Strokes mainly happen in one of two ways. Either there is a blood clot or plaque that blocks an artery in the brain, or a blood vessel in the brain breaks, causing a bleed in the brain.

Both of these stop blood from getting through to the brain to deliver oxygen and nutrients.

This page is about these two types of stroke and how they can happen. Sometimes though, people describe strokes by where they happen in the brain, or by which blood vessels are affected. For more on this, see Brain and arteries.

Diagram of the two types of stroke, a blockage and a bleed.

Blocked artery – ischaemic stroke

A stroke that is caused by the blood supply being blocked is called an ischaemic stroke (is-key-mick). It is the most common type of stroke.

Usually it is a blood clot that blocks the blood supply. Clots help to stop bleeding from wounds, but they can also form within blood vessels, which are the tubes or pipes that carry blood around the body.

Blood carries oxygen and nutrients for your brain cells. Your brain can be injured if blood can’t get through.

Blood vessels include arteries, which carry blood pumped from the heart to the rest of the body, and veins, which return blood to the heart. If a clot blocks an artery to the brain it causes an ischaemic stroke.

Clots that cause strokes usually form either in the heart or in one of the large vessels that supplies blood to the brain. The clot then travels up and blocks a blood vessel in the brain. This is called thromboembolism.

Large artery disease

A common cause of ischaemic strokes is atherosclerosis, which is when cholesterol builds up as plaques on the inner walls of arteries. When these plaques get too big a clot can form.

These clots often form in the carotid arteries in the neck, which are the brain’s main supply of blood. They can also form in arteries within the brain.

Strokes in which a large artery is blocked can do a lot of damage because more of the brain is affected.

However, these strokes can often be prevented by tackling risk factors like high blood pressure and high cholesterol. Find out how to prevent stroke.

Heart conditions

Clots often also form in the heart, due to:

atrial fibrillation (AF), which is an irregular heartbeat. One of the smaller heart chambers doesn’t pump in a coordinated way. Blood then stagnates and clots can form.
patent foramen ovale (PFO), which can connect the two sides of the heart. Clots that form in veins (e.g. deep vein thrombosis or DVT) can then cross to the arteries that supply blood to the brain. Babies need this connection before they’re born. It usually closes after birth, but it can stay open in some people.
damage to the heart, such as from infections or heart attacks, can affect pumping and let clots form where blood flow stagnates.

Small blood vessels

A stroke that blocks narrow arteries deep within the brain is called small vessel disease. The risk factors for small vessel disease are the same as for clots in large arteries.

Some rare genetic disorders can also cause small vessel disease. These include CADASIL, which affects muscle cells in the blood vessel walls.

Other causes

Dissection is when a neck artery tears. A clot can form at the break in the artery wall. Dissections are not common, but they happen more in younger people. The tear can be caused by damage to the neck, or by genetic risk factors such as fibromuscular dysplasia and vascular Ehlers–Danlos syndrome.
Sudden low blood pressure can cause a stroke in rare cases by reducing blood supply. This is called a watershed stroke because it mostly happens in the areas between arteries.
Cerebral vasculitis is a rare autoimmune condition that causes inflammation of blood vessel walls. It can make arteries too narrow and create a clot.
Blood disorders can sometimes make clots more likely. These are also rare, but they can lead to clots in veins within the brain, such as cerebral venous sinus thrombosis.
Reversible cerebral vasoconstriction syndrome is another rare condition in which blood vessels suddenly tighten. It usually causes sudden, severe, ‘thunderclap’ headaches, and can lead to ischaemic or haemorrhagic stroke.

Transient ischaemic attack (TIA)

A transient ischaemic attack (TIA) is when the blood supply to your brain is blocked temporarily. The signs are the same as for a stroke, but they disappear within a short time. Often, the signs are only present for a few minutes. A TIA is sometimes called a "mini-stroke".

After a TIA, your risk of stroke is higher. A TIA is a warning that you may have a stroke, and it is an opportunity to act to prevent this happening. With investigation and treatment, the risk of stroke following a TIA can be reduced by up to 80 per cent.

Bleed in the brain – haemorrhagic stroke

A stroke caused by bleeding is called a haemorrhagic stroke (hemm-orr-ragic).

Blood can leak from a broken or burst blood vessel, which is one of the tubes or pipes that carry blood around the body. The leaking blood is like a bruise that injures part of the brain.

Blood vessels include arteries, which carry blood pumped from the heart to the rest of the body, and veins, which return blood to the heart.

There are two types of haemorrhagic stroke:

High blood pressure

The main cause of haemorrhagic stroke is high blood pressure, which is called hypertension. The high blood pressure breaks major arteries deep within the brain, which causes an intracerebral haemorrhage.

Find out more about reducing high blood pressure.

Cerebral amyloid angiopathy (CAA)

When a protein called amyloid builds up in small blood vessels in the brain it is called cerebral amyloid angiopathy (CAA). The protein can damage the blood vessels and lead to bleeding and a haemorrhagic stroke.

Sometimes the bleeding from CAA is very small, which is known as “microbleeds”. You might not know you have had microbleeds until they are picked up by brain imaging.

CAA is more common in older people and in people who have dementia. But sometimes it can run in families and affect people who are younger.

Blood-thinning medication

Medicine that people take to lower their risk of blood clots and ischaemic stroke can also make bleeding more likely. These medications are known as blood thinners. They may also be called anticoagulants or antiplatelets.

Your doctor will make sure you take the right amount of blood thinner to reduce your risk of both kinds of stroke.

Illegal drugs

Some drugs, such as amphetamine and cocaine, can cause sudden, large rises in blood pressure. This can increase the risk of haemorrhagic stroke.

Vascular malformations

When blood vessels don’t form in the way they are meant to, they can become what is called a vascular malformation.

A vascular malformation is usually there at birth. But it may be that as you get older the blood vessels get bigger and weaker. It can happen anywhere in the body, including in the brain.

An arteriovenous malformation (AVM) is a tangled link between arteries and veins.

A cavernous malformation (cavernoma) is a tangle of tiny blood vessels that forms a weak-walled ‘cavern’ of blood.

If the vascular malformation is in your brain and the blood vessel walls burst, then you will have a haemorrhagic stroke.

Aneurysm

A weak or thin spot in the wall of an artery that balloons out is called an aneurysm. The aneurysm gets weaker as it gets bigger, and there is a risk it will burst and leak blood. Aneurysms often cause a subarachnoid haemorrhage.

Aneurysms develop over a number of years. Most people do not notice any problem until the aneurysm bursts. You can reduce the risk of an aneurysm bursting by controlling blood pressure and not smoking.

Types of stroke

More Resources

Blood pressure

Stroke (Health&)

Heart Foundation

Atrial fibrillation

TIA and minor stroke

Stroke Journey resources

Written by the Mob for the Mob: Stroke booklet

Transient ischaemic attack (TIA)