Subarachnoid haemorrhage is a rare but life-threatening type of stroke that happens when blood spills into the space surrounding the brain, causing sudden and severe symptoms that require immediate medical attention.

Understanding Subarachnoid Haemorrhage

A subarachnoid haemorrhage, often shortened to SAH, occurs when bleeding happens in a very specific area around the brain. Your brain is protected by three thin layers of tissue called meninges, which are membranes that lie between your skull and your brain tissue. The outermost layer is called the dura mater, the middle layer is the arachnoid, and the layer closest to your brain is the pia mater. When blood leaks into the space between the arachnoid layer and the pia mater, this is called a subarachnoid haemorrhage.^[1][3]

This condition is considered a medical emergency because the blood that spills into this narrow space builds up and creates pressure on the brain. When this pressure increases, it can interfere with how your brain works normally. The blood mixing with the fluid that cushions the brain and spinal cord can cause additional problems, and without quick treatment, it can lead to permanent brain damage or death.^[1][5]

Subarachnoid haemorrhage is classified as a type of stroke, specifically a haemorrhagic stroke, which means it involves bleeding rather than a blockage in blood vessels. Overall, about twenty percent of all strokes are haemorrhagic, with subarachnoid haemorrhage and another type called intracerebral haemorrhage each accounting for about ten percent of stroke cases.^[2]

How Common Is Subarachnoid Haemorrhage?

Subarachnoid haemorrhage is relatively rare compared to other types of strokes. In the United States, approximately ten to fourteen people out of every one hundred thousand experience a subarachnoid haemorrhage each year, which translates to roughly thirty thousand cases annually.^[3][4] In the United Kingdom, the incidence is similar, with about eight cases per one hundred thousand people in the population.^[13]

Globally, the burden of subarachnoid haemorrhage varies by region. The age-standardized incidence worldwide is approximately eight to nine cases per one hundred thousand people, with around seven hundred thousand new cases and three hundred fifty thousand deaths reported annually. Some countries, particularly Finland and Japan, see much higher rates of fifteen to twenty-five cases per one hundred thousand people. In contrast, most regions including North America, Europe, and the Middle East report lower rates of five to ten cases per one hundred thousand.^[4]

Subarachnoid haemorrhage can affect anyone, but it most commonly occurs in people between the ages of forty and sixty. The condition is more common in women than in men, with women having about 1.6 times higher risk. Research also shows that Black individuals have a higher risk compared to White individuals.^[3][4][13]

What Causes Subarachnoid Haemorrhage?

The vast majority of subarachnoid haemorrhages happen when a bulge in the wall of a blood vessel in the brain bursts. This bulge is called a brain aneurysm, specifically a saccular or “berry” aneurysm because of its shape. Approximately eighty to eighty-five percent of all non-traumatic subarachnoid haemorrhages are caused by a ruptured brain aneurysm.^[2][4][6]

Brain aneurysms are abnormal, saclike bulges that develop in weak areas of artery walls. Most aneurysms are found in the front part of the brain’s blood supply system, in what doctors call the anterior circulation of the Circle of Willis. This is a ring of arteries at the base of the brain. Aneurysms in the back part of the brain, in the vertebral and basilar arteries, are less common, accounting for only about twelve percent of all brain aneurysms.^[13]

It is not always clear what causes a brain aneurysm to form in the first place. Some people are born with them, while others develop them later in life. Many people who have a brain aneurysm never experience any symptoms related to it unless it ruptures. In most cases, a person does not know they have an aneurysm until it causes bleeding.^[5][6]

Another less common cause of subarachnoid haemorrhage is an arteriovenous malformation, or AVM. This is a tangled, abnormal mesh of blood vessels that incorrectly connects an artery and a vein in the brain. AVMs form accidentally before birth, and bleeding from an AVM most often occurs between the ages of ten and thirty. AVMs can appear in several generations of the same family and are more common in men.^[5]

Subarachnoid haemorrhage can also result from severe head trauma. When bleeding is caused by injury, it most commonly affects older people who have fallen and hit their heads. Among younger people, vehicle crashes are the most common injury leading to subarachnoid haemorrhage.^[3]

