Circle of Willis configuration as a determinant of intracranial dolichoectasia

Abstract

Background: Circle of Willis (COW) variants might influence arterial caliber in the brain. We hypothesized that these variants would be associated with the prevalence of intracranial dolichoectasia (DE).

Methods: We examined COW variants and DE in a sample of stroke-free participants (n = 436) undergoing magnetic resonance angiography (MRA) as part of a population-based study. Large intracranial arterial diameters were obtained when available; if not, the artery was defined as hypoplastic or absent according to its visibility on MRA. Subscores for the anterior and the posterior circulations were created. DE was defined as arterial diameters ≥2 SD above the population mean for that artery, adjusting for intracranial volume. Generalized linear models with a Poisson distribution were used to evaluate predictors of both absent and hypoplastic vessels, and logistic regression was used to assess the odds ratio (OR) and 95% confidence interval (95% CI) of DE depending on COW variants.

Results: Only 44% of the sample had all 14 arteries present, 32% lacked 1 artery, 18% lacked 2 and 6% lacked 3 or more. DE of at least 1 artery was not associated with the total number of hypoplastic or absent arteries, but DE in a posterior circulation artery was weakly associated with the number of absent arteries in the posterior circulation (β coefficient = 0.36, p = 0.06). DE of at least 1 artery was more frequent in those with 1 or more absent arteries (OR 1.27, 95% CI 1.03-1.57). Posterior circulation DE was more frequent in participants with at least 1 or more absent arteries at any location (OR 1.35, 95% CI 1.02-1.78). Participants with an incomplete posterior COW were more likely to have DE in the anterior circulation (OR 1.52, 95% CI 1.01-2.33). Having an absent left anterior cerebral artery (ACA) A1 segment was associated with right ACA DE (OR 34.1, 95% CI 3.16-368.2); an absent right ACA was associated with left ACA DE (OR 14.1, 95% CI 1.69-118.28). Absence of 1 (OR 1.9, 95% CI 1.1-3.4) or 2 (OR 3.0, 95% CI 1.4-6.6) of the 2 arteries connecting the anterior to the posterior circulation was associated with basilar artery DE.

Conclusion: The COW is a pleomorphic structure that allows collateral flow to compensate for an insufficient or absent arterial component at the base of the skull. By presumed flow diversion, arteries might undergo outward remodeling. Whether this compensatory arterial dilatation is beneficial or not remains unknown.

Fig. 1

The five most common COW variants of the anterior and posterior circulation in the NOMAS.

Fig. 2

Supporting evidence of flow-induced intracranial arterial remodeling. a The odds of BA DE are increased in a dose-effect fashion with the absence of 1 or 2 antero-posterior communicating arteries (either in the form of fPCA or Pcomm). b In the absence of the first segment of the ACA (A1), the contralateral diameters are increased compared to when the A1 segment is present. c The degree of lateral deviation of the BA from its midline position relative to the clivus was positively associated with the degree of contralateral VA dominance.

Fig. 3

BA DE and COW variants. Example of the BA diameter influenced by the status of antero-posterior arteries, in the form of Pcomm or fPCA. Top row case: a1 Note the lack of AP arteries in the top-down view of the COW. a2 Note the lateral deviation of the BA and its dilated origin measuring 5 mm. Bottom row case: b1 Complete COW as demonstrated by presence of both Pcomm and both PCA. b2 The BA diameter is small; its pathway is less anfractuous than that of the top row case.