Correlated Vascular Asymmetries in the Circle of Willis

Abstract

The Circle of Willis (CoW) is a cranial vascular structure with documented anatomical variation. The variations of basic structure and collateral flow of the CoW are suggested to play a role in the development of, and protection against, ischemic stroke and injury. Patients with more extensive collateral flow pathways are less likely to suffer from ischemic injury. The internal carotid arteries, vertebral arteries, and basilar artery have been studied for impact of agenesis, aplasia, hypoplasia, and other congenital asymmetries on clinical outcomes. Previous studies have chosen to focus on specific vessels within CoW, particularly on the stenosis or compromise of certain vessels. Far less attention has been given to predictive models of vessel asymmetry. If anastomotic weaknesses and vessel asymmetries tend to co‐occur across different portions of the CoW, then some anatomical variants may carry higher risk of vascular injury than previously recognized. Our aim was to assess the correlations between missing or asymmetric vessels within the CoW, and to relate these to risk of vascular injury using a cadaveric population with known medical histories and causes of death.With permission from the Keck School of Medicine at the University of Southern California and the Anatomical Gift Program at USC, 33 cadavers managed by the Integrative Anatomical Sciences Department in the Keck School of Medicine were utilized in the study. As a part of the ongoing medical curriculum, the brains were removed and dissected from all 33 cadavers, with careful attention to preserving the CoW anatomy. We then used digital calipers to measure the diameter of vessels within the CoW, including the basilar artery, vertebral arteries (VA), posterior cerebral arteries (PCA), middle cerebral arteries (MCA), anterior cerebral arteries (ACA), anterior communicating arteries (Acomm), and posterior communicating arteries (Pcomm). Serial t‐tests with a Bonferroni Correction were employed to compare mean diameters. Reduced Major Axis regression was employed to determine if vessel size or left to right asymmetry in a specified area of the CoW could predict shape in other portions of the CoW.We found that ACA diameter significantly predicted a moderate proportion of the variation in Acomm diameter but that PCA diameters had no correlation with the diameter of the Pcomm vessels or Pcomm asymmetry. We found that the diameter of the right vertebral artery was significantly more predictive of basilar artery diameter than that of the left vertebral artery. Vertebral artery left to right asymmetry was also correlated with left to right asymmetry in the PCA. Unexpectedly, pairwise tests indicated that the difference in mean diameter was greater (and statistically significant) between left and right vessel pairs than in cross quadrant pairs (left ACA vs right PCA, for example). In fact, cross‐quadrant mean diameters did not differ significantly. This implies that right and left halves of the CoW tend to have opposing anteroposterior asymmetry. Our results suggest that the posterior CoW is particularly variable, and that vertebral artery asymmetry is dominated by variability of the left vertebral artery. The apparent counter‐directional asymmetry between left and right halves of the CoW may be an adaptive developmental pattern that promotes collateral flow potential in a highly variable system.Support or Funding InformationAccess to research material was provided by the Anatomical Gift Program at USCThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.