Subarachnoid Hemorrhage: Causes, Symptoms, Warning Signs & Prevention

When you hear the term “subarachnoid hemorrhage”, you may think of a frightening condition that happens without warning.

A subarachnoid hemorrhage is one type of stroke, along with ischemic stroke and intracerebral hemorrhage.

One of the most common causes is the rupture of a brain aneurysm.

Because subarachnoid hemorrhage requires urgent medical attention, it is important to understand the condition correctly. At the same time, many questions can cause unnecessary worry: “Does every headache mean a subarachnoid hemorrhage?” “Will every brain aneurysm eventually rupture?” and “Are there warning signs before it happens?”

In this article, we explain what subarachnoid hemorrhage is, how brain aneurysms are involved, the symptoms and so-called warning signs, how the condition is diagnosed and treated, major risk factors, and everyday health habits worth paying attention to.

What Is a Subarachnoid Hemorrhage?

To understand subarachnoid hemorrhage, it helps to first understand where the bleeding occurs.

The brain is surrounded by three protective membranes called the dura mater, arachnoid mater and pia mater.

The space between the arachnoid mater and pia mater is called the subarachnoid space. It contains cerebrospinal fluid, and blood vessels also run across the surface of the brain in this area.

When bleeding occurs into this space, the condition is called a subarachnoid hemorrhage.

Although all three are types of stroke, what happens inside the brain is different. Ischemic stroke involves a blocked blood vessel, while intracerebral hemorrhage and subarachnoid hemorrhage involve bleeding.

A subarachnoid hemorrhage can also occur after a significant head injury, such as a traffic accident or fall. This is known as a traumatic subarachnoid hemorrhage.

This article focuses mainly on non-traumatic subarachnoid hemorrhage, particularly cases associated with the rupture of a brain aneurysm.

What Causes It? Brain Aneurysms and Other Causes

A common cause of non-traumatic subarachnoid hemorrhage is the rupture of a brain aneurysm.

A brain aneurysm is an area of an artery in the brain that has developed a balloon-like bulge.

Aneurysms often develop around branching points in blood vessels, where the vessel wall may be exposed to greater mechanical stress from blood flow.

One important point is that many brain aneurysms cause no symptoms before they rupture.

For this reason, an unruptured aneurysm may sometimes be discovered unexpectedly during an MRI or MRA performed as part of brain screening or for another medical reason.

If an aneurysm ruptures, blood can spread rapidly into the subarachnoid space and cause a subarachnoid hemorrhage.

If a brain aneurysm is found, will it definitely rupture?

This is one of the most commonly misunderstood points.

Finding a brain aneurysm does not mean that it will inevitably rupture.

Japan's National Cerebral and Cardiovascular Center states that brain aneurysms are found in approximately 3% of the population.

Some unruptured aneurysms remain stable for many years without rupturing.

At the same time, the likelihood of rupture is not identical for every aneurysm.

In other words, it is not as simple as saying that a large aneurysm will definitely rupture or that a small aneurysm is completely safe.

When an unruptured aneurysm is discovered, specialists consider both the estimated risk of rupture and the risks associated with treatment when deciding whether monitoring or treatment may be appropriate.

Brain aneurysm rupture is also not the only possible cause of subarachnoid hemorrhage. Other abnormalities involving blood vessels in the brain can sometimes be responsible.

Symptoms of Subarachnoid Hemorrhage and the Idea of “Warning Signs”

Two of the most common questions people have about subarachnoid hemorrhage are “What symptoms should I watch for?” and “Are there warning signs beforehand?”

The best-known symptom is a sudden and extremely severe headache.

People sometimes describe it as “the worst headache of my life” or as a sudden sensation similar to being struck in the head.

The important feature is not simply that the pain is severe.

The headache typically begins suddenly and may be extremely intense from the outset.

Symptoms can vary considerably from person to person.

It is therefore not safe to assume that subarachnoid hemorrhage can be ruled out simply because every “typical” symptom is not present.

Are there warning signs before a subarachnoid hemorrhage?

You may wonder whether recognising a warning sign could allow you to identify the problem before a major hemorrhage occurs.

However, not everyone experiences a clear warning symptom before a subarachnoid hemorrhage.

For some people, a sudden severe headache is the first noticeable symptom.

In other cases, a small amount of bleeding from an aneurysm may occur before a larger hemorrhage, causing a sudden headache. This is sometimes referred to as a “sentinel headache” or warning headache.

Importantly, this should not be viewed as a harmless sign that a more serious event may happen several days later.

A sudden, unusually severe headache itself may already require urgent medical assessment.

How Is Subarachnoid Hemorrhage Diagnosed?

With subarachnoid hemorrhage, rapid diagnosis and access to specialist treatment are important.

So how is it investigated in hospital?

CT is one of the main initial tests

When subarachnoid hemorrhage is suspected, one of the main imaging tests used is a CT scan of the head.

A CT scan can show blood that has spread into the subarachnoid space.

It is particularly useful early after symptoms begin, although factors such as the amount of bleeding and the time since onset can affect what is visible. In some situations, CT alone may not provide a definite answer.

