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 4 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, as these areas are often weaker.
There are a number of things that can increase your risk of developing a brain aneurysm. These are discussed below.
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 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:
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.
But the increased risk is still small: only around 1 in 50 people with a family history of a ruptured brain aneurysm have a rupture themselves.
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.
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.
In some cases, brain aneurysms are caused by weaknesses in the blood vessels present from birth.
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 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 2 factors increases your risk of developing a brain aneurysm.
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 1 in every 1,000 people is born with ADPKD. Of these people, around 1 in 20 develop an aneurysm in the brain.
This is because these conditions can sometimes cause weaknesses in the walls of your blood vessels.
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.