A subarachnoid haemorrhage is most often caused by a 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 eight out of every 10 subarachnoid haemorrhages happen in this way.
A brain aneurysm doesn't usually cause any symptoms unless it ruptures. However, 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:
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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Less common causes of subarachnoid haemorrhages include:
- arteriovenous malformations – where blood vessels develop abnormally
- a brain tumour damaging the blood vessels – both cancerous and non-cancerous brain tumours can cause a subarachnoid haemorrhage
- a brain infection, such as encephalitis
- fibromuscular dysplasia – a rare condition that can cause the arteries to narrow
- Moyamoya disease – a rare condition that causes blockages inside the brain's arteries
- vasculitis – where the blood vessels inside the brain become inflamed (swollen), which can be caused by a wide range of problems, such as infection or the immune system attacking healthy tissue