brain, spinal cord, and nerve disorders
Ischemic Stroke
Causes of Ischemic Stroke
An ischemic stroke typically results from blockage of an artery that supplies blood to the brain, most commonly a branch of one of the internal carotid arteries. As a result, brain cells are deprived of blood. Most brain cells die if they are deprived of blood for 4.5 hours. Blood is supplied to the brain through two pairs of large arteries: Internal carotid arteries, which carry blood from the heart along the front of the neck Vertebral arteries, which carry blood from the heart along the back of the neck In the skull, the vertebral arteries unite to form the basilar artery (at the back of the head). The internal carotid arteries and the basilar artery divide into several branches, including the cerebral arteries. Some branches join to form a circle of arteries (circle of Willis) that connect the vertebral and internal carotid arteries. Other arteries branch off from the circle of Willis like roads from a traffic circle. The branches carry blood to all parts of the brain. When the large arteries that supply the brain are blocked, some people have no symptoms or have only a small stroke. But others with the same sort of blockage have a massive ischemic stroke. Why? Part of the explanation is collateral arteries. Collateral arteries run between other arteries, providing extra connections. These arteries include the circle of Willis and connections between the arteries that branch off from the circle. Some people are born with large collateral arteries, which can protect them from strokes. Then when one artery is blocked, blood flow continues through a collateral artery, sometimes preventing a stroke. Other people are born with small collateral arteries. Small collateral arteries may be unable to pass enough blood to the affected area, so a stroke results. The body can also protect itself against strokes by growing new arteries. When blockages develop slowly and gradually (as occurs in atherosclerosis), new arteries may grow in time to keep the affected area of the brain supplied with blood and thus prevent a stroke. If a stroke has already occurred, growing new arteries can help prevent a second stroke (but cannot reverse damage that has been done). Commonly, blockages are blood clots (thrombi) or pieces of fatty deposits (atheromas, or plaques) due to Atherosclerosis . Such blockages often occur in the following ways: An atheroma in the wall of an artery may continue to accumulate fatty material and become large enough to block the artery. Even if the artery is not completely blocked, the atheroma narrows the artery and slows blood flow through it, like a clogged pipe slows the flow of water. Slow-moving blood is more likely to clot. A large clot can block enough blood flowing through the narrowed artery that brain cells supplied by that artery die. Or if an atheroma splits open (ruptures), the material in it can trigger formation of a blood clot that can block the artery (see figure How Atherosclerosis Develops ). By traveling from another artery to an artery in the brain:A piece of an atheroma or a blood clot in the wall of an artery can break off and travel through the bloodstream (becoming an embolus). The embolus may then lodge in an artery that supplies the brain and block blood flow there. (Embolism refers to blockage of arteries by materials that travel through the bloodstream to another part of the body.) Such blockages are more likely to occur where arteries are already narrowed by fatty deposits. Blood clots may form in the heart or on a heart valve, particularly artificial valves and valves that have been damaged by infection of the heart's lining ( Infective Endocarditis ). These clots may break off and travel as emboli and block an artery to the brain. Strokes due to such blood clots are most common among people who have recently had heart surgery, who have had a heart attack, or who have a heart valve disorder or an abnormal heart rhythm (arrhythmia), especially a fast, irregular heart rhythm called When an artery that carries blood to the brain becomes clogged or blocked, an ischemic stroke can occur. Arteries may be blocked by fatty deposits (atheromas, or plaques) due to atherosclerosis. Arteries in the neck, particularly the internal carotid arteries, are a common site for atheromas. Arteries may also be blocked by a blood clot (thrombus). Blood clots may form on an atheroma in an artery. Clots may also form in the heart of people with a heart disorder. Part of a clot may break off and travel through the bloodstream (becoming an embolus). It may then block an artery that supplies blood to the brain, such as one of the cerebral arteries. Blood clots in a brain artery do not always cause a stroke. If the clot breaks up spontaneously within less than 15 to 30 minutes, brain cells do not die and people's symptoms resolve. Such events are called Transient Ischemic Attacks (TIAs) (TIAs). If an artery narrows very gradually, other arteries (called collateral arteries—see figure Supplying the Brain With Blood ) sometimes enlarge to supply blood to the parts of the brain normally supplied by the clogged artery. Thus, if a clot occurs in an artery that has developed collateral arteries, people may not have symptoms. The most common causes of ischemic stroke can be classified as Cryptogenic stroke Embolic stroke Lacunar infarction Large-vessel atherosclerosis (the 4th most common cause) Stroke is classified as cryptogenic when no clear cause is identified despite a complete evaluation. Blood clots