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. Around 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:
It's not known exactly why brain aneurysms develop in some people, although certain risk factors have been identified.
Most brain aneurysms won't rupture but a procedure to prevent subarachnoid haemorrhages is sometimes recommended if they're detected early.
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