg, are released into the bloodstream. These pieces can clump together and block an artery. This condition is called fat embolism syndrome.
An ischemic stroke can result if the blood flow to the brain is reduced, as may occur when a person loses a lot of blood or has very low blood pressure. Occasionally, an ischemic stroke occurs when blood flow to the brain is normal but the blood does not contain enough oxygen. Disorders that reduce the oxygen content of blood include severe anemia (a deficiency of red blood cells), suffocation, and carbon monoxide poisoning. Usually, brain damage in such cases is widespread (diffuse), and coma results.
An ischemic stroke can occur if inflammation or infection narrows blood vessels that supply the brain. Drugs such as cocaine and amphetamines cause spasm of the arteries, which can narrow the arteries supplying the brain and cause a stroke.
Symptoms
Most ischemic strokes begin suddenly, develop rapidly, and cause death of brain tissue within minutes to hours. Then most strokes become stable, causing little or no further damage. (Strokes that remain stable for 2 to 3 days are called completed strokes.) Such strokes are more likely to be due to sudden blockage by an embolus. Less commonly, strokes may continue to worsen for several hours to a day or two, as a steadily enlarging area of brain tissue dies. (Such strokes are called evolving strokes.) The progression is usually interrupted by somewhat stable periods, during which the area temporarily stops enlarging or some improvement occurs. Such strokes are more likely to be due to formation of clots in a narrowed artery.
Many different symptoms can occur, depending on which part of the brain is deprived of blood and oxygen. When the arteries that branch from the internal carotid artery are affected, blindness in one eye or abnormal sensations and weakness in one arm or leg or on one side of the body are most common. When the arteries that branch from the vertebral arteries in the back of the brain are affected, dizziness and vertigo, double vision, and generalized weakness of both sides of the body are more common. Many other symptoms, such as difficulty speaking (for example, slurred speech) and loss of coordination, can occur.
Large strokes may lead to stupor or coma. In addition, strokes, even small ones, can cause depression or an inability to control emotions (causing inappropriate crying or laughing).
Strokes can cause swelling in the brain due to accumulation of fluid (edema). Swelling in the brain is particularly dangerous because the skull does not expand. The resulting increase in pressure can cause the brain to shift and damage brain tissue further, making neurologic dysfunction worse, even if the area affected by the stroke itself does not enlarge. If the pressure becomes very high, the brain may be forced downward in the skull, resulting in herniation of the brain (see Section 6, Chapter 87).
People who are immobilized by a stroke may develop certain complications. They may inhale vomit or other irritating materials into the lungs, resulting in aspiration pneumonia. Being in one position too long can result in bedsores (pressure sores). Not being able to move the legs can result in the formation of blood clots in the deep veins of the legs and groin (deep vein thrombosis).
 See the sidebar Recognizing an Ischemic Stroke.
Diagnosis
Doctors can usually diagnose an ischemic stroke based on the history of events and results of a physical examination. Doctors can usually identify which artery in the brain is blocked based on neurologic symptoms (see Section 6, Chapter 82). For example, weakness or paralysis of the left leg suggests blockage of the artery supplying the area on the right side of the brain that controls the left leg's muscle movements. The sounds of turbulent blood flow (bruits) through the internal carotid arteries (heard using a stethoscope) may indicate narrowing.
Magnetic resonance imaging (MRI) or computed tomography (CT) is usually performed to confirm the diagnosis. Usually, MRI can detect ischemic strokes within minutes of their start. CT can detect ischemic strokes within 1 hour of their start. CT or MRI also helps distinguish an ischemic stroke from a hemorrhagic stroke, a brain tumor, an abscess, and other structural abnormalities. Cerebral angiography is performed when surgical removal of fatty deposits or clots (endarterectomy) is being considered or when inflammation of blood vessels (vasculitis) is suspected. Angiography provides detailed information about the blood supply to the brain (see Section 6, Chapter 77). Magnetic resonance angiography or color Doppler ultrasonography, both less invasive procedures than cerebral angiography, are helpful. These imaging procedures can show which large artery is blocked, but they cannot show the medium and small arteries that may be affected by vasculitis.
Identifying the precise cause of the stroke, particularly whether the blockage is a blood clot or a fatty deposit, is important. If the blockage is a blood clot, another stroke is very likely unless the underlying disorder is corrected. For example, blood clots may form in the heart because it is beating irregularly. Treating the irregular heartbeat can prevent new clots from forming and causing another stroke. If doctors suspect an irregular heartbeat, electrocardiography (ECG) is usually performed to look for abnormal heart rhythms. Other procedures used to diagnose heart disorders may also be performed. They include continuous ambulatory ECG (see Section 3, Chapter 21), in which a Holter monitor records the heart rate and rhythms continuously for 24 hours, and echocardiography (see Section 3, Chapter 21), which produces images of the chambers and valves of the heart.
