What is a Stroke
A stroke (also called a "cerebrovascular accident" or CVA) occurs when blood vessels carrying oxygen and other nutrients to a specific part of the brain suddenly burst or become blocked. When blood fails to get through to the affected parts of the brain,
the oxygen supply is cut off, and brain cells begin to die. The blood supply to the brain is very important. Brain cells must have a continuous supply of oxygen and other nutrients from the blood in order to function. To meet this demand, blood is pumped continuously from the heart to the brain via several artery groups. Within the brain, these arteries (known as cerebral arteries) branch into smaller and smaller arteries, and eventually into tiny vessels called capillaries. These thin-walled vessels supply the nutrients to millions of nerve cells within the brain. When this continuous blood supply is disrupted, a stroke results.
Strokes fall into several major categories, based on whether the disrupted blood supply is caused by a blocked blood vessel (also known as an ischemic stroke) or a hemorrhage. Since each type of stroke has a different type of treatment, it is very important for the physician to determine the cause of the stroke, as well as the location, as quickly as possible.
Ischemic Stroke
Ischemic stroke results from a blocked blood vessel, and includes both thrombotic and embolic stroke.
Thrombotic Stroke (or cerebral thrombosis) is the most common type of stroke. In this type of stroke, a blood clot (thrombus) forms inside an artery in the brain, blocking blood flow. Sometimes, the clot occurs in one of the neck (carotid or vertebral) arteries that transport blood from the heart to the brain. Blood clots form most often in arteries damaged by atherosclerosis, a disease in which rough, fatty deposits build up on the walls of the arteries and project into the bloodstream. These deposits gradually narrow the passageway, causing the blood flow to slow down and, sometimes, to completely occlude (block) the artery.
Embolic Stroke (or cerebral embolism) is also caused by a clot; however, unlike cerebral thrombosis, the clot originates somewhere other than the brain. Embolic stroke occurs when a piece of clot (an embolus) breaks loose and is carried by the blood stream to the brain. Traveling through the arteries as they branch into smaller vessels, the clot reaches a point where it can go no further and plugs the vessel, cutting off the blood supply. This sudden blockage is called an embolism.
You may also hear the term cerebral infarction in connection with these two types of stroke. An infarct is an area of necrosis, or tissue death, due to obstruction of a blood vessel by a thrombus or embolus. Thrombotic and embolic strokes are the two types of the cerebral infarction category of stroke.
Hemorrhagic Stroke
The other main category of stroke, hemorrhagic stroke, occurs when a blood vessel in or around the brain ruptures, spilling blood into the brain or the area surrounding the brain. When this occurs, the cells nourished by the artery fail to get their normal supply of nutrients and cease to function properly. Furthermore, the accumulated blood from the ruptured artery soon clots, displacing normal brain tissue and disrupting brain function. Cerebral hemorrhage is most likely to occur in people who suffer from a combination of atherosclerosis and high blood pressure.
There are two main types of hemorrhagic strokes: subarachnoid hemorrhage and intracerebral hemorrhage, which refer to the parts of the brain affected by the bleeding.
- In Subarachnoid Hemorrhage, the bleeding occurs in the space between the brain and the skull.
- Intracerebral Hemorrhage is caused when a defective artery within the brain bursts, flooding the surrounding brain tissue with blood.
Aneurysms and AVMs
Hemorrhagic strokes are frequently caused by the bursting of an aneurysm (an abnormal "bulging" of a blood vessel in the brain) or an arteriovenous malformation (AVM), a cluster of abnormal blood vessels. In the case of an aneurysm, the weak spot in the vessel wall can be stretched out over the years, often by high blood pressure, which ultimately causes it to rupture. While aneurysms may not cause any symptoms until they burst (sometimes causing people to liken them to "time bombs" in the brain), AVMs may have many associated symptoms, including seizures, progressive neurologic problems and severe headaches that are unresponsive even to strong medications.
Until recently, some aneurysms and AVMs were virtually impossible to treat without high risk to the patient. New diagnostic and surgical advances pioneered at the Stanford Stroke Center have made it possible to treat these important causes of stroke and offer patients the likelihood of a cure. Some of these new treatments will be explored further in the section on surgical techniques.
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