Subarachnoid Haemorrhage

Authored by , Reviewed by Dr Adrian Bonsall | Last edited | Meets Patient’s editorial guidelines

A subarachnoid haemorrhage (SAH) is a form of stroke in which bleeding occurs into the fluid-filled space surrounding the brain (called the subarachnoid space). It is usually caused by the rupture of a small berry-like swelling (called a berry aneurysm) on one of the arteries in the brain.

SAH is a medical emergency. It is also a catastrophe: half of people who have SAH will die, and around half of the survivors are left with significant disability.

A subarachnoid haemorrhage (SAH) occurs when there is bleeding around the brain into the subarachnoid space. This is a space between two of the protective layers which surround the brain. These are the pia mater, which covers the brain directly, and the arachnoid mater, which is separated from the pia mater by a space filled with fluid. This fluid, the cerebrospinal fluid, is there to cushion and protect the brain from injury.

The bleeding usually comes from a burst (ruptured) berry aneurysm, most commonly at the back of the brain. Berry aneurysms are small berry-shaped swellings on the arteries which supply the brain, usually at a junction where an artery divides into two. These are relatively common in the general population, particularly in older patients, although most never rupture.

brain and meninges

Subarachnoid haemorrhage (SAH) is a catastrophe. Many people who have an SAH will die, sometimes before reaching hospital. Most others will be left with some degree of permanent disability. This is due to the damage caused to the brain by the initial bleeding and by the shortage of oxygen to the brain which follows it. About 1 in 4 people survive SAH well enough to live independently. This is more likely if the bleed is small, if initial symptoms are less severe and if the patient is younger.

  • SAH usually affects adults under the age of 60 years, with the average age of affected patients around 50 years.
  • Children are not usually affected.
  • Women are affected slightly more often than men.
  • Patients of African descent are slightly more commonly affected than patients of Caucasian descent.
  • The condition is also more common in Finland and Japan, where the rates are around three times those in Europe.
  • Patients who smoke, use excessive alcohol or have high blood pressure are at greater risk.
  • First-degree relatives of people who have had an SAH are at greater risk. 
  • Around 6-10 people of every 100,000 per year will have an SAH.
  • SAH causes around 6 out of 100 of all strokes.
  • This makes SAH relatively rare - but hugely important. It is a very disabling and dangerous type of stroke and it tends to affect younger patients more than other strokes.

More than 8 out of 10 cases of SAH are caused by the bursting (rupture) of berry aneurysms. About 1 in every 25 adults will have one or more berry aneurysms. However, an SAH rupturing is a relatively rare event.

Larger berry aneurysms (bigger than 7 mm) are more likely to rupture (smaller ones can grow larger over time). Rupture is more likely to occur if the elastic tissue in the walls of the berry aneurysm is weakened or strained by:

  • Uncontrolled high blood pressure.
  • High alcohol use.
  • Smoking.
  • Cocaine use.
  • Certain inherited conditions which cause weakening of the blood vessels. These include autosomal dominant polycystic kidney disease (ADPKD). These conditions cause only 1-2 in every 100 cases of SAH.

Other causes of SAH, accounting for fewer than 2 out of every 10 cases, include:

  • Head and neck trauma, when bleeding from injury to the brain or the structures around it can track into the subarachnoid space.
  • Abnormal blood vessels such as those that may be present in a brain tumour.
  • Brain infection such as encephalitis.
  • Vasculitis, when blood vessels inside the brain become swollen and inflamed. This can occur either due to infection or to conditions affecting the immune system.


  • The most characteristic feature is a sudden explosive headache.
  • In 1 in 3 patients this will be the only symptom (the rest will have other symptoms as described below).
  • The headache is severe, often described as the most severe headache ever experienced. It is sometimes referred to as a thunderclap headache. However, the term thunderclap headache is more commonly used for some other types of sudden headache, including primary sexual headache.
  • Usually the headache lasts for a week or two. It is usually a dull, all-over headache. It may occasionally last only a few seconds or even less. The person may even look round and accuse someone of hitting him or her on the back of the head.
  • About 1 in 10 sudden explosive headaches are found to be due to SAH.

If you have an episode of sudden severe headache - coming on in minutes or less and being worse than headaches you have normally experienced - seek medical advice at once.

Being sick (vomiting)

Vomiting is very common in SAH. Vomiting also occurs with migraine and with other severe headaches. However, if you have an unusually severe sudden headache with vomiting you should seek medical advice.

Seizures and altered consciousness

A few people with SAH have a seizure due to irritation of the brain by the bleeding. Confusion or altered consciousness may also develop. If someone is unconscious they cannot tell you they have a headache. If someone complains of a severe headache then becomes unexpectedly drowsy, make sure that medical attendants are aware that the headache came first, as this suggests SAH.

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About 1 in 10 people with SAH have some warning signs in the days or hours before the SAH occurs. The most common are:

  • Brief sudden headaches (the most common symptom).
  • Dizziness.
  • Eye pain.
  • Double vision or lost vision.
  • Loss of sensation or movement.

These symptoms may be caused by early small leaks of blood from the aneurysm as it starts to burst (rupture). However, they may not last long, and of course they may not be recognised as possibly serious by doctors or patients.

If a subarachnoid haemorrhage is suspected then a computerised tomography (CT) scan should be performed as soon as possible. This will nearly always confirm the diagnosis.

Other investigations may include cerebral angiography. A lumbar puncture (spinal tap) may be needed if the CT scan is normal but a subarachnoid haemorrhage is still suspected.