In about fifteen to twenty percent of patients who present with subarachnoid haemorrhage, no vascular lesion such as an aneurysm or AVM is found on initial imaging studies. These cases can be caused by other health conditions that affect blood vessels.^[2]

Risk Factors for Subarachnoid Haemorrhage

Several factors can increase a person’s likelihood of experiencing a subarachnoid haemorrhage. Some of these risk factors relate to having an existing aneurysm, while others involve lifestyle choices and certain medical conditions.^[3]

Having an unruptured aneurysm anywhere in the body, particularly in the brain, is a significant risk factor. The risk of an aneurysm rupturing varies according to its size and location. Aneurysms that are seven millimeters or larger in diameter are more likely to rupture. Aneurysms located in the front and back circulation systems of the brain carry a higher risk of rupture than those in the middle cerebral artery.^[4][5]

High blood pressure, also called hypertension, is a major risk factor for both the formation of aneurysms and their rupture. Keeping blood pressure under control is one of the most important ways to reduce risk.^[3][4]

Cigarette smoking significantly increases the risk of subarachnoid haemorrhage. Smoking damages blood vessel walls and makes aneurysms more likely to form and rupture. The use of cocaine and methamphetamine also increases risk, as does excessive alcohol consumption.^[3][4]

Certain genetic and inherited conditions predispose people to developing brain aneurysms. These include autosomal dominant polycystic kidney disease, Ehlers-Danlos syndrome type IV, Marfan syndrome, neurofibromatosis type 1, and fibromuscular dysplasia. People with a strong family history of aneurysms, meaning at least one first-degree relative such as a parent or sibling has had an intracranial aneurysm, are also at higher risk.^[3][13]

The use of blood-thinning medications such as warfarin can increase the risk of bleeding if an aneurysm ruptures. Additionally, having a history of a previously ruptured brain aneurysm increases the likelihood of experiencing another subarachnoid haemorrhage.^[3]

The prevalence of unruptured intracranial aneurysms in the general population is estimated at three to five percent, with higher rates in individuals who have hypertension, smoke, or have a family history of aneurysm rupture. About twenty percent of patients who have had a subarachnoid haemorrhage have multiple aneurysms.^[4][5]

Recognizing the Symptoms

The hallmark symptom of a subarachnoid haemorrhage is a sudden, extremely painful headache that many people describe as the worst headache of their life. This is often called a thunderclap headache because it comes on as suddenly and intensely as a clap of thunder. The headache starts in a split second and becomes very painful right away, unlike typical headaches that build up gradually.^[1][3][6]

Along with the sudden severe headache, people experiencing a subarachnoid haemorrhage may have several other symptoms. Nausea and vomiting are very common. A stiff neck or neck pain often develops because the blood can spread into the area around the spinal cord, irritating the nerves and causing discomfort. Some people become very sensitive to bright light, a condition called photophobia.^[1][6][10]

Neurological symptoms can include changes in consciousness and alertness. Some people may briefly lose consciousness or faint. Others may feel drowsy, confused, or experience mood and personality changes, including irritability. Visual problems such as blurred vision, blind spots, or double vision can occur. Drooping on one side of the face, slurred speech, or weakness on one side of the body may also happen.^[1][3][6]

Seizures or uncontrollable shaking can occur in some cases. Patients with a severe subarachnoid haemorrhage may present in a state of coma. The presence of neurological deficits such as weakness or speech problems increases the severity grade of the haemorrhage and affects the outlook for recovery.^[2][6]

A subarachnoid haemorrhage can happen at any time, but it may be more likely to occur when a person is doing something that temporarily increases blood pressure, such as coughing, going to the toilet, lifting something heavy, or having sex.^[6]

Sometimes, a person may experience a small subarachnoid haemorrhage with only some of these symptoms, particularly a severe headache that starts suddenly. The symptoms may improve on their own, but this can be a critical warning sign called a sentinel headache. People who have a small subarachnoid haemorrhage may have a second, more severe haemorrhage within the next week if they do not receive medical treatment.^[5]

