Like all organs, brain tissue needs a constant supply of oxygen and nutrients to stay alive. Different parts of the brain controls different functions, like memory, speech, coordination and sensory perception such as taste, vision or hearing. If a part of the brain is severely injured or dies then one or more functions can be affected. This is commonly seen in a stroke which causes loss of sensation, weakness or paralysis, forgetfulness and poor coordination.
What does a stroke mean?
A stroke is a condition where the brain tissue is injured and some brain tissue dies. This type of injury and tissue death occurs when the blood supply to the brain is interrupted either by a blockage or a burst blood vessel. The disturbance in brain circulation affects the oxygen supply to the brain leading to tissue injury (known as ischemia) and tissue death (known as infarction).
The symptoms of a stroke depends on which area of the brain is affected. It often presents with one-sided tingling/numbness, muscle weak/paralysis, confusion and sensory impairment. A stroke on the other hand causes permanent damage to brain tissue although a person can recover significantly with minimal symptoms persisting for life.
What does a TIA mean?
Sometimes there is only a temporary disturbance in the blood flow to the brain. This is sometimes referred to as a pre-stroke or mini-stroke. However, both terms can be misleading because there is usually no brain tissue death like a stroke and therefore no permanent impairment.
The symptoms of these episodes may be similar to a stroke but it only lasts for a short while and resolves once blood flow is restored. These episodes are known as transient ischemic attack (TIAs) and can occur several times over months or years before a stroke occurs, if at all.
How Does A Stroke Happen?
A stroke can strike suddenly, often with no warning and may not only lead to permanent disability but can result in death. However, in most cases the problems that resulted in a stroke arise days, months or even years prior to the stroke occurring. It is important to understand the two types of stroke in order to understand what happens in a stroke. In both types, the blood supply to the brain is interrupted.
Read more on types of strokes.
Blockage of Blood to the Brain
All organs in the body require a constant supply of blood. Arteries carry blood that is rich in oxygen and nutrients to the various parts of the body. However, the brain is highly oxygen sensitive. Even a few seconds without oxygen-rich blood can disrupt brain activity and brain death occurs within minutes. This is where the problem occurs in the two types of strokes.
The most common type of stroke is known as an ischemic stroke. In this type there is a blockage in an artery that supplies blood to the brain. This blockage may be caused by a blood clot which either develops at the site where it causes a blockage (this type of clot is known as a thrombus). When this occurs, the stroke is referred to as a thrombotic stroke.
Alternatively a clot may form elsewhere in the body, break away and travel through the bloodstream and eventually block an artery to the brain (this type of clot is known as an embolus). When this causes a stroke, it is then referred to as an embolic stroke. Sometimes it is not a blood clot that travels through the bloodstream to block an artery to the brain. Other type of debris or masses traveling in the bloodstream can also cause an embolic stroke.
It is important to understand that both a thrombotic stroke and embolic stroke are sub-types of an ischemic stroke. About 80% of strokes are ischemic strokes.
Break in a Blood Vessel to the Brain
The other type of stroke which is far less common is a hemorrhagic stroke. In this type of stroke there is a leak or break in a blood vessel. Therefore parts of the brain beyond the break cannot receive oxygen-rich blood and the tissue is injured and parts may die. Hemorrhagic strokes may occur when there is a leak or rupture of a blood vessel in the brain or near the brain.
Why do strokes occur?
The ways a stroke happens has been explained but why it occurs can vary. A stroke can occur at any age but 3 in 4 strokes occur in people over the age of 65 years. Smoking, excessive alcohol consumption, being overweight or obese and physical inactivity are the leading risk factors. This means that people with one or more of these risk factors have a high chance of developing a stroke.
Read more on warning signs of a stroke.
Narrowed Blood Vessel
Although a blood clot can occur for a number of reasons, one of the common underlying problems that leads to a stroke is a narrowed artery. The most likely cause of a narrowed artery is atherosclerosis. This is where fatty plaques develop in the wall of the artery thereby narrowing it.
High Blood Pressure
Hypertension is another common factor that increases the risk of a stroke. Not only does the high pressure damage the inner walls of blood vessels but it also increases the risk of atherosclerosis and blood clot formation. Hypertension is a common cardiovascular problem, especially with advancing age.
Blood clots can arise anywhere in the body for various reasons, apart from its primary function which is to plug broken blood vessels. In a heart condition known as atrial fibrillation, the risk of a stroke increases almost five fold. This fluttering heart problem contributes to blood clot formation which may then block a narrowed artery. A host of other heart diseases can also contribute to a stroke through a number of mechanisms.
There are several ways in which arteries can be weakened and then become more likely to rupture. An aneurysm is a bulging in one part of an artery. This area is then weakened. Sometimes blood vessels form with thinner walls and become entangled. This is known as an arteriovenous (AV) malformation. The risk of a hemorrhagic stroke is further increased by conditions like hypertension or with the use of anti-clotting drugs.