Atypical variants of the cerebral arterial circle beyond the Lazorthes classification: a retrospective morphologic...

To characterize atypical variants of the cerebral arterial circle that fall outside the Lazorthes classification and to interpret them in a contemporary neuroradiologic framework.

This retrospective morphologic-neuroradiologic study analyzed 650 cases collected between 1 September 2023 and 1 March 2026: 40 dissections, including 10 injected with plastic material, 10 digital subtraction angiograms, 300 computed tomography angiograms, and 300 magnetic resonance angiograms. Typical configurations were separated from non-classifiable variants using the 22-variant Lazorthes system as the reference framework.

Of the 650 cases, 523 (80.5%) were assignable to the Lazorthes classification, whereas 127 (19.5%) showed atypical configurations. These included six-sided anterior variants, anatomical absence or imaging non-visualization of one or two arterial segments, and incomplete posterior anastomotic patterns caused by segmental hypoplasia. The most frequent atypical pattern was unilateral anatomical absence or imaging non-visualization of a posterior communicating artery (43 cases; 6.6%), followed by unilateral anatomical absence or imaging non-visualization of a P1 segment (17 cases; 2.6%) and bilateral anatomical absence or imaging non-visualization of the posterior communicating arteries (11 cases; 1.6%).

A substantial proportion of cerebral arterial circles cannot be adequately described within the classical Lazorthes typology. These atypical variants are best interpreted as patterns of altered collateral design with potential relevance for CTA and MRA interpretation, cross-flow potential, vascular territory dependence, and cerebrovascular risk assessment.