Brain Aneurysm: Why Size and Location Affect Risk
Medical review: This article has been reviewed by Mr Ahilan Kailaya-Vasan, Consultant Neurosurgeon and Neurovascular Surgeon in London, and reflects current medical understanding at the time of publication.
Key Points
The risk associated with a brain aneurysm depends on multiple factors, particularly size and location.
Smaller aneurysms often carry a lower risk, especially if stable over time.
Some areas of the brain carry a higher risk than others.
Risk assessment is individual and helps guide decisions about monitoring or treatment.
When a brain aneurysm is identified, one of the most common questions is: how risky is it?
No single factor determines the answer. Instead, specialists assess a combination of features, with size and location playing a central role in diagnosis and treatment decisions.
Understanding how these factors influence risk can help place findings into context and support informed decision-making.
What Determines Aneurysm Risk?
A brain aneurysm is a weakness in the wall of a blood vessel that causes it to bulge outward. While many aneurysms remain stable, some can rupture.
Risk is assessed based on several features, including:
Size
Location
Shape and appearance
Changes over time
Individual patient factors
Among these, size and location are two of the most important considerations.
How Aneurysm Size Affects Risk
In general, larger aneurysms are associated with a higher risk of rupture compared to smaller ones.
Aneurysms are commonly found incidentally and, in many cases, remain stable over time. These are described as an unruptured brain aneurysm, and are frequently managed with monitoring rather than immediate treatment of brain aneurysms.
However, size alone does not determine risk. For example:
A small aneurysm in a higher-risk location may still require careful assessment
A larger aneurysm in a lower-risk setting may be managed differently depending on other factors
Another important consideration is whether the aneurysm changes over time. Evidence of growth can increase concern, regardless of the starting size.
Why Location Matters
Not all areas of the brain carry the same level of risk when it comes to aneurysms.
The brain’s blood supply is divided into different regions, and aneurysms can occur along various arteries. Some locations are associated with a higher likelihood of rupture, even when the aneurysm is relatively small.
This is why location is always considered alongside size when assessing overall risk.
Posterior vs Anterior Circulation
A key distinction in neurovascular assessment is between the anterior circulation and the posterior circulation.
Anterior circulation
This includes arteries at the front of the brain, such as:
The internal carotid arteries
The anterior cerebral arteries
The middle cerebral arteries
Aneurysms in this region are more common and, in many cases, are associated with a lower overall risk of rupture, particularly when small.
Posterior circulation
This includes arteries at the back of the brain, such as:
The basilar artery
The vertebral arteries
Posterior cerebral arteries
Aneurysms in the posterior circulation are less common but are generally considered to carry a higher risk, even at smaller sizes.
Why Posterior Aneurysms May Carry Higher Risk
The exact reasons are complex, but several factors are thought to contribute:
Differences in blood flow dynamics
Structural characteristics of the vessel walls
The anatomical environment in this region
From a practical perspective, this means that:
A smaller aneurysm in the posterior circulation may be treated differently from a similar-sized aneurysm in the anterior circulation.
How Size and Location Are Considered Together
In clinical practice, size and location are not assessed in isolation. Instead, they are considered together to build a more complete picture of risk.
For example:
A small anterior circulation aneurysm may be suitable for monitoring
A similarly sized posterior aneurysm may require closer follow-up or consideration of treatment
Larger aneurysms, particularly in higher-risk locations, are more likely to be considered for intervention
This combined assessment helps ensure that decisions are tailored to the individual rather than based on a single measurement..
Does Higher Risk Always Mean Treatment?
Not necessarily. Even when certain features suggest a higher level of risk, the decision to treat depends on balancing:
The potential benefit of intervention
The risks associated with treatment
The individual patient’s overall health and preferences
Treatment options may include monitoring or procedures such as endovascular coiling or surgical clipping, depending on the individual case.
When Specialist Review Is Helpful
Understanding aneurysm risk can be complex, particularly when multiple factors are involved.
Specialist review may be helpful when:
The significance of size or location is unclear
Different opinions have been given
You are unsure whether monitoring or treatment is appropriate
A neurovascular specialist can explain how these factors apply in your specific situation and help guide next steps.
What This Means in Everyday Terms
Many brain aneurysms, particularly smaller ones, remain stable and do not cause problems.
While size and location are important factors, they are only part of a broader assessment. Risk develops over time and varies between individuals.
With careful evaluation and appropriate follow-up, most aneurysms can be managed in a considered and measured way.
About the Specialist
Mr Ahilan Kailaya-Vasan is a Consultant Neurosurgeon and Neurovascular Surgeon in London with over 17 years of experience in the assessment and treatment of complex brain and cerebrovascular conditions. He is recognised for his measured, evidence-based approach and is frequently consulted for specialist opinions in cases where diagnosis or management is uncertain.
As a private neurosurgeon, he works with patients in the UK and internationally, providing specialist review of imaging and clear guidance in complex neurovascular surgery cases.
If you would like a specialist opinion or further clarification regarding your diagnosis or treatment options, please get in touch.
