Circle of Willis and Collateral Circulation in Stroke: A Narrative Review

Background: Stroke is a leading cause of death and long-term disability worldwide. Variability in clinical presentation and outcomes among patients with similar vascular occlusions is increasingly attributed to differences in cerebrovascular anatomy and collateral circulation, particularly the configuration of the Circle of Willis. Objective: To synthesize existing evidence on the anatomy, embryological development, anatomical variants, and functional significance of the Circle of Willis and collateral circulation, and to examine their relevance to stroke pathophysiology and clinical management. Methods: A narrative review of peer-reviewed literature was conducted using PubMed/MEDLINE, Scopus, Google Scholar, and the Cochrane Library. Anatomical, embryological, imaging, and clinical studies relevant to cerebrovascular anatomy, collateral circulation, and stroke were qualitatively synthesized. Results: Anatomical variations of the Circle of Willis are common and significantly influence collateral capacity. Primary and secondary collateral pathways play a key role in maintaining cerebral perfusion, determining infarct core–penumbra dynamics, and influencing response to reperfusion therapies. Adequate collateral circulation is associated with delayed infarct progression and improved outcomes, whereas poor collateralization predicts rapid neurological deterioration. Conclusion: Stroke is shaped not only by time but also by cerebrovascular anatomy and physiological reserve. Variations in the Circle of Willis and collateral networks critically influence infarct evolution and clinical outcome, underscoring the importance of anatomy-guided assessment in modern stroke management.