What is a subarachnoid haemorrhage?

Diagram showing subarachnoid space: brain and spine

A subarachnoid haemorrhage is a leakage of blood that occurs over the surface of the brain, most commonly originating from a weakened artery deep within the brain.  

Membranes, one of which is called the arachnoid, cover the surface of the brain.  The space beneath this membrane is known as the subarachnoid space, also contained within this area is clear fluid (cerebro-spinal fluid) that flows over the surface of the brain and around the spinal cord.  In a subarachnoid haemorrhage, blood ruptures into this subarachnoid space and the fluid contained within it. 

Subarachnoid haemorrhage affects 10-16 per 100,000 people each year in the UK, which is approximately 8,500 people each year.  It is a potentially life threatening condition occurring suddenly, and without warning.  It can occur at any age but most commonly occurs between the ages of 40-65 years, and is very rare in children.  It affects females more often than males, at a ratio of 3:2.


In most cases the blood that haemorrhages onto the surface of the brain causes a very sudden, and severe headache, often described as feeling like you had been hit over the back of the head.   The headache persists and can often be accompanied by nausea, vomiting and neck stiffness.  In some cases collapse, seizure and/or loss of consciousness may occur, depending on the severity of the bleed, and often people can become disorientated.