How AVMs Affect Blood Flow in the Brain
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
An arteriovenous malformation, or brain AVM, changes the normal flow of blood through the brain.
Blood bypasses the tiny capillaries that normally deliver oxygen to brain tissue.
AVMs can affect blood flow in different ways, but many cause no symptoms.
Understanding how an AVM changes circulation helps guide decisions about monitoring and treatment.
An arteriovenous malformation (AVM) is an abnormal connection between arteries and veins in the brain. Rather than following its usual route through a network of tiny blood vessels called capillaries, blood flows directly from an artery into a vein.
An AVM on the brain affects part of the brain’s wider circulatory system, which carries blood, oxygen and nutrients to tissues throughout the body.
This change in circulation alters the way blood moves through the brain. While most AVMs remain stable and never cause problems, others can affect surrounding brain tissue or increase the risk of complications.
Understanding how an AVM affects blood flow helps explain why treatment decisions vary from one person to another.
How Blood Normally Flows Through the Brain
The brain forms part of the central nervous system and relies on a continuous supply of oxygen and nutrients to function normally.
Under normal circumstances:
Arteries carry oxygen-rich blood from the heart to the brain.
Blood then passes through tiny blood vessels called capillaries, where oxygen and nutrients are delivered to brain tissue.
Veins return oxygen-depleted blood back towards the heart.
Arteries, veins and capillaries are different types of blood vessels, each with a specific role in circulation. The network of small blood vessels connecting arteries and veins is known as the capillary bed.
This carefully regulated circulation allows the brain to receive the blood supply it needs while maintaining stable pressure within the blood vessels.
What Happens in an AVM?
An AVM changes this normal pattern of blood flow.
Instead of passing through the capillary network, blood travels directly from an artery into a vein through a tangled collection of abnormal blood vessels known as the nidus.
A brain AVM is often described as a congenital defect, meaning the abnormal blood vessel connections are generally thought to develop before birth or in early childhood. However, the precise reason why many AVMs form is not fully understood.
Because arteries normally carry blood at much higher pressure than veins, this direct connection exposes the veins to pressures they were not designed to withstand.
At the same time, surrounding brain tissue may receive less blood than usual because some of the circulation is diverted through the AVM. Specialists sometimes describe this as a “steal phenomenon”, although its significance varies between individuals.
How Does an AVM Affect the Brain?
The effects of an AVM depend on several factors, including:
Its size
Its location within the brain
The pattern of blood flow
Whether bleeding has occurred
These features form part of the wider risk factors a specialist considers when assessing an AVM.
Many AVMs cause no symptoms and are discovered incidentally during brain imaging performed for unrelated reasons.
When symptoms do occur, they may result from:
Changes in normal blood flow
Bleeding from the AVM
Seizures caused by irritation of the surrounding brain
Not every AVM behaves in the same way, which is why assessment is always individual.
Does Altered Blood Flow Damage the Brain?
Not necessarily.
Although an AVM alters normal circulation, this does not automatically mean the brain is being damaged.
In many people, the brain adapts to these changes and continues to function normally. Some AVMs remain stable over the long term without affecting neurological function.
The main concern is understanding whether the AVM increases the risk of future problems, rather than assuming that altered blood flow is causing ongoing injury.
Why Blood Flow Matters When Deciding on Treatment
One of the main goals of specialist assessment is to understand how the AVM is affecting the surrounding brain.
Detailed imaging helps assess:
The size of the AVM
How blood enters and leaves the malformation
Which areas of the brain are involved
Whether the AVM has features associated with a higher risk of bleeding
These findings help determine whether monitoring or treatment is likely to offer the greatest benefit.
Does Every AVM Need Treatment?
No. Many AVMs can be monitored safely without immediate intervention.
Treatment decisions depend on balancing:
The risk of future bleeding
The location of the AVM
The complexity of treatment
The person’s age and overall health
Whether symptoms are present
When treatment is recommended, options may include microsurgery, endovascular embolisation, stereotactic radiosurgery, or a combination of these approaches.
When Specialist Review Is Helpful
AVMs are among the most complex neurovascular conditions because no two are exactly alike.
Specialist review is particularly valuable when:
An AVM has been discovered incidentally.
Different treatment options have been suggested.
The risks and benefits of treatment are unclear.
You would like a second opinion before making a decision.
A neurovascular surgeon can explain how an AVM affects blood flow in your individual case and whether treatment or monitoring is the most appropriate course of action.
Putting This Into Perspective
Although an AVM changes the way blood flows through the brain, this does not necessarily mean it will cause symptoms or require treatment.
Understanding how an AVM affects circulation is one part of a much broader assessment that includes its size, location, and overall risk profile.
With careful evaluation and specialist guidance, many people with an AVM can make informed decisions about monitoring or treatment with confidence.
About the Neurovascular Specialist
Mr Ahilan Kailaya-Vasan is a Consultant Neurosurgeon and Neurovascular Surgeon 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 cases.
If you would like a specialist opinion or further clarification regarding your diagnosis or treatment options, please get in touch.