Causes of subarachnoid haemorrhages

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

A Aneurysm, abdominal aortic 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.

Abrain aneurysm doesn't usuallycause 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:

• smoking
• high blood pressure
• excessive alcohol consumption
• a family history of the condition
• some rarer conditions, such as 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.

• treating brain aneurysms

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