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A subarachnoid haemorrhage is most often caused by a brain aneurysm.

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:

  • 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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Other causes

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
Page last reviewed: 14/01/2016
Next review due: 01/01/2019