What causes a stroke?
Thrombotic stroke: The blockage of an artery in the brain by a clot (thrombosis) is the most common cause of a stroke. The part of the brain that is supplied by the clotted blood vessel is then deprived of blood and oxygen. As a result of the deprived blood and oxygen, the cells of that part of the brain die and the part of the body that it controls stops working. Typically, a cholesterol plaque in one of the brain's small blood vessels ruptures and starts the clotting process.
Risk factors for narrowed blood vessels in the brain are the same as those that cause narrowing blood vessels in the heart and heart attack (myocardial infarction). These risk factors include:
* high blood pressure (hypertension),
* high cholesterol,
* diabetes, and
* smoking.
Embolic stroke: Another type of stroke may occur when a blood clot or a piece of atherosclerotic plaque (cholesterol and calcium deposits on the wall of the inside of the heart or artery) breaks loose, travels through the bloodstream, and lodges in an artery in the brain. When blood flow stops, brain cells do not receive the oxygen and glucose they require to function and a stroke occurs. This type of stroke is referred to as an embolic stroke. For example, a blood clot might originally form in the heart chamber as a result of an irregular heart rhythm, like atrial fibrillation. Usually, these clots remain attached to the inner lining of the heart, but occasionally they can break off, travel through the bloodstream (embolize), block a brain artery, and cause a stroke. An embolism, either plaque or clot, may also originate in a large artery (for example, the carotid artery, a major artery in the neck that supplies blood to the brain) and then travel downstream to clog a small artery within the brain.
Cerebral haemorrhage: A cerebral haemorrhage occurs when a blood vessel in the brain ruptures and bleeds into the surrounding brain tissue. A cerebral haemorrhage (bleeding in the brain) causes stroke symptoms by depriving blood and oxygen to parts of the brain in a variety of ways. Blood flow is lost to some cells. Additionally, blood is very irritating and can cause swelling of brain tissue (cerebraledema). Edema and the accumulation of blood from a cerebral haemorrhage increases pressure within the skull and causes further damage by squeezing the brain against the bony skull. This further decreases blood flow to brain tissue and its cells.
Subarachnoid haemorrhage: In a subarachnoid haemorrhage, blood accumulates in the space beneath the arachnoid membrane that lines the brain. The blood originates from an abnormal blood vessel that leaks or ruptures. Often this is from an aneurysm (an abnormal ballooning out of the blood vessel). Subarachnoid haemorrhages usually cause a sudden severe headache, nausea, vomiting, light intolerance, and stiff neck. If not recognized and treated, major neurological consequences, such as coma, and brain death may occur.
Vasculitis: Another rare cause of stroke is vasculitis, a condition in which the blood vessels become inflamed causing decreased blood flow to parts of the brain.
Migraine headache: There appears to be a very slight increased occurrence of stroke in people with migraine headache. The mechanism for migraine or vascular headaches includes narrowing of the brain blood vessels. Some migraine headache episodes can even mimic stroke with loss of function of one side of the body or vision or speech problems. Usually, the symptoms resolve as the headache resolves.
What are the risk factors for stroke?
Overall, the most common risk factors for stroke are:
* high blood pressure,
* high cholesterol,
* smoking,
* diabetes, and
* increasing age.
Heart conditions like atrial fibrillation, patent foramen ovale, and heart valve disease can also be the potential cause of stroke.
When stroke occurs in younger individuals (less than 50 years old), less common risk factors to be considered include illicit drugs, such as cocaine or amphetamines, ruptured aneurysms, and inherited (genetic) predispositions to abnormal blood clotting.
An example of a genetic predisposition to stroke occurs in a rare condition called homocystinuria, in which there are excessive levels of the chemical homocystine in the body. Scientists are trying to determine whether the non-hereditary occurrence of high levels of homocystine at any age can predispose to stroke.
What is a transient ischemic attack (TIA)?
A transient ischemic attack (TIA, mini-stroke) is a short-lived stroke that gets better and resolves spontaneously. It is a short-lived episode (less than 24 hours) of temporary impairment of brain function that is caused by a loss of blood supply. A TIA causes a loss of function in the area of the body that is controlled by the portion of the brain affected. The loss of blood supply to the brain is most often caused by a clot that spontaneously forms in a blood vessel within the brain (thrombosis). However, it can also result from a clot that forms elsewhere in the body, dislodges from that location, and travels to lodge in an artery of the brain (emboli).
