Table of ContentsView AllTable of ContentsAnatomyFunctionVariationsAssociated ConditionsTreatment
Table of ContentsView All
View All
Table of Contents
Anatomy
Function
Variations
Associated Conditions
Treatment
The circle of Willis is a group of blood vessels in the brain that form a continuous structure resembling a circle. Thesearteriessupply blood to a large portion of the brain and function as a “failsafe” system if any of the arteries become blocked or narrowed.
The circle of Willis is named after Thomas Willis (1621–1675), an English physician who first identified this structure of blood vessels in 1664.
This article describes the anatomy and function of the circle of Willis, including medical conditions that can adversely affect it.
Anatomy of the Circle of Willis
The circle of Willis is made of up arteries. Arteries are the blood vessels that transport oxygenated blood from the heart to tissues throughout the body.
The circle of Willis is a relatively small, symmetrical structure with a circumference of just a few centimeters. Each blood vessel in the circle has a diameter of between 1.5 and 2.5 millimeters.
Phil Boorman / Getty Images
The circle of Willis is composed of the following arteries:
The circle of Wills is completed with the two “communicating” arteries:
A Guide to the Anatomy of the Brain
Function of the Circle of Willis
The function of the circle of Willis is to enable “collateral circulation.” This is when blood flow is redirected whenever there is an obstruction of any sort.
If one part of the circle becomes blocked (referred to asischemia) or narrowed (referred to asstenosis) or one of the arteries supplying the circle becomes blocked or narrowed, blood from the other vessels can “work around it,” providing enough blood to avoid tissue damage.
This can limit the damage caused during serious events like stroke.
Redundant Blood FlowThe circle of Willis serves as a backup system (redundancy) if blood flow is suddenly reduced. Should the PCA, ICA, or ACA ever become blocked, blood can be rerouted through the communicating arteries to compensate partially or fully for the loss.
Redundant Blood Flow
The circle of Willis serves as a backup system (redundancy) if blood flow is suddenly reduced. Should the PCA, ICA, or ACA ever become blocked, blood can be rerouted through the communicating arteries to compensate partially or fully for the loss.
What Is Brain Ischemia?
Structural Differences
The circle of Willis can vary considerably from one person to the next, with only around 30% of people having a classic (intact) structure.This may explain why some people are more vulnerable to severe injury from a stroke or aneurysm—simply because blood is not rerouted as effectively as it could be.
One of the more common variations is the absence of a posterior communicating artery. Another is having a large posterior communicating artery and a small PCA, which requires the ICA to service a large part of the back of the brain.Both can have serious consequences if the PCA ever becomes blocked.
Another potentially serious variation is anazygosanterior cerebral artery (azygos ACA) in which a single ICA supplies both ACAs. A blockage of the ICA can cause problems on both sides of the brain.
Not every variation has serious consequences or any consequences at all. These include variations where communicating arteries are duplicated or split into multiple branches. These can still provide collateral circulation in the event of arterial ischemia or stenosis.
Brain Aneurysm
Abrain aneurysmis a defect in which an artery in the brain develops a bulging pouch. The bulge can press on certain nerves, causing them to malfunction.
An aneurysm in the circle of Willis can impinge on the optic chiasm, causing vision loss in yourcentral visionorperipheral visionin one or both eyes. Pressure on the pituitary gland can cause the overproduction or underproduction of hormones that keep the body functioning normally.
Small brain aneurysms, which are often described asberry aneurysms, can cause headaches, fatigue, dizziness, and trouble concentrating.
A bigger concern is the spontaneous rupture of an aneurysm. If it occurs in the circle of Wilis, you would likely experience a stiff neck, severe headache, vomiting, numbness, weakness, abnormal pupils, and loss of consciousness.Death is likely in 35% to 50% of cases.
Stroke
A stroke is the interruption of blood flow to the brain preventing the delivery of oxygen and nutrients that the brain needs to function and survive. The loss of blood causes tissues to die, leading tosymptoms of a stroke.
Even with an intact circle of Willis, the interruption of blood flow can’t always be fully compensated if the disruption is severe.
There are two main types of stroke that can involve the circle of Wilis:
Traumatic Brain Injury
A severe head trauma can stretch or tear an artery in the circle of Willis. If the artery bleeds, the built-in redundancy may not be able to compensate for the disruption and affect blood circulation downstream of the circle.
Even if bleeding does not occur, inflammation of the brain can press on vessels in the circle of Willis and reduce blood flow. One example of a non-hemorrhagictraumatic brain injuryis aconcussion, leading to symptoms like headaches, dizziness, nausea, vomiting, and confusion.
Subclavian Steal Syndrome
Subclavian steal syndromeoccurs when thesubclavian arteryin the arm becomes narrow, causing blood to backflow into a PCA rather than carrying blood away from it. This can “steal” blood from other arteries of the circle of Willis, leading to widespread neurological symptoms like dizziness, vision changes, hearing loss,difficulty walking, anddifficulty speaking.
Most cases are relatively mild, but some may require medical intervention.
Treatment and Rehabilitation
If you have a disease or injury that affects the circle of Willis, treatment can often be quite complex.
For example, if you have an aneurysm, your medical team may decide to take a watch-and-wait approach due to the risks associated withaneurysm surgery. On the other hand, they may need to act immediately if there is a high risk of a rupture.
For other defects, less invasive procedures likeangioplastyandstentingmay help reverse stenosis. Rather than accessing the brain through the skull, a narrow catheter (tube) can be threaded through a blood vessel in your neck or groin to the site of the narrowing.
In the event of a stroke, the first line of treatment is to limit the brain injury withanticoagulants(blood thinners) and other drugs. Thereafter, you may require extensive rehabilitation depending on which blood vessels are affected.
For example, if you have had a stroke of the ACA, you may need morecognitive rehabilitation. If you had a stroke affecting the PCA, you may need moremotor rehabilitation.
Brain Aneurysm Prognosis
Summary
The circle of Willis is a circle of arteries that act as a backup system should an artery in the brain become blocked or narrowed. If this occurs, blood can be rerouted so that damage to brain tissues is avoided or reduced.
Certain medical conditions can affect the circle of Willis and cause the system to break down. These include stroke, brain aneurysms, traumatic brain injuries, and subclavian steal syndrome. Certain treatments can limit the damage or avoid more serious complications.
12 Sources
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