Ischemic Strokes And Tias - Subtypes And Causes Of Stroke: Stroke

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Ischemic strokes and TIAs

Each year about 560,000 people in the United States have ischemic strokes. But not all ischemic strokes are alike: Each subtype develops for different reasons and poses particular challenges (see "Ischemic stroke subtypes"). Although the exact treatment will depend on the type of ischemic stroke you have, clot-busting drugs and anticoagulants (antithrombotics), followed by preventive medications, make up the most common treatment regimen (see "Treating ischemic stroke").

Ischemic stroke subtypes Large-artery atherothrombotic stroke (A) most often occurs when a blood clot forms in an artery supplying blood to the brain and breaks off, traveling downstream to lodge in an area where the artery is already narrowed by plaque. These account for roughly 15% of ischemic strokes. Small-artery or lacunar stroke is caused by the blockage of one of the smaller blood vessels that arise from the main arteries at the base of the brain. A small hole made of scar tissue may result. These account for about 25% of ischemic strokes. Embolic stroke (B) occurs when a clot that has formed elsewhere in the body — typically in the heart or the ascending part of the aorta — breaks off and travels through the bloodstream until it blocks an artery to the brain. Embolic stokes account for almost 60% of ischemic strokes. Dissection stroke occurs because of a tearing, or dissection, of the carotid artery. This is one of several other types of pathologies that account for less than 3% of ischemic strokes.

Large-artery atherothrombotic strokes

Atherosclerosis is a medical term for the buildup of fatty deposits in the arteries. When this process occurs in arteries feeding the heart, it accounts for more than 90% of heart attacks. But when the same process affects arteries in the brain, it accounts for only 15% of ischemic strokes. That is because the brain has a rich supply of collateral blood vessels, the most significant being the Circle of Willis. These other blood vessels take over and compensate for the vessel that becomes clogged with plaque through the atherosclerotic process.

Strokes that are caused by atherosclerosis are known as large-artery atherothrombotic strokes. These types of stroke are often heralded by a TIA. Although atherosclerotic plaque can form in any artery, it may lead to stroke when it occurs at any of four strategically focal locations: the origin of the internal carotid artery, the siphon portion of the internal carotid artery, the middle cerebral artery stem, and the junction of the distal vertebral arteries and the basilar artery (see Figure 5).

Figure 5: Blood vessels affected by ischemic stroke Large-artery atherothrombotic strokes generally occur at the origin of the internal carotid artery, the siphon portion of the internal carotid artery, the middle cerebral artery stem, or the junction of the distal vertebral arteries and the basilar artery. Small-vessel lacunar strokes typically result from obstruction of a single blood vessel that branches off from the Circle of Willis, the middle cerebral artery stem, the basilar artery, or the distal vertebral arteries. Embolic strokes are caused when a blood clot obstructs a major blood vessel at the base of the brain, such as the basilar artery or the middle cerebral stem. Dissection strokes occur after a large extracranial artery, such as the carotid or vertebral artery, tears.

Sometimes atherosclerotic plaques cause stroke or a TIA by producing a critical narrowing in the artery, limiting blood flow to the specific area of the brain that is supplied by that artery. This is known as a low-flow stroke and generally occurs when collateral flow to the same area through other vessels, such as the Circle of Willis, is limited. These strokes are usually progressive.

But atherosclerotic plaques are not just passive plugs that block arteries like a cork in a bottle. They are active, dynamic lesions teaming with cholesterol and inflammatory cells. The most common type of large-artery atherothrombotic stroke occurs when a plaque deposit ruptures and develops a blood clot (thrombus). Although the blood clot may not plug the artery completely, it can break off (embolize) and travel from the site of origin to the part of the brain being supplied by that particular artery. This is known as artery-to-artery embolic stroke. At other times, the blood clot may grow large enough to obstruct the blood vessel at the site of the atherosclerotic process, and then extend up into the brain to precipitate a stroke. This occurs mainly in the basilar artery and somewhat less commonly in the carotid artery or middle cerebral artery stem.

Damage from large artery-to-artery embolic stroke may grow worse for three reasons. First, the same underlying mechanism, atherosclerosis, may trigger a second stroke and expand the area of blocked blood flow. Second, brain swelling can occur as a result of the initial stroke and expand pressure inside the skull. In this case, the surgeons may have to remove part of the skull to relieve pressure caused by the swelling. Third, the blood clot that broke off and triggered an artery-to-artery embolic stroke can further fragment. Although this restores blood flow to the damaged brain, the sudden increase in blood flow can also give rise to little points of bleeding in smaller blood vessels such as arterials and capillaries. When this occurs, it is known as hemorrhagic conversion of an ischemic stroke. When the bleeding is severe and forms a large clot, brain compression results.

As might be expected, the advice about how to prevent large-artery atherothrombotic stroke is the same advice given to people at risk for heart attack: Most important, keep blood pressure and cholesterol levels within healthy parameters, stop smoking, and lose excess weight. For more information, and other advice about prevention, see "Preventing stroke."

Small-vessel lacunar stroke

Lacunar strokes, sometimes known as small-vessel strokes, account for 25% of ischemic strokes. This type of stroke is often heralded by a TIA. A lacunar stroke results from the obstruction of a single blood vessel that branches off from the Circle of Willis, the middle cerebral artery stem, the basilar artery, or the distal vertebral arteries (see Figure 5). These arteries are single vessels that penetrate into the brain. They do not branch. Such vessels may become obstructed in several ways.

