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What is a subarachnoid haemorrhage (SAH)?

A subarachnoid haemorrhage (SAH) is a sudden leak of blood over the surface of the brain. The brain is covered by layers of membranes, one of which is called the arachnoid. An SAH occurs beneath this layer. The blood vessels supplying blood to the brain lie in this space, surrounded by clear cerebrospinal fluid.

SAH is a medical emergency. It is a serious, life-threatening condition.

What causes SAHs?

In 75% of cases, there is a weakness in the wall of one of the blood vessels supplying blood to the brain. The resulting balloon-like swelling is called an aneurysm. The haemorrhage occurs when the aneurysm wall tears because of the pressure of the blood as it is pumped through the brain. When this happens, blood bursts into the surrounding brain tissue.

Aneurysms can also occur in blood vessels in other parts of the body but there is no link between these aneurysms and brain aneurysms. We do not fully understand why aneurysms develop, but there is a greater risk of them occurring in people with high blood pressure and in people who smoke. They also become more common as people get older. There is no
established link between aneurysms and stress.

A very small number of SAHs are caused by arteriovenous malformations (AVMs). AVMs are an abnormal arrangement of blood vessels in the head.
(You might be interested in reading our booklet with information on vascular malformations of the brain for further details.)

Could I have prevented it?

No. People who smoke and people with high blood pressure have a greater risk of developing aneurysms but will not necessarily do so. Aneurysms can occur in people without these known risk factors.

Are there warning signs?

Very rarely, an aneurysm can press on a particular part of the brain and symptoms might develop as a result. However, usually, aneurysms go undetected and there are no symptoms until they burst.

Why me?

We do not know why it happens and, in most cases, there is no way of identifying the people who are at risk. There is a slight hereditary factor: a very small number of people can inherit from their family a greater risk of having an aneurysm.

Are my family at risk?

Because of the slight hereditary risk, it might be that members of your family are advised to have an MRA scan (see Tests and Investigations) to check for possible aneurysms. This would usually happen if more than one person in your close family has had an SAH. The people advised to have scans would be your first line relatives – parents, children, brothers and sisters.

Why now?

There is no reason why the haemorrhage occurs on one day rather than another. The bleed often, though not always, happens at a time of physical effort like coughing, going to the toilet, heavy lifting, straining, or during sex.

What are the symptoms?

Most people have a sudden, severe headache, often at the back of the head, followed by vomiting (being sick). The headache usually persists for more than an hour. People usually describe it as the worst headache they have ever had. It is common to have a stiff neck. People might also slur their speech, experience a disturbance in their vision, or have physical problems like weakness in an arm or leg. In more severe cases, people can collapse and lose consciousness. Some people might also have a seizure (a “fit”).

What happens in hospital?

Most people are admitted to their local hospital where the haemorrhage is confirmed by a CT scan of the brain. You might also have a sample of fluid taken from your spine (lumbar puncture).

You will then usually be seen at a regional centre by a neurosurgeon who has particular expertise in dealing with SAHs. You will be admitted to a neurosurgical ward for further investigations which could include an angiogram, an MRI scan, an MRA scan, or a CTA scan (see Tests and Investigations).

After an SAH there is a risk that the blood vessels in the brain will contract suddenly and block the blood supply to the brain. This might cause a stroke. The sudden contraction of blood vessels is called vasospasm. The drug Nimodipine is used to reduce the risk. It is given for three weeks after the bleed by which time the risk of vasospasm is usually over.Doctors usually rate the severity of SAHs using a grading system. A common grading system ranks haemorrhages from Grade 1 (minor) to Grade 5 (very serious). You might be told what grade your haemorrhage is, or hear the grade referred to in hospital.

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Last updated 16 June 2010