Blood tests are performed to be sure that the stroke was not caused by a deficiency of red blood cells (anemia), polycythemia, cancer of the white blood cells (leukemia), or an infection. Rarely, a spinal tap (lumbar puncture) is performed--for example, after CT, when doctors still need to determine whether a stroke is due to an infection (such as herpes simplex) or whether a subarachnoid hemorrhage is present (see Section 6, Chapter 86). This procedure can be performed only if doctors are sure that the brain is not under excess pressure (usually determined by CT or MRI).
Treatment
Symptoms that suggest an ischemic stroke require immediate medical attention; doctors can sometimes reduce the damage or prevent further damage by acting quickly.
When a person who has had a stroke arrives at the hospital, the first steps in treatment are to restore the person's breathing, heart rate, blood pressure, and temperature to normal. Fluids are given if blood pressure is low. Drugs (such as a beta-blocker) are given to stabilize the heart rate if it is too fast; a pacemaker may be inserted if the heart rate is too slow. If the person has a fever, it may be lowered using acetaminophen, ibuprofen, or a cooling blanket. An increase in body temperature by even a few degrees can dramatically worsen brain damage due to an ischemic stroke. Oxygen is usually administered through a face mask or nasal prongs, and an intravenous line to provide fluids and drugs is inserted. Generally, doctors do not immediately treat high blood pressure unless blood pressure is higher than 170/110 mm Hg, because if blood pressure becomes too low, brain tissue may not receive enough oxygen-rich blood.
A drug that breaks up clots (a thrombolytic drug), such as tissue plasminogen activator (tPA), can be given intravenously to help restore blood flow to the brain. Because thrombolytic drugs can cause bleeding in the brain and elsewhere, they should not be given to people who have had a hemorrhagic stroke. So before a thrombolytic drug is given, CT or MRI is performed to make sure that there is no bleeding in the brain. To be effective, a thrombolytic drug given intravenously must be started within 3 hours of the beginning of an ischemic stroke. However, most people who have had a stroke arrive at the hospital between 3 to 6 hours afterward--too late to be given a thrombolytic drug intravenously. Some of such people can be given a thrombolytic drug through an artery rather than through a vein (intravenously), so that a more concentrated dose of the drug can be applied directly to the clot. To give the drug through an artery, doctors make an incision in the skin and insert a thin, flexible tube (catheter) into an artery. The catheter is then threaded through other arteries to the clot.
For an evolving stroke, anticoagulants such as heparin may be given, but their effectiveness has not been proved. However, after the stroke is completed, anticoagulants are given to prevent subsequent strokes in people who have atrial fibrillation or a heart valve disorder. Because these drugs increase the risk of bleeding into the brain, doctors usually wait at least 24 hours after thrombolytic therapy is ended before anticoagulants are started. Anticoagulants are not given to people who have uncontrolled high blood pressure or who have had a hemorrhagic stroke.
Other new experimental measures that may improve the chances of a favorable outcome involve blocking the receptors of certain neurotransmitters in the brain. However, these measures are not yet available for routine use.
Once an ischemic stroke is completed, some brain tissue is dead, so reestablishing its blood supply by surgical removal of the blockage (endarterectomy) in an internal carotid artery cannot restore the lost function. Therefore, endarterectomy is not usually performed. However, removing blockages after a small stroke may reduce the risk of subsequent strokes.
If a stroke is very severe, drugs such as mannitol may be given to reduce swelling and the increased pressure on the brain. Some people need a ventilator to breathe adequately.
Measures to prevent aspiration pneumonia (see Section 4, Chapter 42) and bedsore (pressure sores (see Section 18, Chapter 205)) are started early. Heparin, injected under the skin, may be given to help prevent deep vein thrombosis (see Section 3, Chapter 36). The person is closely monitored to determine whether the bladder and intestines are functioning. Often, other disorders such as heart failure, abnormal heart rhythms, and lung infections must be treated. High blood pressure is often treated after the stroke has been stabilized. Because a stroke often causes mood changes, especially depression, family or friends should inform the doctor if the person seems depressed. Depression can be treated with drug therapy and psychotherapy (see Section 7, Chapter 101).
After a stroke, some people are given antiplatelet drugs or the anticoagulant warfarin to help prevent subsequent strokes.
Prognosis
About 10% of people who have an ischemic stroke recover almost all normal function, and about 25% recover most of it. About 40% of people have moderate to severe impairments requiring special care, and about 10% require care in a nursing home or other long-term care facility. Some people are physically and mentally devastated and unable to move, speak, or eat normally. About 15% of people who have a stroke die in the hospital. The proportion is higher among older people.
During the first few days after an ischemic stroke, doctors usually cannot predict whether a person will improve or worsen. About 50% of people with one-sided paralysis and most of those with less severe symptoms recover some function by the time they leave the hospital, and they can eventually take care of their basic needs. They can think clearly and walk adequately, although they may have limited use of the affected arm or leg. Use of an arm is more often limited than use of a leg.
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