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Subarachnoid Hemorrhage

Medically reviewed by Last updated on Feb 28, 2022.

What is a Subarachnoid Hemorrhage?

Harvard Health Publishing

A subarachnoid hemorrhage is bleeding from a damaged artery at the surface of the brain. This bleeding often causes a sudden, severe headache. It is a medical emergency. Subarachnoid hemorrhage is a type of stroke. It can cause permanent brain damage. 

Blood from a subarachnoid hemorrhage pulses into the space between the brain and the skull. It mixes with the cerebrospinal fluid that cushions the brain and spinal cord. As blood flows into the cerebral spinal fluid, it increases the pressure that surrounds the brain. The increased pressure can interfere with brain function.  

In the days that immediately follow the bleeding, chemical irritation from clotted blood around the brain can cause brain arteries to go into spasm. An artery spasm can cause additional new brain damage.  

Most often, a subarachnoid hemorrhage happens because a bulge in the wall of an artery ruptures. The sac-like bulge is called a saccular aneurysm. A subarachnoid hemorrhage also can occur because blood leaks from an abnormal tangle of blood vessels called an arteriovenous malformation (AVM). 

Ruptured Aneurysm

Several large arteries form a circle at the base of your brain. When a subarachnoid hemorrhage is caused by a ruptured saccular aneurysm, the aneurysm usually is located where a blood vessel branches from one of these large arteries. About 20% of patients who have had a subarachnoid hemorrhage have multiple aneurysms.  

Although it is not possible to predict whether an aneurysm will rupture, an aneurysm is more likely to rupture when it has a diameter of 7 millimeters or more. In most cases, a person who has a brain aneurysm never has a symptom related to it. 

Leaking AVM

An AVM is a tangled, abnormal mesh of blood vessels that connects an artery and a vein in the brain. AVMs form accidentally prior to birth. An AVM can form almost anywhere in the brain or spinal cord, but they usually are near the back of the brain.  

AVMs can appear in several generations of the same family and are more common in men. Bleeding from an AVM most often occurs between the ages of 10 and 30. If an AVM is too deep to cause bleeding onto the outer surface of the brain (a subarachnoid hemorrhage), it can cause bleeding inside the brain itself (intracerebral hemorrhage). 


The symptoms of a subarachnoid hemorrhage can include:  

  • Very severe, sudden headache (Some people say the start of this headache is like a thunderclap.) 
  • Nausea and vomiting  
  • Inability to look at bright light  
  • Stiff neck  
  • Dizziness  
  • Confusion  
  • A seizure  
  • Loss of consciousness  

If you have only a small subarachnoid hemorrhage, you may have several of these symptoms, particularly a severe headache that starts suddenly. However, your symptoms may improve on their own. This can be a critical warning sign, called a sentinel headache. People who have a small subarachnoid hemorrhage may have a second hemorrhage within the next week if they do not get medical treatment. 

In the several days that follow a subarachnoid hemorrhage, it is possible to develop more typical symptoms of a stroke, from damage of specific areas of the brain. Examples of these symptoms include: 

  • Weakness, paralysis or numbness of one side or one part of the body 
  • Difficulty speaking 
  • Difficulty walking 
  • Coma 


If you have a subarachnoid hemorrhage, you will be ill enough that you need to be evaluated in an emergency room. The doctor will do a physical examination, including a neurological examination, and will check to see if your neck movement is limited or uncomfortable, which can be a sign that there is an irritant such as blood in the spinal fluid. 

One of two brain imaging tests will be used: either a computed tomography (CT) scan or magnetic resonance imaging (MRI) scan. CT scans provide an image more quickly.  

A lumbar puncture, also called a spinal tap, can confirm that you have a subarachnoid hemorrhage even when the hemorrhage is too small to appear on a CT scan. This test also can test whether you have meningitis, an infection that can cause similar symptoms. During a lumbar puncture, a small sample of cerebrospinal fluid is removed through a needle inserted into your back. This fluid is examined to see if it contains blood.  

Two other tests, called brain angiography and MRI angiography, can evaluate blood flow patterns within your brain. One of these tests can be used to detect an aneurysm or AVM abnormality.  

You also may have an electrocardiogram (EKG). The dramatic stress on the brain during a subarachnoid hemorrhage can trigger chemical changes within your heart muscle. These changes can stress the heart muscle and may result in a heart attack, even if your heart arteries are not narrowed by atherosclerosis. 

Expected Duration

For those who survive a subarachnoid hemorrhage, recovery is slow. Most people do not recover complete functioning within months of a subarachnoid hemorrhage. Up to 50% of people who survive subarachnoid hemorrhage will have neurological disabilities that last much longer or become permanent. 


