Brain aneurysms are caused by a weakness in the walls of blood vessels in the brain. There are several reasons why this may happen, although an exact cause isn't always clear.

The brain requires a large supply of blood delivered via four main blood vessels that run up the neck and into the brain.

These blood vessels divide into smaller and smaller vessels in the same way the trunk of a tree divides into branches and twigs.

Most aneurysms develop at the points where the blood vessels divide and branch off, because these areas are often weaker.

Increased risk

There are a number of things that can increase your risk of developing a brain aneurysm. These are discussed below.

Smoking

Smoking tobacco can significantly increase your risk of developing a brain aneurysm.

Studies have shown the majority of people diagnosed with a brain aneurysm smoked, or had done so in the past.

The risk is particularly high in people with a family history of brain aneurysm.

Exactly why smoking increases the risk of brain aneurysms is unclear. It may be that the harmful substances in tobacco smoke damage the walls of your blood vessels.

High blood pressure

High blood pressure can place increased pressure on the walls of the blood vessels inside the brain, increasing your chances of developing an aneurysm.

You're more likely to develop high blood pressure if you:

• are overweight
• have a relative with high blood pressure
• are of African or Caribbean descent
• eat a lot of salt
• don't eat enough fruit and vegetables
• don't do enough exercise
• drink a lot of coffee or other caffeine-based drinks
• drink a lot of alcohol
• are aged over 65

Family history

Having a first-degree relative, such as a parent, brother or sister, with a history of a brain aneurysm means you're more likely to develop one than someone with no family history of the condition.

However, the increased risk is still small: only around one in 50 people with a family history of a ruptured brain aneurysm have a rupture themselves.

Age

Your risk of developing a brain aneurysm increases as you get older, with most cases diagnosed in people over the age of 40.

This may be because the walls of the blood vessels are weakened over time by the constant pressure of blood flowing through them.

Sex

Women are more likely to develop a brain aneurysm than men. This may be because levels of a hormone called oestrogen lower significantly after the menopause. Oestrogen is thought to help maintain the elasticity of the blood vessels.

Pre-existing weakness in the blood vessels

In some cases, brain aneurysms are caused by weaknesses in the blood vessels, present from birth.

Severe head injury

A brain aneurysm can develop after a severe brain injury if the blood vessels in the brain are damaged, although this is very rare.

Cocaine abuse

Cocaine abuse is considered to be another risk factor for brain aneurysms. Cocaine can inflame the walls of the blood vessels and raise your blood pressure. The combination of these two factors increases your risk of developing a brain aneurysm.

Autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a genetic condition that causes multiple cysts to develop on the kidneys. Cysts are small sacs filled with fluid.

Around one in every 1,000 people is born with ADPKD. Of these people, around one in 20 develop an aneurysm in the brain.

Body tissue disorders

Your risk of developing a brain aneurysm can be higher if you have a condition that affects your body tissues, such as Ehlers-Danlos syndrome or Marfan syndrome.

This is because these conditions can sometimes cause weaknesses in the walls of your blood vessels.

Coarctation of the aorta

People with coarctation of the aorta are also at an increased risk of developing a brain aneurysm.

Coarctation of the aorta is the term used to describe narrowing of the main artery in the body (the aorta), which is present from birth (congenital). It is a common type of congenital heart disease.

A brain aneurysm is usually diagnosed using an MRI scan and angiography (MRA), or a CT scan and angiography (CTA).

An MRI scan is usually used to look for aneurysms in the brain that haven't ruptured. This type of scan uses strong magnetic fields and radio waves to produce detailed images of your brain.

A CT scan is usually preferred if it's thought the aneurysm has ruptured and there's bleeding on the brain (subarachnoid haemorrhage).

This type of scan takes a series of X-rays, which are then assembled by a computer into a detailed 3D image.

In some cases, a ruptured aneurysm is not picked up by a CT scan. If a CT scan is negative but your symptoms strongly suggest you have a ruptured aneurysm, a test called a lumbar puncture will usually be carried out.

A lumbar puncture is a procedure where a needle is inserted into the lower part of the spine to remove a sample of the fluid (cerebrospinal fluid) that surrounds and supports the brain and spinal cord. This fluid can be analysed for signs of bleeding.

Planning treatment

If the results of scans or a lumbar puncture suggest you've either had a brain haemorrhage or have an unruptured brain aneurysm, a further test called an angiogram or arteriogram may be carried out to help plan treatment.

An angiogram or arteriogram involves inserting a needle, usually in the groin, through which a narrow tube called a catheter can be guided into one of your blood vessels.

Local anaesthetic is used where the needle is inserted, so you won't feel any pain.

Using a series of X-rays displayed on a monitor, the catheter is guided into the blood vessels in the neck that supply the brain with blood.

Once in place, special dye is injected into the arteries of the brain through the catheter.

This dye casts a shadow on an X-ray, so the outline of the blood vessels can be seen and if an aneurysm is present it can be recognised.

Screening

There's no routine screening programme for brain aneurysms and it's unlikely that one will be introduced in future.

This is because researchers have calculated that routine screening would do little to prevent deaths, but would place a significant drain on NHS resources.

Screening is only recommended for people thought to have a significant risk of having a brain aneurysm that could rupture at some point in future.

This would usually only apply to you if you had two or more first-degree relatives (father, mother, sister or brother) who had experienced a subarachnoid haemorrhage.

If this applies to you then contact your GP. They'll be able to refer you to a specialist clinic for further assessment.

Discovering you have an aneurysm unsuitable for surgical treatment can cause worry and distress, even though the risk of it rupturing is small. Some people have reported regret at getting screened.

There are no right or wrong answers but it's important you discuss the potential implications of screening with the staff at the clinic.

