There are two main types of haemorrhagic stroke, intracerebral haemorrhage (ICH) and subarachnoid haemorrhage (SAH). These two types of stroke are very different and have different causes.

Main causes of intracerebral haemorrhage (ICH)

Cerebral small vessel disease

Cerebral small vessel disease means having damage to very small blood vessels in your brain. This can lead to blood leaking into the brain tissue.

There are two main types:

• Small vessel disease due to hardening of small deep arteries (sometimes called ‘arteriolosclerosis’)

High blood pressure (hypertension) is a risk factor for this type of small vessel disease in the brain. Over a long period, high blood pressure damages the small, deep blood vessels inside your brain, making them stiffen, and causing their walls to become fragile and prone to bleeding. If one of these small arteries blocks, then small areas of damage develop, due to a loss of blood flow to brain cells. This can look like tiny scars (called ‘lacunes’) on a brain scan.

Small vessel disease can affect your thinking ability and your mood, and it’s linked to memory problems and dementia. It also makes a brain haemorrhage more likely to happen in deep parts of the brain.

• Cerebral amyloid angiopathy (CAA)

This is the other common type of small vessel disease, especially in older people.

CAA is where a protein called amyloid beta builds up inside the small blood vessels near the surface of the brain. This damages the blood vessels, making them more likely to bleed. This can cause a bleed into the brain (ICH) leading to a stroke or can also cause bleeding over the brain surface.

When bleeding is on the surface of the brain, it can trigger recurrent, short-lived attacks of symptoms such as pins and needles, numbness or weakness. These are known as transient focal neurological episodes (TFNEs) or amyloid spells.  

CAA is common in people with dementia, but you can have CAA without having dementia. Although there are not any direct treatments for CAA, controlling your blood pressure can help reduce the risk of bleeding in the brain.

Abnormal blood vessels in the brain

Rare abnormalities in the blood vessels of the brain can sometimes cause ICH. These are known as vascular malformations, and they can include tangles of blood vessels or enlarged blood vessels. There are two main types:

• Arteriovenous malformation (AVM)

In an AVM, the blood vessels carrying blood to and from the brain grow together in a tangle, instead of linking to the full network of smaller blood vessels in the brain. An AVM can reduce blood flow and compress the surrounding brain tissue. Inside the AVM, blood flows at high pressure into weak blood vessels, which can sometimes lead to bleeding.

• Cavernous malformation, or cavernoma

A cavernoma is a cluster of enlarged blood vessels (capillaries), often said to look like a raspberry. It’s made up of a series of connected small ‘bubbles’ or ‘caverns’, filled with blood. Often these do not cause any symptoms, but the blood vessel walls can be weak, making a bleed more likely.

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Main causes of subarachnoid haemorrhage (SAH)

SAH is often due to a burst aneurysm along a blood vessel that runs in the subarachnoid space. A brain aneurysm is a swelling in a blood vessel that has ballooned out. The walls of an aneurysm are weak and they sometimes burst, causing bleeding into the layer of fluid around the brain.

What causes a brain aneurysm?

An aneurysm can be present from birth, or it can develop later in life. A specific cause is not identified for most aneurysms. However, some of the causes may include:

• Autosomal dominant polycystic kidney disease (ADPKD)

This kidney condition can make you more likely to have a brain aneurysm.

• Body connective tissue disorders

Some rare disorders affecting connective tissues in your body can make you more likely to have a brain aneurysm. Connective tissues are part of the structure of blood vessels and other tissues like skin and tendons. Body tissue disorders include Ehlers-Danlos syndrome and Marfan syndrome.

• Coarctation of the aorta

If someone is born with narrowing of the main artery in their body (the aorta), it can make a brain aneurysm more likely.

• Other factors

Other things can increase your risk of developing a brain aneurysm, such as smoking, high blood pressure, taking drugs such as cocaine or having a severe head injury.