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A subarachnoid haemorrhage is most often caused by a burst blood vessel in the brain (a ruptured brain aneurysm).

A brain aneurysm is a bulge in a blood vessel caused by a weakness in the blood vessel wall, usually at a point where the vessel branches off.

As blood passes through the weakened vessel, the pressure causes a small area to bulge outwards like a balloon.

Occasionally, this bulge can burst (rupture), causing bleeding around the brain. More than 8 out of every 10 subarachnoid haemorrhages happen in this way.

A brain aneurysm doesn't usually cause any symptoms unless it ruptures.

But some people with unruptured aneurysms experience symptoms such as:

• sight problems
• pain on one side of the face or around the eye
• persistent headaches

It's not known exactly why brain aneurysms develop in some people, although certain risk factors have been identified.

These include:

• smoking
• high blood pressure
• excessive alcohol consumption
• a family history of the condition
• severe head injury
• autosomal dominant polycystic kidney disease (ADPKD)

Most brain aneurysms won't rupture but a procedure to prevent subarachnoid haemorrhages is sometimes recommended if they're detected early.

Read more about:

• diagnosing brain aneurysms
• treating brain aneurysms