Screening may also be recommended if you have a condition that increases your chances of developing a brain aneurysm, such as autosomal dominant polycystic kidney disease.

Brain aneurysms can be treated using surgery if they've burst (ruptured) or there's a risk they will.

Preventative surgery is usually only recommended if there's a high risk of a rupture.

This is because surgery has it's own risk of potentially serious complications, such as brain damage or stroke.

Assessing your risk

If you're diagnosed with an unruptured brain aneurysm, a risk assessment will be carried out to assess whether surgery is necessary.

The assessment process is usually based on the following factors:

• your age – research has found the risks associated with surgery in older adults often outweigh the potential benefits (extending natural lifespan)
• the size of the aneurysm – aneurysms larger than 7mm often require surgical treatment, as do aneurysms larger than 3mm in cases where there are other risk factors
• the location of the aneurysm – brain aneurysms located on larger blood vessels have a higher risk of rupture
• family history – brain aneurysms are considered to have a higher risk of rupturing if you have a history of ruptured brain aneurysm in your family
• underlying health conditions – some health conditions increase the risk of a rupture, such as autosomal dominant polycystic kidney disease (ADPKD) or poorly controlled high blood pressure

After these factors have been taken into consideration, your surgical team should be able to tell you whether the benefits of surgery outweigh the potential risks in your case.

Active observation

If the risk of rupture is considered low, a policy of active observation is normally recommended.

This means you won't receive immediate surgery, but you'll be given regular check-ups so your aneurysm can be carefully monitored.

You may also be given medication to lower your blood pressure.

Your doctor may discuss lifestyle changes that can help lower the risk of a rupture, such as losing weight and reducing the amount of fat in your diet.

Surgery and procedures

If preventative treatment is recommended, the two main techniques used are called neurosurgical clipping and endovascular coiling.

Both techniques help prevent ruptures by stopping blood from flowing into the aneurysm.

Neurosurgical clipping

Neurosurgical clipping is carried out under general anaesthetic, so you'll be asleep throughout the operation.

A cut is made in your scalp, or sometimes just above your eyebrow, and a small flap of bone is removed so the surgeon can access your brain.

When the aneurysm is located, the neurosurgeon will seal it shut using a tiny metal clip that stays permanently clamped on the aneurysm. After the bone flap has been replaced, the scalp is stitched together.

Over time, the blood vessel lining will heal along the line where the clip is placed, permanently sealing the aneurysm and preventing it from growing or rupturing in the future.

Clipping the artery on which the aneurysm is formed on, as opposed to clipping the aneurysm itself, is rarely necessary. This is usually only carried out if the aneurysm is particularly large or complex.

When this is necessary, it's often combined with a procedure called a bypass. This is where the blood flow is diverted around the clamped area using a blood vessel removed from another place in the body, usually the leg.

Endovascular coiling

Endovascular coiling is also usually carried out using general anaesthetic.

The procedure involves inserting a thin tube called a catheter into an artery in your leg or groin.

The tube is guided through the network of blood vessels, up into your head and finally into the aneurysm.

Tiny platinum coils are then passed through the tube into the aneurysm.

Once the aneurysm is full of coils, blood cannot enter it. This means the aneurysm is sealed off from the main artery, which prevents it from growing or rupturing.

Coiling versus clipping

Whether clipping or coiling is used often depends the size, location and shape of the aneurysm.

Talk to your healthcare team about your treatment options. If it's possible to have either procedure, you should discuss the risks and benefits of both procedures.

Coiling has generally been shown to have a lower risk of complications, such as seizures, than clipping in the short-term, although the benefits in the long-term are not certain.

With coiling, there's also a small chance you'll need to have the procedure more than once to best reduce your chances of the aneurysm rupturing.

About one in five people who have the coiling procedure need further treatment.

However, as coiling is a less invasive procedure, you can usually leave hospital sooner after the operation.

After clipping, you'll usually need to stay in hospital for around four to six days, whereas you can usually go home one or two days after coiling.

The time it takes to fully recover is also typically shorter with coiling. Many people make a recovery within a few weeks of coiling, whereas recovering from clipping may take longer.

Emergency treatment

If you require emergency treatment because of a ruptured brain aneurysm, you'll initially be given a medication called nimodipine to reduce the risk of the blood supply to the brain becoming severely disrupted (cerebral ischaemia).

Either coiling or clipping can then be used to repair the ruptured brain aneurysm. The technique used will usually be determined by the expertise and experience of the surgeons available.

In such emergency cases, the differences between the techniques are less important because things such as your recovery time and hospital stay depend more on the severity of the rupture than the type of surgery carried out.

You cannot always prevent brain aneurysms, but you can lower your risk by not smoking and reducing high blood pressure.

Smoking

If you smoke, stopping can significantly reduce your risk of developing a brain aneurysm.

If you want to give up smoking a good first step is to contact Help Me Quit on 0800 085 2219. 

Alternatively, your GP can provide help and advice about quitting.

See Treatment for quitting smoking for more information about giving up.

High blood pressure

Having high blood pressure can also significantly increase your chance of developing a brain aneurysm.

You can help reduce high blood pressure by:

• eating a healthy diet – in particular, cutting down on salt and eating plenty of fruit and vegetables
• moderating your alcohol intake – men and women are advised not to regularly drink more than 14 units a week
• maintaining a healthy weight – even losing just a few pounds will make a big difference to your blood pressure and overall health
• exercising regularly – being active and taking regular exercise lowers blood pressure by keeping your heart and blood vessels in good condition
• cutting down on caffeine – it's fine to drink tea, coffee and other caffeine-rich drinks as part of a balanced diet, but it's important these drinks aren't your only source of fluid

Read more about preventing high blood pressure.