A subarachnoid hemorrhage occurs when blood leaks into the space between two of the membranes surrounding the brain. A swollen blood vessel, or aneurysm, usually ruptures and causes the condition.

A hemorrhage of this type can lead to a stroke and often has severe consequences. Bleeding can happen suddenly, causing an extreme headache. Frequently, the cause is a ruptured cerebral aneurysm or a head injury.

Subarachnoid hemorrhage occurs in approximately 10 in every 100,000 people in the United States, equivalent to around 30,000 new cases each year.

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A subarachnoid hemorrhage can lead to a stroke.

Three membranes surround the brain. The pia mater is the innermost one while the dura mater is the outermost layer.

In between these two is the arachnoid membrane.

The bleeding in subarachnoid hemorrhage occurs in the arteries just below the arachnoid membrane and above the pia mater.

Cerebrospinal fluid fills the part of the brain known as the subarachnoid space. During a subarachnoid hemorrhage, the cerebrospinal fluid in the subarachnoid space fills with blood.

Subarachnoid hemorrhages are responsible for around 5 percent of all strokes and around one in every four deaths caused by or related to strokes.

The first symptom of a subarachnoid hemorrhage is often a sudden and severe “thunderclap headache.”

People describe the pain as similar to receiving a blow on the head and the worst headache of their life. The headache typically throbs near the back of the head.

Other symptoms include:

  • stiff neck
  • nausea
  • vomiting
  • slurred speech
  • depression, confusion, delirium, or apathy
  • impaired consciousness, sometimes loss of consciousness
  • seizures
  • intraocular hemorrhage, or bleeding into the eyeball
  • occasional difficulty lifting an eyelid
  • sharp rise in blood pressure

The headache and stiff neck are similar to meningitis symptoms. However, a subarachnoid hemorrhage does not cause a skin rash or fever.

Several factors may lead to a subarachnoid hemorrhage.


Up to 80 percent of subarachnoid hemorrhages occur when a cerebral aneurysm ruptures.

An aneurysm occurs when part of a blood vessel swells. This can be due to a weakness in the blood vessel wall. As blood pressure grows, the blood vessel balloons at its weakest point. The larger the swelling, the greater the risk of bursting or rupturing.

Aneurysms associated with subarachnoid hemorrhages tend to occur in a circle of arteries that supply blood to the brain. These are known as the circle of Willis.

In some people, a congenital anomaly leads to weak and thin blood vessel walls, increasing the risk of an aneurysm.

Smoking, drinking alcohol regularly in large quantities, or having uncontrolled high blood pressure are also risk factors for an aneurysm.

Arteriovenous malformations

An arteriovenous malformation is present from birth. Scientists believe it may arise from fetal health issues during development in the womb.

Arteriovenous malformation is a complex, tangled web of abnormal arteries and veins that affects blood vessels in the spinal cord, brainstem, or brain.

Symptoms do not often become apparent until a hemorrhage occurs.

Other causes

A severe head injury, such as a blow to the head, can cause a subarachnoid hemorrhage.

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Smoking increases the risk of subarachnoid hemorrhage.

Lifestyle choices and contributing factors that can increase the risk of subarachnoid hemorrhage include:

  • tobacco use
  • consuming cocaine
  • heavy alcohol use
  • high blood pressure
  • a close relative who also had a subarachnoid hemorrhage

If a person experiences a stiff neck and severe headache with no other known cause, these may be signs of a subarachnoid hemorrhage.

Subarachnoid hemorrhage is an emergency. Anyone showing signs of the condition should go to the emergency room immediately.

An MRI scan provides a detailed image of the inside of the skull. It may help identify the hemorrhage and other blood vessel problems.

A CT scan can detect blood around the brain and any problems that may follow. The doctor may inject a dye to reveal the source of the bleeding.

A lumbar puncture involves doctors using a needle to take a sample of cerebrospinal fluid from the lower spine to check for the presence of blood.

A neurologist may use a doppler ultrasound to monitor the flow of blood in the brain. Unusual changes in the rate of blood flow could indicate that the arteries in the brain are in spasm. This can result in further hemorrhaging.

To prevent the blood vessels near a ruptured aneurysm from going into spasm, doctors may administer a drug called nimodipine. Nimodipine treats hypertension and prevents spasms. A course of this drug may continue for around 3 weeks.

A doctor may use morphine to treat the head pain a person experiences from a subarachnoid hemorrhage.

Treatment can sometimes involve surgeons applying neurosurgical clipping to an aneurysm. This mechanism seals the blood vessel closed with a small metal clip.

Endovascular coiling is another option. This procedure involves surgeons inserting a catheter or small plastic tube into an artery, usually in the groin or leg. The surgeon then threads the tube through blood vessels until it reaches the part of the brain where they have located the aneurysm.

After this, the treating doctor threads platinum coils into the aneurysm through the tube. These stop the blood flow into the aneurysm, effectively halting the hemorrhage.

This intervention has a better success rate when compared to neurological clipping, and people may recover more quickly.

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Around 5 percent of people with subarachnoid hemorrhage will develop epilepsy.

The most dangerous complication of subarachnoid hemorrhage is cerebral vasospasm. This is when the blood vessel near an aneurysm goes into spasm, making a bleed worse.

It can lead to coma and death if a person does not receive treatment. The drug nimodipine can help significantly reduce the risk of this complication.

People with vasospasm need strong blood flow to reach the brain. One approach is to raise blood pressure by pumping fluids into the blood or by transfusing blood and plasma into the body.

This is known as Triple H Therapy, referring to hypervolemia, hemodilution, and hypertension. However, research has not yet confirmed its effectiveness.

If this treatment does not show results, the surgeon may implant a small balloon into an artery and inflate it. This opens up the vessel and returns blood supply.

According to the National Stroke Association, around 5 percent of people who have a stroke experience a seizure within a few weeks. A seizure is more likely when a person has had a stroke as a result of bleeding.

Also, around 5 percent of people with subarachnoid hemorrhage will develop epilepsy. The first seizure can typically occur in the year following the hemorrhage.

Hydrocephalus is a further possible complication of subarachnoid hemorrhage. This condition can occur when too much cerebrospinal fluid builds up in the ventricles of the brain. This pressure can cause damage to the brain.

To treat hydrocephalus, doctors may have to drain fluid by placing a tube or shunt into the brain.

A subarachnoid hemorrhage is extremely dangerous, and the long-term outcomes depend on the complications that may arise.

According to one 2015 study, 18 percent of people who reach a hospital with the condition do not survive.

An earlier study from 2006 reported that 12 percent of cases were fatal before people reached a hospital with a further 10 percent becoming fatal within 24 hours of hospital admission.

Nearly every death from subarachnoid hemorrhage occurred within 3 weeks, due to rebleeding, according to this study. Around one-third of survivors had cognitive complications afterward, and two-thirds had reduced life quality.

It is vital that people with any of the risk factors discuss ways of reducing their risk with a doctor.

Controlling high blood pressure and avoiding drug and excess alcohol use can help protect people against subarachnoid hemorrhage.


How do I perform first aid on a person having a subarachnoid hemorrhage?


It is critical to get emergency help (911 in the U.S.) to have professionals transport them to a stroke center right away. If the hemorrhage is severe enough, the person may stop breathing, or their heart may stop.

In this case, trained personnel should start performing CPR.

Seunggu Han, MD Answers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.
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