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There are 2 main types of stroke: ischaemic strokes and haemorrhagic strokes. They affect the brain in different ways and can have different causes.

Ischaemic strokes are the most common type of stroke. They happen 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 (plaques). This process is known as atherosclerosis.

Your arteries may naturally become narrower as you age, but there are some things that dangerously speed up this process.

These include:

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

Another possible cause of ischaemic stroke is a type of irregular heartbeat called atrial fibrillation.

This can cause blood clots in the heart that break apart and end up in the blood vessels that supply the brain.

Haemorrhagic strokes (also known as cerebral haemorrhages or intracranial haemorrhages) are less common than ischaemic strokes.

They happen when a blood vessel inside 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 more likely to split or rupture.

Things that increase the risk of high blood pressure include:

• being overweight
• drinking excessive amounts of alcohol
• smoking
• a lack of exercise
• stress

Haemorrhagic strokes can also be caused by the rupture of a balloon-like expansion of a blood vessel (brain aneurysm) or abnormally formed blood vessels in the brain.

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

These include:

• age – you're more likely to have a stroke if you're over 55, although about 1 in 4 strokes happen to 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're 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

But it's possible to significantly reduce your risk of having a stroke by making lifestyle changes to avoid problems such as atherosclerosis and high blood pressure.

You should also get medical advice if you think you may have an irregular heartbeat (arrhythmia).

This can be a sign of atrial fibrillation, which increases the risk of a stroke.

Find out more about how to prevent strokes.