The brain is the most complex organ in the body. It’s divided into two sides, or hemispheres, each controlling the opposite side of the body and different areas of activity.
The left hemisphere controls cognition (thinking) and language, plus movement and sensation on the right side of the body. The right hemisphere controls functions involved in more visual-spatial skills, such as the ability to judge distances, size, form and where things are in space (which may affect skills such as map reading, for example), as well as movement and sensation on the left side of the body.
The brain regulates absolutely everything your body does – breathing, moving, sweating, sleeping, waking, feeling, your moods, thoughts and speech. To perform all these functions, it must have a constant supply of blood to deliver oxygen and nutrients to the brain cells.
There are two types of stroke. Ischaemic strokes are the most common, accounting for 80 per cent of cases. 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). Cerebral embolisms happen when a blood clot forms elsewhere in the body and is swept into the arteries serving the brain, travelling to a blood vessel small enough to block its passage. The clot lodges there, blocking the blood vessel and causing a stroke.
In atrial fibrillation, where the two upper chambers of the heart – the atria – beat irregularly instead of beating in a normal rhythmn, blood is not properly pumped out of the heart. As a result, a clot may form – if this lodges in an artery in the brain, a stroke may result. The American Heart Association says around 15 per cent of strokes are caused in this way, a cardioembolic stroke.
Blood clot strokes can be the result of unhealthy blood vessels clogging with a build-up of fatty deposits and cholesterol. Such material is called atheroma. (The body regards these build-ups as multiple, tiny and repeated injuries to the blood vessel wall and reacts as it would to bleeding from a wound, by forming clots.) Such narrowings are made worse by atherosclerosis – hardening of the arteries. Fatty tissue or air bubbles may also form emboli, which cause stroke, especially after major trauma.
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 longer 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 that all is not well with the blood supply to the brain and may be a sign of an imminent full-blown stroke. 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.
In the remaining 20 per cent of cases, strokes are caused by blood vessels in or around the brain rupturing and 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.
There are two types of haemorrhagic stroke: subarachnoid and intracerebral.
In intracerebral haemorrhage, bleeding occurs from vessels within the brain itself. High blood pressure (hypertension) is the primary cause of this type of haemorrhage.
In subarachnoid haemorrhage, an aneurysm bursts in a large artery on or near the delicate membrane (the subarachnoid space) surrounding the brain. Blood spills into the area around the brain, which is filled with a protective fluid, causing the brain to be surrounded by blood-contaminated fluid.
Strokes caused by the breakage or blow-out of a blood vessel in the brain can be the result of number of things:
- A cerebral aneurysm (ballooning of a weakened blood vessel in the brain), which is left untreated
- High blood pressure
- A cluster of abnormally formed blood vessels, where some blood vessels may be dilated or have thinner than normal walls (arteriovenous malformation); this means they are are more liable to burst
Aneurysms develop over a number of years and do not usually cause detectable problems until they break.
Causes of stroke
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
Symptoms generally appear suddenly and without warning, and the signs and symptoms of a stroke and a transient ischaemic attack (TIA) are the same. They include:
- Sudden weakness, numbness or paralysis often down one side of the body, affecting the face, arm, leg or whole side
- Sudden trouble seeing in one or both eyes
- Sudden severe headache with no known cause
- Sudden confusion, trouble speaking or ability to understand what others are saying, or an alteration in speech, such as slurring words
- Sudden difficulty with walking, dizziness, loss of balance or co-ordination
- Swallowing difficulties
Other less obvious symptoms include difficulties in perception or thinking, mood swings and personality change.
If a stroke is suspected, it’s vital to get medical help quickly. The sooner treatment is given the less damage there is likely to be to the brain tissue and the better the odds of a good recovery.
Who is at risk of a stroke?
Anyone of any age, including children, can have a stroke. However nine out of 10 of people affected are over 55. Certain risk factors increase the chances of someone having a stroke.
Stroke and lifestyle factors
Some risk factors are within your control. These are lifestyle factors such as:
- Smoking – smokers are at twice the risk of stroke
- Inactivity – people who are physically inactive are at twice the risk of stroke as those who are moderately active
- Alcohol – binge drinking and regular heavy alcohol intake increase the risk of stroke
- Diet – a diet high in salt and fatty foods is linked to high blood pressure and atherosclerosis, which increase the risk of stroke
Stroke and medical factors
Some illnesses and medical conditions increase the risk of stroke. They include:
- High blood pressure – persistent, untreated hypertension is the major risk factor for stroke and transient ischaemic attack (TIA)
- Cardiovascular problems – existing blood vessel disease,angina, heart attack or a previous TIA and atrial fibrillation(a type of irregular heart beat) increase the risk of stroke
- Diabetes – diabetes doubles the risk of a stroke
Most of these can be treated.
Other stroke risks
Some risk factors are beyond your control and can’t be medically treated. They include:
- Age – stroke is more common in people over the age of 55
- Gender – in the under-75s, men are more likely than women to have a stroke
- Ethnic background – south Asians, Africans and African-Caribbeans are at a higher risk of stroke
- Genetic inheritance – if you have a first-degree relative who had a stroke at an early age (under 50) you’re at higher risk