The circle of Willis is an arterial anastomosis at the base of the brain. Which of the following is a true clinical implication of this structure?

• A A complete circle ensures no ischemia during unilateral carotid occlusion
• B The posterior communicating artery is the most common site for berry aneurysm formation in the circle
• C An incomplete circle (anatomical variant) is present in most individuals, reducing collateral flow capacity ✓
• D The anterior cerebral arteries do not communicate through the circle

Explanation

While the circle of Willis theoretically provides collateral blood supply between anterior (carotid) and posterior (vertebrobasilar) systems, anatomical studies show that a complete circle is present in only ~20-35% of individuals. Hypoplastic or absent communicating arteries are common variants, reducing the effectiveness of collateral flow during vascular occlusion. The most common site for berry aneurysms is the anterior communicating artery (31%), followed by the posterior communicating artery (25%), not exclusively the PCOM. This anatomical variability explains why some patients develop infarction despite apparent collateral pathways.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.