Treatment of SAH

Initial treatment is aimed at:

  • Stabilising the person.
  • Preventing further bleeding
  • Preventing further brain injury.

Medicines are used to:

  • Reduce blood pressure and improve blood supply to damaged parts of the brain.
  • Prevent seizures which can worsen brain damage.

Surgery is used in some patients:

  • People with SAH are usually referred to a specialist neurosurgical unit.
  • Treatment may not always be appropriate. Sadly, in some cases, assessment will show that severe brain damage has already occurred. If this is the case, there is no further treatment that can help.
  • If treatment is to go ahead, it usually consists of blocking off the aneurysm, either:
    • Through blocking it from the inside by using a special platinum coil; or
    • Through clipping it closed.

Patients need to be monitored for some time to avoid complications. These are fluid on the brain (which can be drained), further shortage of oxygen to the brain and low blood pressure.

Long-term management of SAH

This is aimed at:

  • Reducing risk of recurrence - treating high blood pressure, reducing alcohol intake and, if the person smokes, stopping smoking.
  • Specialist rehabilitation service for disabilities.
  • Not everyone can be saved. Irreversible brain damage can occur before the patient reaches medical help. Only half of affected people will survive the first month.
  • In those who survive the first day, the risk of further bleeding is still very high.
  • Between four weeks and six months after the SAH, the risk of having another SAH gradually decreases, although it never returns completely to zero.
  • Improvement will continue for about 18 months after the SAH.
  • Most affected people will have some impairment but some do well.
  • The things which increase the chance of doing better are:
    • A smaller bleed.
    • A younger age.
    • Remaining conscious at the time of the SAH.
  • The overall death rate is about 1 in 2.
  • Of those who survive, a third will be dependent on carers.
  • In a survey of patients who survived SAH and were interviewed several years later:
    • 1 in 4 had stopped working.
    • 1 in 4 worked shorter hours or had a position with less responsibility.
    • 1 in 4 said they had completely recovered.
  • Personality change is common after SAH, most often increased irritability or increased emotional swings.

It is not known why some people with berry aneurysms in the brain go on to develop SAH. However, it is known that certain avoidable risk factors make it more likely:

  • High blood pressure
  • Smoking
  • Excessive alcohol intake
  • Cocaine use

Avoiding these risk factors is the first way to reduce the risk of SAH.

The factors that affect the health of the walls of your brain's (cerebral) blood vessels are the same factors that affect your risk of heart disease (cardiovascular risk). Therefore, lifestyle measures which generally reduce cardiovascular risk will help prevent SAH. These include:

Berry aneurysms are common. They are present in around 4 in every 100 adults, being more common with greater age. They are probably the result of wear and tear on the cerebral arteries at their point of greatest weakness - the point where they divide into two.

Aneurysms less than 7 mm in diameter tend not to burst (rupture). An aneurysm probably behaves rather like a balloon. When you blow up a balloon you reach a point when the elastic strength of the balloon wall 'gives' and the balloon expands more quickly because the wall is weakened. It seems likely that the same happens to an aneurysm, and that it's only when it grows in this way that it risks rupture. Why some aneurysms grow is not fully understood - but it seems likely that high blood pressure and smoking contribute.

If you have an aneurysm which is less than 7 mm in size, you may be advised that, as long as you keep your blood pressure well controlled and maintain a healthy lifestyle, your risk of SAH is no greater than that of the general population. The decision is more difficult if your aneurysm is larger than 7 mm, or you have several aneurysms. You might be offered repeat scanning, at intervals, to see if your aneurysm is growing, and to review the decision.

The alternative is to clip or block the aneurysm, even whilst it is small. The risks of surgery are significant, particularly if you are older. The risk of rupture also increases with age. These are not easy decisions.

Incidentally found aneurysms

Because they are common, berry aneurysms are often found incidentally. That is, you are found to have a berry aneurysm when having a CT scan for another reason. If this happens you will need specialist advice as to whether you need to consider operative treatment for your aneurysm. Many factors will be taken into account when advising you, including:

  • Your age.
  • The number of aneurysms you have.
  • Where the aneurysms are.
  • How large they are.
  • Any underlying conditions that might put you at greater risk.

People who have had an SAH and who have unruptured aneurysms

Those who have already had an SAH are usually offered treatment for other aneurysms, as they have already proved that they have aneurysms which tend to grow.

Screening is sometimes offered to first-degree relatives of patients with SAH. However, very few apparently dangerous lesions are found and so this is probably not a good use of resources or time. It is likely that the period of time between an aneurysm starting to 'look' dangerous (because it has grown) and it actually bursting (rupturing) is short. Therefore, screening is unlikely to catch aneurysms at this stage.

Further reading and references

  • Ziu E, Mesfin FB; Subarachnoid Hemorrhage. StatPearls Publishing 2017 Oct 6.

  • Yew KS, Cheng EM; Diagnosis of acute stroke. Am Fam Physician. 2015 Apr 1591(8):528-36.

  • D'Souza S; Aneurysmal Subarachnoid Hemorrhage. J Neurosurg Anesthesiol. 2015 Jul27(3):222-40. doi: 10.1097/ANA.0000000000000130.

  • Schattlo B, Fathi AR, Fandino J; Management of aneurysmal subarachnoid haemorrhage. Swiss Med Wkly. 2014 Apr 29144:w13934. doi: 10.4414/smw.2014.13934. eCollection 2014.