The Circle of Willis: Neurovascular Architecture

1. Overview

The Circle of Willis (Circulus Arteriosus Cerebri) is a ring-like arterial anastomosis located at the base of the brain, specifically in the interpeduncular cistern. It serves as a safety mechanism, providing collateral blood flow if one of the major arteries is occluded.

2. Components

The circle is formed by the anastomosis between the internal carotid arteries (ICA) and the vertebrobasilar system:

• Anterior Communicating Artery: Connects the two anterior cerebral arteries.
• Anterior Cerebral Arteries (ACA): Supply the medial surface of the brain.
• Internal Carotid Arteries: Provide the primary anterior circulation.
• Posterior Communicating Artery: Connects the ICA system to the posterior cerebral system.
• Posterior Cerebral Arteries (PCA): Terminal branches of the basilar artery.

3. Variations and Anomalies

Complete circles are found in only about 20-25% of the population. Hypoplasia of the posterior communicating artery is a common anatomical variation.

4. Clinical Significance

• Berry Aneurysms: Most common at the junctions of the circle, particularly the anterior communicating artery. Rupture leads to Subarachnoid Hemorrhage (SAH).
• Stroke: If the circle is well-formed, an occlusion in the ICA may remain asymptomatic due to collateral flow from the contralateral side.

5. Quiz

1. Which artery connects the anterior and posterior circulations?
2. Where is the most common site for a saccular aneurysm?
3. The PCA is a terminal branch of which artery?