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A new classification system for the anatomical variations of the human circle of Willis: A systematic review

Ayre, James R.; Bazira, Peter J.; Abumattar, Mohammed; Makwana, Haran N.; Sanders, Katherine A.


James R. Ayre

Mohammed Abumattar

Haran N. Makwana

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Dr Kat Sanders
Senior Lecturer in Clinical Anatomy


The circle of Willis (CoW) is an anastomotic arterial network located on the base of the brain. Studies have shown that it demonstrates considerable anatomical variation in humans. This systematic review aimed to identify and catalogue the described anatomical variations of the CoW in humans to create a new, comprehensive variation classification system. An electronic literature search of five databases identified 5899 studies. A two-phase screening process was performed, and studies underwent quality assessment. A total of 42 studies were included in the review. Data were extracted and circles were reconstructed digitally using graphics software. The classification system contains 82 CoW variations in five continuous groups. Group one contains 24 circles with one or more hypoplastic segments only. Group two contains 11 circles with one or more absent segments only. Group three contains 6 circles with hypoplastic and absent segments only. Group four contains 26 circles with one or more accessory segments. Group five contains 15 circles with other types of anatomical variation. Within each group, circles were subcategorised according to the number or type of segments affected. An original coding system was created to simplify the description of anatomical variations of the CoW. The new classification system provides a comprehensive ontology of the described anatomical variations of the CoW in humans. When used with the coding system, it allows the description and categorisation of recorded and unrecorded variants identified in past and future studies. It is applicable to current clinical practice and the anatomical community, including human anatomy education and research.


Ayre, J. R., Bazira, P. J., Abumattar, M., Makwana, H. N., & Sanders, K. A. (2022). A new classification system for the anatomical variations of the human circle of Willis: A systematic review. Journal of anatomy, 240(6), 1187-1204.

Journal Article Type Review
Acceptance Date Dec 10, 2021
Online Publication Date Dec 21, 2021
Publication Date 2022-06
Deposit Date Jan 4, 2022
Publicly Available Date Jan 5, 2022
Journal Journal of Anatomy
Print ISSN 0021-8782
Electronic ISSN 1469-7580
Publisher Wiley
Peer Reviewed Peer Reviewed
Volume 240
Issue 6
Pages 1187-1204
Keywords Cerebrovascular anatomy; Classification; Neuroanatomy; Variation
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© 2021 The Authors. Journal of Anatomy published by John Wiley & Sons Ltd on behalf of Anatomical Society.

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

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