What if CT does not provide a clear diagnosis?

If symptoms strongly suggest subarachnoid hemorrhage but no definite bleeding is seen on CT, doctors may consider additional investigations such as MRI or lumbar puncture, depending on the person's condition and the timing of symptoms.

During a lumbar puncture, cerebrospinal fluid is collected through a needle inserted in the lower back and examined for signs of bleeding.

The appropriate tests vary according to the individual situation, and the medical team determines which investigations are necessary.

Finding the source of the bleeding

If subarachnoid hemorrhage is confirmed, doctors then investigate where the bleeding came from.

If a brain aneurysm is suspected, its location, size and shape are assessed in detail to help determine the most appropriate treatment.

Tests may include CT angiography or catheter-based cerebral angiography.

Treatment for Subarachnoid Hemorrhage

When subarachnoid hemorrhage is caused by a ruptured brain aneurysm, one of the major early treatment goals is to prevent further bleeding.

Rebleeding from a ruptured aneurysm can cause rapid deterioration, so once the diagnosis is made, the medical team manages the patient's overall condition while planning specialist treatment as quickly as appropriate.

The two main approaches used to treat a ruptured aneurysm are open neurosurgery and endovascular treatment.

Surgical clipping

Surgical clipping involves opening the skull, identifying the aneurysm directly and placing a small specialised clip across the base of the aneurysm.

This prevents blood from continuing to enter the aneurysm and helps reduce the risk of further bleeding.

Endovascular treatment, including coil embolisation

Coil embolisation is an endovascular procedure in which a catheter is guided through the blood vessels to the aneurysm. Fine coils or other devices are then used to reduce or block blood flow into the aneurysm.

Unlike surgical clipping, this approach treats the aneurysm from inside the blood vessels without opening the skull.

Which treatment is better?

You may wonder whether clipping or coil embolisation is the better option.

There is no single treatment that is best for every patient.

The choice depends on factors such as the location, size and shape of the aneurysm, the extent of bleeding, the patient's age and their overall medical condition.

Even among people with the same diagnosis, the severity of bleeding and characteristics of the aneurysm can differ significantly, so treatment plans vary.

For the general public, the most important issue is therefore not deciding which treatment would be best, but getting suspected symptoms assessed by specialist medical services as quickly as possible.

So far, we have looked at what subarachnoid hemorrhage is, its relationship with brain aneurysms, symptoms and warning signs, diagnosis and treatment.

Next, we will look at the major risk factors associated with subarachnoid hemorrhage and the everyday health habits worth considering.

Who Is More Likely to Develop It? Major Risk Factors and Lifestyle Habits

Subarachnoid hemorrhage often occurs suddenly, but this does not mean that there are never underlying factors associated with its development.

A number of factors are known to be associated with the formation or rupture of brain aneurysms and with subarachnoid hemorrhage.

Among the most important modifiable factors are high blood pressure, smoking and alcohol consumption.

Having one or more of these factors does not mean that a person will definitely experience a subarachnoid hemorrhage.

Likewise, the condition can sometimes occur in people without an obvious risk factor.

Rather than thinking of risk as simply “present” or “absent,” it is more useful to distinguish between factors we cannot change and those we can influence through health management and lifestyle.

Blood pressure is an important place to start

High blood pressure is one of the most important risk factors for stroke in general.

It is also associated with subarachnoid hemorrhage.

One challenge is that high blood pressure often causes no noticeable symptoms.

Feeling well therefore does not necessarily mean that your blood pressure is within a healthy range. Regular health checks and, where appropriate, home blood pressure monitoring can help you understand your usual blood pressure.

If you are already receiving treatment for high blood pressure, do not reduce or stop prescribed medication on your own. Follow the advice of your doctor or other healthcare professional.

Smoking is also an important vascular risk factor

Smoking is often associated primarily with lung disease, but its effects are not limited to the lungs.

Smoking also affects the cardiovascular system and blood vessels.

It is an important risk factor for stroke and is also associated with subarachnoid hemorrhage.

For people who currently smoke, stopping smoking is an important health measure when considering the risk of stroke and many other diseases.

Alcohol: “moderate drinking” should not automatically be considered healthy

Our understanding of alcohol and health has changed over time.

Although older advice sometimes suggested that small amounts of alcohol could be beneficial, current health guidance increasingly emphasises that if you drink alcohol, drinking less is generally better for health.

Heavy alcohol consumption is associated with an increased risk of stroke, including hemorrhagic stroke and subarachnoid hemorrhage.

It can therefore be helpful to look not only at what you drink, but also at how much and how frequently you drink.

Some risk factors cannot be changed

Age and family history are examples of factors that cannot be changed.

Family history and genetic factors may play a role in brain aneurysms and subarachnoid hemorrhage, but having a family member who has experienced subarachnoid hemorrhage does not mean that you will inevitably develop the condition yourself.

Rather than allowing these factors to create unnecessary anxiety, it is more useful to focus on risk factors that can be addressed.

What can you do in everyday life?

