Subarachnoid haemorrhages are mainly caused by aneurysms.
Aneurysm
Around 8 out of 10 subarachnoid haemorrhages are caused by brain aneurysms.
It is not known why brain aneurysms (sometimes referred to as “berry aneurysms”) occur, although smoking, high blood pressure, family history, excess alcohol, and some rarer conditions such as autosomal dominant polycystic kidney disease, increase the risk.
If an aneurysm bursts, a subarachnoid haemorrhage will occur. Not everyone who has an aneurysm will experience a haemorrhage, perhaps only a third of people with intracranial aneurysms will experience a haemorrhage, and many people with an aneurysm will have no symptoms at all.
The risk of an aneurysm bursting is increased by:
- smoking
- excessive alcohol consumption
- poorly controlled high blood pressure
After it has burst, the aneurysm will often seal itself and the bleeding will stop. However, there is a high risk that without treatment, the aneurysm will burst again and cause more bleeding.
Other causes
Other less common causes of subarachnoid haemorrhages include:
- arteriovenous malformations – which is where blood vessels develop abnormally
- severe head injury
- 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, damaging the blood vessels
- fibromuscular dysplasia – which is a rare genetic disorder that can cause narrowing of the arteries
- Moyamoya disease – another rare condition, thought to be genetic, that causes progressive blockages inside the arteries inside the brain
- the blood vessels inside the brain become inflamed (swollen) which is known as vasculitis; vasculitis can have a wide range of underlying causes such as infection or the immune system attacking healthy tissue
