Stroke - Causes of stroke

There are two main types of stroke – ischaemic strokes and haemorrhagic strokes – which affect the brain in different ways and can have different causes.

Ischaemic strokes

Ischaemic strokes are the most common type of stroke. They occur when a blood clot blocks the flow of blood and oxygen to the brain.

These blood clots typically form in areas where the arteries have been narrowed or blocked over time by fatty deposits known as plaques. This process is known as atherosclerosis.

As you get older, the arteries can naturally narrow, but certain things can dangerously accelerate the process. These include:

smoking
high blood pressure (hypertension)
obesity
high cholesterol levels
diabetes
an excessive alcohol intake

Another possible cause of ischaemic stroke is a type of irregular heartbeat called atrial fibrillation, which can cause blood clots in the heart that break up and escape from the heart and become lodged in the blood vessels supplying the brain.

Atrial fibrillation can have a number of different causes, including lung disease, heart valve disease, excessive alcohol intake, coronary heart disease, and an overactive thyroid gland (hyperthyroidism). Read more about the causes of atrial fibrillation.

Haemorrhagic strokes

Haemorrhagic strokes (also known as cerebral haemorrhages or intracranial haemorrhages) are less common than ischaemic strokes. They occur when a blood vessel within the skull bursts and bleeds into and around the brain.

The main cause of haemorrhagic stroke is high blood pressure, which can weaken the arteries in the brain and make them prone to split or rupture.

Things that increase the risk of high blood pressure include:

being overweight or obese
drinking excessive amounts of alcohol
smoking
a lack of exercise
stress, which may cause a temporary rise in blood pressure

Haemorrhagic strokes can also occur as the result of the rupture of a balloon-like expansion of a blood vessel (brain aneurysm) and badly-formed blood vessels in the brain.

Can I reduce my risk?

It's not possible to completely prevent strokes because some things that increase your risk of the condition cannot be changed, including:

age – you are more likely to have a stroke if you are over 65 years old, although about a quarter of strokes happen in younger people
family history – if a close relative (parent, grandparent, brother or sister) has had a stroke, your risk is likely to be higher
ethnicity – if you are south Asian, African or Caribbean, your risk of stroke is higher, partly because rates of diabetes and high blood pressure are higher in these groups
your medical history – if you have previously had a stroke, transient ischaemic attack (TIA) or heart attack, your risk of stroke is higher

However, in most cases it is possible to significantly reduce your risk of having a stroke by making lifestyle changes to avoid problems such as atherosclerosis and high blood pressure. This includes things such as having a healthy diet, exercising regularly, stopping smoking if you smoke and cutting down on the amount of alcohol you drink.

As atrial fibrillation can also significantly increase your risk of having a stroke, it is also important to seek medical advice if you think you may have an irregular heartbeat. If you are diagnosed with the condition, you should talk to your doctor about the option of taking anticoagulant medications to lower your stroke risk.