How does a brain aneurysm form?

Arteries that carry blood into the brain are usually strong but elastic and they can stretch to accommodate large volumes of blood.

An aneurysm is a point in the wall of a blood vessel that is weak and does not stretch and then spring back into place properly.

Weak points tend to occur where blood vessels branch and can get weaker as we age and if blood pressure is high over a long period of time.

The weak point of the blood vessel can swell and bulge out and this can be detected in an MRI scan or CT scan. Small aneurysms can exist without causing symptoms and small and large brain aneurysms can press on brain tissue. Not all aneurysms burst.

The three main types of brain aneurysm are:

Saccular or berry aneurysms 

These are small sacs of blood that come out of a blood vessel, usually at the branching point, and are attached by a small stem. They look a bit like berries hanging from a fruit bush and are most common on the arteries at the base of the brain.

Lateral aneurysms 

Bulges or balloon-like swellings in one side of the artery wall.

Fusiform aneurysms 

Cause a full diameter swelling of the blood vessel.

Small brain aneurysms are smaller than about a centimetre in diameter; large ones can be up to 2.5 centimetres across and giant aneurysms are generally over 2.5 centimetres.

An aneurysm of any size can burst if the wall of the blood vessel breaks down, causing symptoms of stroke as blood is released into the brain.

What causes a brain aneurysm?

Around 10 in every 100,000 people experience a brain aneurysm. It tends to affect people between the ages of 55 and 65 years, but is sometimes seen in younger adults.

Below the age of 40, men and women are equally affected by brain aneurysm, but women are more likely to experience the condition in older age.

Potential causes of brain aneurysm include:

• Genetics: having a family member who has had a brain aneurysm is an established risk factor. People with adult polycystic kidney disease or Marfan syndrome are also at a greater risk
• Atherosclerotic plaques that build up in the blood vessels that supply the brain
• Brain infections that can be due to either bacteria or fungi
• Brain tumours
• Trauma, such as an accident or impact to the head

Environmental factors also have a role in the development of brain aneurysms and people are more at risk if they:

• Have high blood pressure (hypertension)
• Smoke: the risk of a brain aneurysm is 10 times higher in a smoker compared to a non-smoker
• Have high levels of cholesterol in the blood (hypercholesterolaemia)
• Drink too many units of alcohol on a regular basis
• Use illegal or hard drugs such as cocaine, crack or heroin

What are the symptoms of a brain aneurysm?

A brain aneurysm can be very serious but symptoms may not appear until the brain aneurysm bursts or presses on nearby nerves.

It depends to a large extent where the aneurysm is. Some people with a brain aneurysm that has not yet burst may have severe pain that sits above or behind one eye.

They might also have a dilated pupil in just one eye and complain of problems with their vision.

If a brain aneurysm bursts it can be life threatening, particularly if it is a large aneurysm. The result is a haemorrhagic stroke that leads to symptoms such as:

• A very severe headache or facial pain that comes on without warning
• Neck stiffness or pain, with sensitivity to light and dizziness
• Paralysis or difficulty moving, often down one side of the body
• Sickness
• Feeling very confused, finding it hard to talk, having seizures, or even lapsing into unconsciousness

How is a brain aneurysm treated?

The outcomes of brain aneurysm can be serious. If it bursts this can cause subarachnoid haemorrhage or acute haemorrhagic stroke and can increase the pressure inside the skull. 

Once a brain aneurysm ruptures, it causes death in 25–40% of those affected within 24 hours so someone showing any of the symptoms should be given emergency medical attention. Prompt treatment can save lives.