It is almost impossible to prevent subarachnoid hemorrhage caused by an aneurysm or AVM. These blood vessel abnormalities usually do not cause any symptoms before the hemorrhage occurs. Smoking has been shown to increase the risk of forming an aneurysm, so avoiding smoking may prevent some cases of hemorrhagic stroke. 

Some people have proposed screening tests, such as MRI angiography, that would identify aneurysms before they cause a problem. However, this idea has been impractical for most people, because surgery to remove an aneurysm can leave you with decreased function after your recovery. For most people, this is a risk that is not worth taking, since most aneurysms never cause serious bleeding.  

Screening and surgery are not recommended for people who have a single close relative who has a brain aneurysm or a subarachnoid hemorrhage. Screening and surgery may make sense for people who have two or more close relatives who have had bleeding, since this family is at an especially high risk. Because aneurysm screening is controversial, you should consider the risks of surgery carefully with your doctor before you ask for a screening test.  


When a large hemorrhage occurs in or around the brain, the entire brain is in danger because of increasing pressure within the skull. Much of the emergency treatment for hemorrhagic stroke involves measuring and lowering this pressure. Depending upon the patient's level of consciousness and intracranial pressure, he or she may need to be hooked up to a breathing machine. Also intravenous medications can be given to decrease the pressure.  

Blood pressure must be kept in a tight range. Blood pressure that gets too low can be even more dangerous than high readings. 

Subarachnoid hemorrhages commonly cause arteries nearest to the site of bleeding to spasm and become narrower. The calcium channel blocker called nimodipine is usually given to help prevent the spasm from further damaging the brain.   

A subarachnoid hemorrhage frequently causes shifts in the level of blood chemicals called electrolytes. Your doctor will adjust the amount of chemicals in the intravenous fluids based upon results of daily blood tests. 

If bleeding occurred because of an abnormally formed blood vessel, you will likely need a procedure to prevent recurrent bleeding. Ideally the procedure is performed when your condition is more stable.  

For an aneurysm, your doctor may recommend endovascular coiling or aneurysm clipping.   

  • Endovascular coiling is a less invasive procedure. A specially trained doctor inserts a thin flexible tube (called a catheter) with a metal coil at the end into a blood vessel. The doctor threads the catheter into the brain at the site of the aneurysm. The metal coil is left behind. It blocks blood flow to the aneurysm. The pressure inside the aneurysm will be much lower and the risk of recurrent bleeding is reduced. 
  • Aneurysm clipping requires brain surgery. The brain surgeon places a small metal clip across the base of the aneurysm.  

An AVM sometimes can be destroyed by a carefully directed beam of radiation or can be removed through surgery. Another technique is called embolization. A catheter is threaded through a blood vessel and guided to the AVM. The doctor injects special material or a chemical into the AVM to block off blood supply. 

Occupational and physical therapy will likely be needed if neurological impairment has occurred. The therapists are professionals who help the person improve daily function and regain strength after brain injury. Commonly, hospitalization is followed by a period of residence at a rehabilitation center, where additional intensive therapy may be provided. The goal of rehabilitation is to help the patient recover as much physical and speaking function as possible.  

Treatment options

The following list of medications are in some way related to or used in the treatment of this condition.

When To Call A Professional

Call for emergency help immediately whenever a friend or family member shows any unexpected symptoms that suggest to you they may be having a subarachnoid hemorrhage, especially if that person loses consciousness or suddenly develops a seizure. Call your own doctor promptly or have a friend bring you to the emergency room if you develop a severe headache and you are vomiting. Less-intense headaches should be evaluated by your doctor if you have them often or if they are accompanied by symptoms such as nausea, vomiting, weakness or numbness in any part of the body.  

If you have a sudden, very severe headache but it goes away, it is critically important to discuss this with your doctor. Sometimes blood leaks briefly from a blood vessel 6 to 20 days before a subarachnoid hemorrhage occurs. The headache this leak causes is called a sentinel headache. Your doctor may be able to provide treatment that can prevent more serious bleeding.  


Many patients with a subarachnoid hemorrhage do not survive long enough to reach a hospital. Of those who do, the prognosis depends upon the severity of the initial brain damage. If the person is awake and has minimal or no neurological abnormalities, the immediate prognosis is good. However, the person's condition can change rapidly. 

Without treatment of an aneurysm or AVM that bled, the risk of rebleeding is significant. Successful treatment greatly lowers this risk.

Additional Info

National Institute of Neurological Disorders and Stroke

National Stroke Association

Learn more about Subarachnoid Hemorrhage

Treatment options

Care guides

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.