Preventing Subarachnoid Haemorrhage

While it is not always possible to prevent a subarachnoid haemorrhage, especially when it is caused by a congenital aneurysm or unknown factors, there are several important steps people can take to reduce their risk.^[3]

The most important preventive measure is to control blood pressure. High blood pressure significantly increases the risk of both forming aneurysms and causing them to rupture. Regular monitoring of blood pressure and following medical advice to keep it within a healthy range can make a substantial difference.^[3]

Stopping smoking is crucial. Cigarette smoking is one of the strongest modifiable risk factors for subarachnoid haemorrhage. Smoking damages blood vessel walls and makes aneurysms more likely to develop and burst. Quitting smoking at any age can help reduce risk.^[3]

Avoiding or limiting alcohol consumption is also important. Excessive drinking can contribute to high blood pressure and increase the risk of bleeding. Similarly, avoiding cocaine and methamphetamine use is essential, as these substances can cause sudden spikes in blood pressure and dramatically increase the risk of aneurysm rupture.^[3]

Staying well hydrated is a simple but helpful measure. Maintaining good hydration supports overall vascular health and may help reduce risk.^[18]

For people who have a known unruptured brain aneurysm, regular medical follow-up is essential. Healthcare providers can monitor the aneurysm over time and recommend preventive treatment if it grows or shows signs of increased risk for rupture. In some cases, doctors may recommend surgery or an endovascular procedure to repair an aneurysm before it ruptures.^[6]

People with a family history of brain aneurysms or certain genetic conditions that increase risk should discuss screening options with their healthcare providers. While routine screening for brain aneurysms is not recommended for the general population, it may be appropriate for those with multiple risk factors or strong family history.^[13]

How Subarachnoid Haemorrhage Affects the Body

When a subarachnoid haemorrhage occurs, it triggers a complex cascade of events that affect both the brain and the entire body. Understanding these changes helps explain why the condition is so serious and why intensive medical care is necessary.^[2][13]

The initial bleeding episode causes immediate mechanical injury to the brain. As blood spills into the subarachnoid space, it increases the pressure surrounding the brain, a condition called increased intracranial pressure. This pressure can compress brain tissue and interfere with normal brain function. The blood also mixes with cerebrospinal fluid, the clear fluid that normally cushions the brain and spinal cord.^[1][5]

In the hours and days following the initial bleeding, the clotted blood around the brain causes chemical irritation. This irritation can cause the brain’s arteries to go into spasm, a condition called vasospasm. When blood vessels narrow due to spasm, blood flow to parts of the brain decreases, which can cause additional brain damage called delayed cerebral ischemia. This complication is one of the major causes of poor outcomes after subarachnoid haemorrhage.^[1][5][13]

The bleeding can also cause a buildup of cerebrospinal fluid in the brain’s ventricles, the hollow spaces that normally contain this fluid. When the fluid cannot drain properly, it accumulates and causes a condition called hydrocephalus. This further increases pressure on the brain and can worsen symptoms.^[6][12]

Subarachnoid haemorrhage is not just a brain problem; it affects multiple organ systems throughout the body. The sudden stress on the body can cause heart problems, including abnormal heart rhythms and changes in heart function. Lung complications can develop, and liver function may be affected. These systemic effects occur because the brain injury triggers widespread inflammatory and stress responses.^[12][13]

Some patients develop an additional complication called Terson syndrome, which involves bleeding into the eye that occurs at the same time as the brain hemorrhage. About eighty percent of patients who develop Terson syndrome need regular follow-up visits with an eye specialist, though most do not require additional treatment for the eye bleeding.^[2]

The severity of brain damage and the extent of complications depend on several factors, including the location of the bleeding, the amount of blood, how quickly treatment is started, and whether complications like vasospasm or rebleeding occur. Some parts of the brain may be more directly damaged, leading to specific problems with movement, sensation, speech, memory, thinking, or emotions.^[5][12]