A subarachnoid haemorrhage is a bleed that occurs in the subarachnoid space that lines the brain.

Subarachnoid haemorrhage usually comes on suddenly and can be life-threatening but treatment is available and many people make a good recovery.

The subarachnoid space protects the brain

The brain is essential for everything we do in life, so it is no surprise that it is carefully protected. The skull is the major source of that protection but the brain is cushioned inside the bone by the meninges, which are formed from three separate membranes:

  • The dura mater, which lies immediately beneath the skull.
  • The arachnoid mater, so-called because it resembles a spider’s web. This is between the other two membranes.
  • The pia mater, which is attached to the brain.

To further cushion the brain, a viscous cerebrospinal fluid (CSF) flows around and between the different layers of membranes. This fluid fills the subarachnoid space, the small gap between the arachnoid mater and the pia mater. Blood vessels that supply the brain are also present in the subarachnoid space.

Subarachnoid haemorrhage: a bleed on the brain

A subarachnoid haemorrhage is a sudden bleed that occurs in the subarachnoid space. It occurs when one of the blood vessels ruptures and blood seeps into the subarachnoid space. This causes brain damage and increases the pressure inside the skull.

A subarachnoid haemorrhage can be due to:

  • A weakness in the wall of a blood vessel in the subarachnoid space, which then swells and forms a bubble called an aneurysm. The walls of an aneurysm are weak and can burst, causing a bleed. This is the cause of subarachnoid haemorrhage in 75% of cases.
  • An accident that involves a blow to the head.
  • An arteriovenous malformation (AVM), a rare condition where the veins and arteries in the subarachnoid space do not connect properly and can bleed easily.
  • Cavernomas that form in the subarachnoid space.
  • A brain tumour is a very rare cause of subarachnoid haemorrhage.

The key symptom of subarachnoid haemorrhage is a sudden and extremely painful headache. People with a subarachnoid haemorrhage may also:

  • Feel sick or vomit
  • Have a stiff neck or pain in their back or legs
  • Find it uncomfortable to look at bright light
  • Find it hard to speak or see, or slur their speech
  • Become unconscious

About half of people who have a subarachnoid haemorrhage get a headache a few hours before a major bleed, and some may experience regular headaches in the few months before. About a quarter of people have a seizure prior to the subarachnoid haemorrhage. These signs are called prodromal symptoms and are usually only recognised with hindsight; it is almost impossible to predict when and if a subarachnoid haemorrhage will take place.

Subarachnoid haemorrhage is rare

Subarachnoid haemorrhage is not a common condition as it only affects around 49 out of every 100,000 people. Women are more likely to suffer from a subarachnoid haemorrhage and most occur around the age of 50.

It is also more common in people who smoke, have high blood pressure, or high levels of cholesterol. There seems to be a genetic component as subarachnoid haemorrhage can run in families.

What is the impact of a subarachnoid haemorrhage?

A subarachnoid haemorrhage is a serious vascular event and although some people recover completely, others can be left with serious complications. Sometimes the blood vessels around the site of injury can constrict, causing a vasospasm. This can starve the brain of blood and cause an ischaemic stroke or transient ischaemic attack. A subarachnoid haemorrhage is the cause of around 5% of all strokes.

The bleed itself can also lead to an increase in pressure inside the skull, which can damage the brain. Inflammation around the bleed can add to the increased pressure.

Many people who have had a subarachnoid haemorrhage have no major long-term effects but others can experience symptoms that are similar to those of stroke such as paralysis, sight problems, loss of speech and memory problems.