can form in the heart, especially in people who have or have had the following: Atrial Fibrillation and Atrial Flutter None Heart (usually Acute Coronary Syndromes (Heart Attack; Myocardial Infarction; Unstable Angina) None Infective Endocarditis None Noncancerous primary heart tumors None Repairing or replacing a heart valve None Mechanical circulatory assist devices (such as a left Heart Transplantation ) Tiny pieces of these blood clots can break off and travel to small arteries in the brain (as emboli). Lacunar infarction refers to tiny ischemic strokes, typically no larger than about a third of an inch (1 centimeter). In lacunar infarction, one of the small arteries deep in the brain becomes blocked when part of its wall deteriorates and is replaced by a mixture of fat and connective tissue—a disorder called lipohyalinosis. Lipohyalinosis is different from atherosclerosis, but both disorders can cause arteries to be blocked. Lacunar infarction can also occur when tiny pieces of fatty material that has been deposited in arteries (atheromas or Plaque formation ) break off and travel to small arteries in the brain. Lacunar infarction tends to occur in older people with diabetes or poorly controlled high blood pressure. Only a small part of the brain is damaged in lacunar infarction, and the prognosis is usually good. However, over time, many small lacunar infarcts may develop and cause problems, including problems with thinking and other mental functions (cognitive impairment). In large-vessel Atherosclerosis , atherosclerotic plaques develop in the walls of large arteries, such as those that supply the brain (cerebral arteries). The plaques can gradually enlarge and cause the artery to narrow. As a result, tissues supplied by the artery may not receive enough blood and oxygen. Plaques tend to split open (rupture). Then material inside the plaque is exposed to the bloodstream. The material triggers the formation of blood clots (called thromboses). These blood clots can suddenly block all blood flow through an artery. Sometimes the blood clots break off, travel through the bloodstream and block an artery that supplies blood to the brain (called emboli). Both thromboses and emboli can cause a stroke by blocking the blood supply to an area of the brain. Several conditions besides rupture of an atheroma can trigger or promote the formation of blood clots, increasing the risk of blockage by a blood clot. They include the following: Some disorders, such as an excess of red blood cells ( Polycythemia Vera ), Oral Contraceptives : An ischemic stroke can also result from any disorder that reduces the amount of blood supplied to the brain. For example, An ischemic stroke can occur if inflammation of blood vessels ( Overview of Vasculitis ) or infection (such as In Atrial Fibrillation and Atrial Flutter , the heart does not contract normally, and blood can stagnate and clot. A clot may break loose, then travel to an artery in the brain, and block it. Sometimes the layers of the walls of an artery that carries blood to the brain (such as arteries in the neck) separate from each other (called dissection) and interfere with blood flow to the brain. Migraine headaches or drugs such ascocaineand amphetamines can cause spasm of the arteries, which can narrow the arteries supplying the brain long enough to cause a stroke. Rarely, a stroke results from a general decrease in blood flow, as occurs when people lose a lot of blood, become severely dehydrated, or have very low blood pressure. This type of stroke often occurs when arteries supplying the brain are narrowed but had not previously caused any symptoms and had not been detected. 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 a severe deficiency of red blood cells ( Overview of Anemia ), suffocation, and Sometimes a blood clot in a leg vein ( Deep Vein Thrombosis (DVT) ) or, rarely, small pieces of fat from the marrow of a broken leg bone move into the bloodstream. Usually, these blood clots and pieces of fat travel to the heart and block an artery in the lungs (called Some risk factors for ischemic stroke can be controlled or modified to some extent—for example, by treating the disorder that increases risk. Themajor modifiable risk factorsfor ischemic stroke are Atrial Fibrillation and Atrial Flutter None Narrowing (stenosis) of a carotid artery in the neck Dyslipidemia None Overview of Coronary Artery Disease (CAD) None High Blood Pressure None Diabetes Mellitus (DM) None Insulinresistance (an inadequate response toinsulin), which occurs in type 2 diabetes Cigarette Smoking and Other Tobacco Use None Obesity , particularly if the excess weight is around the abdomen Obstructive sleep apnea None Consumption of too much alcohol Lack of physical activity An unhealthy diet (such as one that is high in saturated Types of Fat , trans fats, and calories) Depression or other mental stresses Heart disorders that increase the risk of blood clots forming in the heart, breaking off, and traveling through the blood vessels as emboli (such a Acute Coronary Syndromes (Heart Attack; Myocardial Infarction; Unstable Angina) an abnormal heart rhythm called Infective Endocarditis (infection of the heart's lining and usually of the heart valves) Use of Cocaine or Inflammation of blood vessels ( Overview of Vasculitis ) Excessive Clotting that result in excessive clotting Use of estrogen therapy, including oral contraceptives Risk factors that cannot be modifiedinclude Having had a stroke previously Being male Being older Having relatives who have had a stroke 