Types of stroke

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.

This stops blood from getting through, stopping the delivery of oxygen and nutrients.

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 can stay open in some people.
• damage to the heart, such as from infections or heart attacks, that affects pumping and can 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 are shared with clots in large arteries.

Small vessel disease can also be caused by rare genetic disorders like CADASIL, which affects muscle cells in blood vessels.

Other causes

A dissection or tear in a neck artery can form a clot at the break in the artery wall. Although dissections are uncommon, 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 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.

A rare autoimmune condition that causes inflammation of blood vessel walls, called cerebral vasculitis, can make arteries too narrow and create a clot.

Some blood disorders can 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.

Another rare condition is a sudden constriction of blood vessels called reversible cerebral vasoconstriction syndrome. It usually causes sudden, severe, ‘thunderclap’ headaches, and can lead to ischaemic or haemorrhagic stroke.

Transient ischaemic attack (TIA)

A transient ischaemic attack (TIA) happens 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, they 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 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 percent.

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:

• Intracerebral haemorrhage (ICH) is when an artery inside the brain bursts and bleeds into the brain itself.
• Subarachnoid haemorrhage (SAH) is bleeding on the surface of the brain, between two of the layers of membranes that cover it.

High blood pressure

The main cause of haemorrhagic stroke is high blood pressure, or hypertension. The pressure breaks major arteries deep within the brain, causing 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 (haemorrhagic stroke).

Sometimes the bleeding from CAA is very small and 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 when they’re younger.

Blood-thinning medication

Medications that are taken to lower the risk of blood clots and ischaemic stroke can also make bleeding more likely. These medications are known as blood thinners, and 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 illegal drugs such as amphetamine and cocaine can also increase the risk of haemorrhagic stroke by causing sudden severe rises in blood pressure.

Vascular malformations

When blood vessels don’t form properly they can become a vascular malformation.

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

An arteriovenous malformation (AVM) is an abnormal tangled connection between arteries and veins.

A cavernous malformation is a tangle of tiny blood vessels creating a weak-walled ‘cavern’ of blood.  

If the vascular malformation is in the 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. Controlling blood pressure and not smoking reduce the risk of aneurysm rupture.