A stroke is the sudden death of brain cells due to lack of oxygen. A stroke is usually defined as one of two types:
- Ischemic (caused by a blockage in an artery)
- Hemorrhagic (caused by a tear in the artery's wall that produces bleeding into or around the brain)
The consequences of a stroke, the type of functions affected, and the severity, depend on where in the brain it has occurred and the extent of the damage.
Blood Flow Blockage. Strokes are caused by either blood flow blockage to the brain (ischemic stroke) or the sudden rupture of an artery in the brain (hemorrhagic stroke). Brain cells require a constant supply of oxygen to stay healthy and function properly. Therefore, blood needs to be supplied continuously to the brain through two main arterial systems:
The Circle of Willis is the joining area of several arteries at the bottom (inferior) side of the brain. At the Circle of Willis, the internal carotid arteries branch into smaller arteries that supply oxygenated blood to over 80% of the cerebrum.
- The carotid arteries come up through either side of the front of the neck. (To feel the pulse of a carotid artery, place your fingertips gently against either side of your neck, right under the jaw.)
- The basilar artery forms at the base of the skull from the vertebral arteries, which run up along the spine, join, and come up through the rear of the neck.
Blockage of blood flow to the brain for even a short period of time can be disastrous and cause brain damage or even death.
Click the icon to see an image of the brain.
Ischemic strokes are by far the more common type, causing nearly 90% of all strokes. Ischemia means the deficiency of oxygen in vital tissues. Ischemic strokes are caused by blood clots that are usually one of three types:
- Thrombotic stroke
- Embolic stroke
- Lacunar stroke
Thrombotic or Large-Artery Stroke and Atherosclerosis. The thrombotic stroke accounts for about 60% of all strokes. It usually occurs when an artery to the brain is blocked by a thrombus (blood clot) that forms as the result of atherosclerosis (commonly known as hardening of the arteries). These strokes are also sometimes referred to as large-artery strokes. The process leading to thrombotic stroke is complex and occurs over time:
- The arterial walls slowly thicken, harden, and narrow until blood flow is reduced, a condition known as stenosis.
- As these processes continue, blood flow slows. In addition, other events contribute to the coming stroke:
- The arteries become calcified, lose elasticity, and become susceptible to tearing. In this event, the thrombus (blood clot) forms.
- The blood clot then blocks the already narrowed artery and shuts off oxygen to part of the brain. A stroke occurs.
Embolic Strokes and Atrial Fibrillation. An embolic stroke is usually caused by a dislodged blood clot that has traveled through the blood vessels (an embolus ) until it becomes wedged in an artery. Embolic strokes may be due to various conditions:
- In about 15% of embolic strokes, the blood clots originally form as a result of a heart rhythm disorder known as atrial fibrillation.
- Emboli can originate from blood clots that form at the site of artificial heart valves.
- Patients with heart valve disorders such as mitral stenosis are at increased risk for clots when they also have atrial fibrillation.
- Emboli can also occur after a heart attack or in association with heart failure.
- Rarely, emboli are formed from fat particles, tumor cells, or air bubbles that travel through the bloodstream.
Lacunar Strokes. Lacunar infarcts are a series of very tiny, ischemic strokes, which cause clumsiness, weakness, and emotional variability. They make up the majority of silent brain infarctions and are probably a result of chronic high blood pressure. They are actually a subtype of thrombotic stroke. They can also sometimes serve as warning signs for a major stroke.
Silent Brain Infarctions. Many elderly people have silent brain infarctions, small strokes that cause no apparent symptoms. They are detected in up to half of elderly patients who undergo imaging tests for problems other than stroke. The presence of silent infarctions indicates an increased risk for future stroke, and are often contributors to mental impairment in the elderly. Smokers and people with hypertension are at particular risk.
Transient Ischemic Attacks (TIAs)
A transient ischemic attack (TIA) is an episode in which a person has stroke -like symptoms that typically last for a few minutes and usually less than 1-2 hours. Transient ischemic attacks (TIAs) are caused by tiny emboli (clots often formed of pieces of calcium and fatty plaque) that lodge in an artery to the brain. They typically break up quickly and dissolve but they do temporarily block the supply of blood to the brain.
TIAs do not cause lasting damage but they are a warning sign that a true stroke may happen in the future if something is not done to prevent it. TIA should be taken very seriously and treated as aggressively as a stroke. About 10 - 15% of patients who have a TIA have a stroke within 3 months, with half of these strokes occurring within 48 hours after the TIA.
About 10% of strokes occur from hemorrhage (sudden bleeding) into or around the brain. While hemorrhagic strokes are less common than ischemic strokes, they tend to be more deadly.
Hemorrhagic strokes are categorized by how and where they occur.
- Parenchymal, or intracerebral, hemorrhagic strokes. These strokes occur from bleeding within the brain tissue. They are most often the result of high blood pressure exerting excessive pressure on arterial walls already damaged by atherosclerosis. Heart attack patients who have been given drugs to break up blood clots or blood-thinning drugs have a slightly increased risk for this type of stroke.
- Subarachnoid hemorrhagic strokes. This kind of stroke occurs when a blood vessel on the surface of the brain bursts, leaking blood into the subarachnoid space, an area between the brain and the skull. They are usually caused by the rupture of an aneurysm, a bulge in a blood vessel, which creates a weakening in the artery wall.
- Arteriovenous malformation (AVM) is an abnormal connection between arteries and veins. If it occurs in the brain and ruptures, it can also cause a hemorrhagic stroke.