What is a stroke?
A stroke is when an area of the brain is deprived of its blood supply - usually because of a blockage or burst blood vessel - for long enough to cause vital brain tissue to die. It's essentially the same as what happens in the arteries leading to the heart when someone has a heart attack, which is why a stroke is now often described as a 'brain attack'.
If brain cells lose their supply of oxygen from the blood, they may be damaged or die. When this happens, it's so sudden there's little medical science can do to prevent it.
Dead brain cells can't start working again. However, surrounding these dead cells is an area of tissue where blood supply is poor but not lost completely, so the nerve cells are receiving barely enough oxygen to stay alive.
One of the main aims of treating a stroke is to act fast enough to save this threatened brain tissue by restoring blood flow to the area and minimising the damage. As the inflammation and swelling caused by the stroke subside, brain cells near the dead cells may recover and begin working again.
There are two main types of stroke:
In this, the most common type of stroke, the artery is blocked by a blood clot, which interrupts the brain's blood supply.
This may be due to a cerebral thrombosis (sometimes called a thrombotic stroke), where a blood clot forms in one of the main arteries leading to the brain, or to a cerebral embolism (sometimes called an embolic stroke), in which a blood clot forms elsewhere in the body and is swept into the arteries serving the brain.
Fatty tissue or air bubbles may also form emboli which cause stroke, especially after major trauma.
Another type of thrombotic ischaemic stroke is called a lacunar stroke. In this form, one of the tiny blood vessels deep inside the brain tissue becomes blocked, leading to the death of the small area of tissue that it supplies. Lacunar strokes are usually less severe.
In this type of stroke, a blood vessel in or around the brain ruptures causing bleeding, or a haemorrhage. The build-up of blood presses on the brain, damaging its delicate tissue. Meanwhile, other brain cells in the area are starved of blood and damaged.
In an intracerebral haemorrhage, the bleeding occurs inside the brain itself. In a subarachnoid haemorrage, the burst blood vessel bleeds into the subarachnoid space surrounding the brain.
Transient ischaemic attack (TIA)
A transient ischaemic attack, often known as a mini-stroke, is a brief episode where some brain function is temporarily lost because of a short-lived disruption of the blood supply.
Symptoms, such as weakness of a limb, last for just minutes (typically two to 15 minutes) before the blood supply returns and everything returns to normal, because the brain cells haven't suffered permanent damage.
Traditionally it has been said that if symptoms last less than 24 hours it's a TIA, but when symptoms persist for more than 24 hours then a stroke has occurred. But with more powerful and sophisticated brain-scanning techniques, it has become possible to show that permanent damage (the real hallmark of a stroke) can usually be detected when symptoms last more than an hour or so.
TIAs are an important warning sign that all is not well with the blood supply to the brain. The risk of suffering a complete stroke within the first month after a TIA may be as high as 20 per cent, with the risk being even greater in the first few days following a TIA.
Why the brain needs a constant blood supply
The brain is the most complex organ in the body. It regulates absolutely everything your body does - breathing, moving, sweating, sleeping, waking, feeling, your moods, thoughts and speech.
To perform all these functions, the brain must have a constant supply of blood to deliver oxygen and nutrients to the brain cells. If the blood supply fails, as in a stroke, the brain cells become damaged or die within a very short space of time. Unlike other cells in the body, once they've died brain cells can't regrow.
What are the causes?
Each type of stroke has different causes. They include:
Diseased arteries - blockage of the arteries is usually the result of athersclerosis, furring and narrowing of the artery walls with a mixture of cholesterol and other debris.
Aneurysm - a weakened spot on an artery wall causes it to stretch. The vessel wall may become so thin it bursts, causing bleeding into the brain (haemmorhagic stroke).
Atrial fibrillation - this kind of irregular heartbeat (arrhythmia) can cause a blood clot to form in the heart, which then travels to the brain.