Circle of Willis

What is the circle of Willis?

The circle of Willis (CoW) is a ring- or pentagon-shaped structure of arteries located at the base of your brain. It’s where two essential blood flow circulations for your brain connect — the internal carotid artery pathway and the vertebral artery pathway. The internal carotid arteries supply fresh, oxygen-rich blood to most of the front of your brain. The vertebral arteries do the same for most of the back of your brain.

A complete circle of Willis functions as a fail-safe. If one of these two blood vessel pathways becomes damaged, such as from a ruptured brain aneurysm or stroke, the other pathway can still provide blood flow to the affected part of your brain via the CoW.

The circle of Willis gets its name from the anatomist Thomas Willis, who contributed much to our understanding of this anatomical feature today. Other names for the CoW include:

• Cerebral arterial circle.
• Willis polygon.
• Loop of Willis.

Variations of the circle of Willis

Researchers estimate that a complete circle of Willis exists in less than half of the general population. There are several possible structural variations, which often result in an incomplete circle of Willis.

The most common variations involve changes in the anterior communicating artery (a blood vessel in the front part of the CoW), including:

• Fenestration: This affects up to 21% of the general population. Fenestration means that a single blood vessel divides into two channels that later fuse back together.
• Duplication: This affects up to 18% of the general population. Duplication happens when two arteries with distinct origins join together to form a single segment.

Even if your CoW is incomplete, you can still be healthy and have healthy brains. However, it may increase the risk of severe effects if you get cerebrovascular disease, like a stroke. There’s nothing you can do to prevent or change an incomplete CoW — it’s just how your blood vessels naturally form during fetal development.

How common is an incomplete circle of Willis?

Researchers believe that an incomplete circle of Willis is common. It’s difficult to know exactly how common it is, though. Only a small percentage of the population gets or needs brain imaging that can evaluate their CoW. Various studies suggest that 50% to 90% of adults have an incomplete CoW.

What is the function of the circle of Willis?

To understand the function of the circle of Willis, it helps to think of this structure as a traffic roundabout.

Imagine that each of the arteries that make up the CoW is a different road. The two main “highways” that provide the most traffic (blood flow) are the internal carotid artery pathway and the vertebral artery pathway. The other, smaller roads in the CoW form a roundabout for these two highways to connect and exchange traffic.

The CoW helps ensure that if there’s a roadblock or “accident” in one of the main highways (one of your brain’s main arteries), the other highway can help move traffic (blood) to the affected part of your brain. In other words, a complete circle of Willis offers a detour route in the case of a blockage or issue.

If you have a complete circle of Willis, it may prevent you from experiencing severe effects from disease or damage to the arteries of the CoW. These conditions may include:

• Plaque buildup inside the arteries (atherosclerosis).
• Brain bleed (intracranial hemorrhage), like from a traumatic injury.
• Carotid artery disease (carotid artery stenosis).
• Inflammation of the brain vessels (Central nervous system vasculitis).
• Ruptured brain aneurysm.
• Stroke.
• Transient ischemic attack (TIA) or mini-stroke.

If you have an incomplete CoW, this roundabout doesn’t work as a fail-safe. It may increase your risk for more severe effects of these cerebrovascular conditions, especially stroke.

Where is the circle of Willis located?

The circle of Willis sits at the base of your brain, in the space just outside the brain tissue (subarachnoid space). It surrounds the stalk of your pituitary gland, just below your hypothalamus. The circle of Willis is also near several cranial nerves.

When would I hear about the circle of Willis from my healthcare provider?

There are a few reasons you may hear about the circle of Willis from your healthcare provider, including if you:

• Have a brain aneurysm: The CoW is one of the most common locations for intracranial aneurysms. Researchers estimate that about 85% of intracranial aneurysms happen in the anterior (front) circulation of the CoW.
• Have a stroke: You can have a stroke due to a blockage or bleeding in one of the CoW arteries.
• Have moyamoya disease: This is a rare brain condition. It happens when you have progressive narrowing or blockage in the arteries of your CoW that supply mostly the front of the brain
• Have subclavian steal syndrome: This happens when your subclavian artery narrows or has a blockage and “steals” blood that’s supposed to go to your brain via your vertebral artery — one of the main arteries of your CoW.
• Need surgery in or around the base of your brain: Surgery in this part of your brain — like to treat an aneurysm or a pituitary gland condition — carries the risk of damaging your CoW and other critical structures around it. Damage to the CoW itself may result in brain ischemia (lack of blood flow) or infarction (blood flow blockage).
• Get brain imaging: If you get a brain imaging test, like a cerebral angiogram or brain MRI (magnetic resonance imaging), your report may include information about your CoW and its structure.

What happens when the circle of Willis is blocked?

If you have a blockage in your circle of Willis, it’ll likely cause a stroke. But if you have a complete circle of Willis, it may protect you from experiencing severe symptoms of stroke.

What is the circle of Willis syndrome?

Some healthcare providers or medical sources may call moyamoya disease the “circle of Willis syndrome.” Another name for moyamoya disease is “spontaneous occlusion of the circle of Willis.”

What can I do to keep my circle of Willis healthy?

The main thing you can do to keep your circle of Willis — and other brain blood vessels — healthy is to manage any risk factors you have for cerebrovascular disease. These risk factors include:

• High blood pressure.
• High cholesterol.
• Diabetes.
• Heart disease.
• Chronic stress.
• Obesity.
• Smoking (nicotine use disorder).
• Alcohol use disorder.

Talk to your healthcare provider about strategies for managing these conditions.