Sometimes a small penetrating vessel becomes damaged by a unique pathologic process known as lipohyalinosis, in which endothelial cells (cells that line the blood vessel walls) are injured and degenerate as a result of persistent hypertension. Less commonly, atherosclerotic plaque can originate at the penetrating artery origin and obstruct it. In rare instances, a tiny embolic fragment may break away from a blood clot elsewhere and then travel down the arterial tree to block a tiny penetrating vessel.

However it occurs, the blockage starves a small part of the brain of blood flow, leaving a tiny area of scar tissue. Because the area of the damaged or infarcted brain is small, recovery is often significant. But some lacunar strokes can lead to disabling paralysis or a painful sensory syndrome.

Fortunately, in some cases prevention is possible. The lipohyalinotic process responsible for many lacunar strokes is directly related to hypertension and diabetes, which can damage blood vessel walls. Controlling blood pressure is the most important step you can take to help prevent this type of lacunar stroke (see "Preventing stroke").

Embolic stroke

Embolic strokes account for almost 60% of all ischemic strokes in the United States. This type of stroke occurs when an embolic fragment, usually a blood clot, breaks away from the heart, aorta, or an unknown source. When the blood clot is small, it travels to a more distant vessel in the brain and blocks it, resulting in a smaller stroke and limited area of brain damage. Larger clots, however, can block larger blood vessels and cause more damage. If the clot is between 2½ and 3 millimeters (about the size of the tip of a pencil), it can block major blood vessels at the base of the brain, such as the basilar artery or the middle cerebral stem (see Figure 5). The result can be a devastating and disabling stroke. The clots can develop out of the blue, changing a person's life forever. Prevention, whenever possible, is vital.

Treatment of embolic stroke may involve clot-busting agents such as tissue plasminogen activator, anticoagulation therapy with heparin and warfarin (Coumadin), and antiplatelet therapy with aspirin or other agents (see "Treating ischemic stroke").

Although not all embolic strokes can be prevented, some can. Although it has not yet been proved, strokes that occur because of atherosclerosis in the aorta possibly may be prevented by taking some of the same steps to reduce your risk of heart disease: controlling high blood pressure, keeping cholesterol levels healthy, and not smoking.

It is also possible to prevent embolic strokes that are caused by fragments originating inside the heart, particularly in the atria and the left atrial appendage. This type of stroke is most likely to occur in people with a heart rhythm disturbance known as atrial fibrillation (the most common cause of embolic stroke, accounting for fully 15% of all ischemic strokes). Studies at Massachusetts General Hospital and elsewhere have clearly demonstrated that, in people with atrial fibrillation who are over 65, anticoagulation therapy with warfarin (Coumadin) can be highly protective against embolic strokes. The MGH study, for example, found that people with atrial fibrillation who took warfarin were 86% less likely than others with this heart condition to suffer an embolic stroke.

The use of warfarin and other anticoagulants to prevent other types of cardiac-source embolic strokes is less well studied. However, warfarin may also be used in people who have recently received a heart-valve replacement, suffered a heart attack, or have poor cardiac output or a clot inside the heart.

People with other types of heart conditions may also be at increased risk of embolic stroke, but it is unclear whether the best approach is to use anticoagulation or antiplatelet therapy, or to recommend invasive therapies such as surgery. One example: People with a hole in the heart between the right and left atrial chambers, a condition called patent foramen ovale, are at increased risk for an embolic stroke. Possible treatments include antiplatelet therapy with aspirin, anticoagulant therapy with warfarin (Coumadin), or a surgical procedure to close the hole. The issue is under study, but until more is known, doctors must make their therapy decisions based on each patient.

Other subtypes of ischemic stroke

Other subtypes of ischemic stroke include inflammation of small blood vessels known as arteritides, genetic factors affecting small blood vessels, or trauma leading to dissection (tearing) of a large extracranial artery, such as the carotid or vertebral artery (see Figure 5). These other subtypes account for less than 3% of ischemic strokes. The likelihood of experiencing one of these stroke subtypes can vary, depending on your risk for developing the specific pathologic processes that can trigger them. Age, race, and some as yet unknown genetic factors all play a role.

Dissection lesions are extremely important because they involve major arteries, and therefore may cause major damage. In this type of stroke, the blood vessel tears (dissects), damaging the inside of the artery. A blood clot, or thrombus, develops, which may then break off and travel to the brain. Low-flow or embolic TIAs often herald a more devastating stroke.

A dissection stroke can occur out of the blue and often affects younger people. In fact, it is one of the most significant causes of devastating ischemic stroke in children and young adults who are very active. This type of stroke can be caused by arterial injury due to whiplash, chiropractic neck manipulation, or a trauma suffered during vigorous exercise (such as a skiing accident or strenuous weightlifting). Even serious retching or coughing may be enough to tear a blood vessel.

The best way to prevent a dissection stroke is to prevent trauma to the arteries. Anticoagulation therapy may help prevent further clots or embolism (see "Treating ischemic stroke").

Last updated:	September 05, 2008