There is no special lifestyle that can guarantee prevention of subarachnoid hemorrhage.

The basic approach is to look after overall cardiovascular health and manage known stroke risk factors.

Focus on the whole diet rather than a “special food”

When people start reading about blood vessels and cardiovascular health, it is easy to become interested in claims that a particular food might prevent disease.

However, no single food can guarantee protection against subarachnoid hemorrhage.

A more useful approach is to avoid excessive salt intake and focus on the overall balance of your diet.

There is no need to completely avoid restaurant meals or processed foods, but it can be helpful to consider whether your overall diet regularly contains too much salt or fat.

Long-term habits that are realistic and sustainable are generally more useful than trying to make every meal perfect.

Exercise: consistency matters more than intensity

Regular physical activity can be part of general health management, including maintaining healthy blood pressure and body weight.

You do not necessarily need an intensive exercise programme. Walking and other everyday forms of activity can be easier to maintain.

The important thing is to choose activity that is appropriate for your health and fitness level and that you can continue over time.

If you are being treated for high blood pressure, heart disease or another medical condition, avoid suddenly increasing your exercise intensity without appropriate medical advice.

Health checks are not only about finding disease

Some risk factors, including high blood pressure, may not cause noticeable symptoms.

Regular health checks are therefore useful not only for identifying medical problems, but also for understanding your current health and identifying areas where lifestyle changes may be worthwhile.

Rather than waiting until symptoms appear, it is helpful to have a general understanding of your usual blood pressure and overall health.

What Can Brain Screening Show? Understanding Unruptured Brain Aneurysms

After learning about subarachnoid hemorrhage, you may wonder whether having a brain scan or screening test can prevent it.

In Japan, comprehensive MRI/MRA-based brain screening is often referred to as a “brain dock.” Such screening can sometimes identify an unruptured brain aneurysm in a person who has no symptoms.

Brain aneurysms are also sometimes discovered incidentally when imaging is performed for another medical reason.

However, one important distinction should be kept in mind.

The fact that brain imaging can sometimes detect an aneurysm does not mean that screening can reliably prevent every case of subarachnoid hemorrhage.

The Japan Brain Dock Society publishes guidelines intended to support the quality and effectiveness of brain screening, including the 2026 Guidelines for Brain Dock.

If an unruptured aneurysm is found, does it need immediate surgery?

An important point to remember is that finding an unruptured brain aneurysm does not automatically mean that immediate surgery is necessary.

Brain aneurysms are not exceptionally rare. Japan's National Cerebral and Cardiovascular Center states that they are found in approximately 3% of the population.

Some remain unruptured for many years.

At the same time, the likelihood of rupture is not the same for every aneurysm.

Decisions about treatment or monitoring take into account factors such as the aneurysm's size, location, shape and whether it has grown, as well as the person's age and overall health.

Depending on these factors, treatment may be considered, or the aneurysm may instead be monitored with periodic imaging.

It is therefore not accurate to think “an aneurysm means immediate surgery” or “a small aneurysm is completely safe.”

What happens if monitoring is recommended?

If an unruptured aneurysm is monitored rather than treated immediately, MRI or MRA may be repeated at intervals determined by the treating doctor to check for changes in size or shape.

At the same time, managing high blood pressure appropriately and stopping smoking are examples of modifiable risk factors that can be addressed as part of general health management.

Summary: What Matters More Than Searching for “Warning Signs”

Subarachnoid hemorrhage is a type of stroke in which bleeding spreads into the subarachnoid space between the arachnoid mater and pia mater surrounding the brain.

A ruptured brain aneurysm is one of the most common causes of non-traumatic subarachnoid hemorrhage, and a characteristic symptom is a sudden, extremely severe headache.

A warning or sentinel headache may occur before a major hemorrhage in some people, but not everyone experiences a clear warning sign beforehand.

In other words, subarachnoid hemorrhage is not a condition where memorising a checklist of warning signs can guarantee reassurance.

The purpose of learning about subarachnoid hemorrhage is not to become anxious about every minor change in how you feel.

The important distinction is between recognising symptoms that require urgent medical attention and, in everyday life, managing health factors such as blood pressure, smoking and alcohol consumption.

These are two different issues, and both are worth understanding.

If you suddenly experience an extremely severe headache unlike anything you have experienced before, do not continue searching online for the cause or simply wait at home to see whether it improves. Contact emergency medical services immediately.

Important note
This article is intended to provide general information about subarachnoid hemorrhage and brain aneurysms. It is not a substitute for individual medical diagnosis or treatment. If you have symptoms or questions about testing or treatment, please consult an appropriate healthcare professional.

References and Further Information

• Ministry of Health, Labour and Welfare, Japan – Cerebrovascular Disease and Stroke
• Japan Stroke Association – Ten Rules for Stroke Prevention 2025
• National Cerebral and Cardiovascular Center – Brain Aneurysm
• Japan Brain Dock Society – Brain Dock Guidelines
• Japan Stroke Society – Japanese Guidelines for the Management of Stroke 2021, revised 2025