Infarct Laterality Patterns in Relation to A1 Segment Hypoplasia/Aplasia According to Etiological Subtype

Background: The Circle of Willis (CoW) is a key collateral pathway that enables communication between the anterior and posterior cerebral circulations. However, anatomical variations in the A1 segment of the anterior cerebral artery, such as hypoplasia or aplasia, can alter hemodynamics and may compromise this collateral function. While incomplete CoW configurations have been linked to aneurysm formation and altered patterns of hemorrhage, their role in the distribution of cerebral infarctions remains controversial. We aimed to explore the association between A1 segment hypoplasia/aplasia and infarct laterality across different etiological subtypes. Methods: We retrospectively analyzed patients with unilateral anterior circulation infarction admitted between April 2017 and March 2023. The CoW was assessed by magnetic resonance angiography (MRA). A1 segment hypoplasia was defined as a segment diameter <1 mm, and A1 aplasia was defined as non-visualization on MRA. The side with hypoplasia or aplasia was defined as the minor side, and the contralateral side as dominant. We assessed whether infarction occurred on the minor or dominant side. Results: Among 198 patients with unilateral anterior circulation infarction classified as lacunar, cardioembolic stroke (CES), or embolic stroke of undetermined source (ESUS), 30% had A1 hypoplasia or aplasia, with similar prevalence across subtypes. Infarcts occurred on the A1 dominant side in 53% of lacunar, 55% of ESUS, and 75% of CES cases. Although this difference did not reach statistical significance (p = 0.43), it should be interpreted with caution given the limited sample size. Conclusions: The rates of A1 hypoplasia and aplasia were similar across stroke types. No statistically significant association was identified. The findings remain inconclusive given the limited sample size. These results should be considered exploratory and hypothesis-generating.