The layouts of the carotid and vertebral systems are opposite. The CCA and the ICA bifurcate. The VA, on the contrary, converges with the other VA to become a single BA.ⁱ The fact that both VAs supply the BA presents a problem when we are trying to determine which VA is the source of an embolic infarction in the cerebellum or brain stem because an embolus lodging in the BA or any of its branches could have originated from either VA. An exception to this statement exists in those individuals who have one hypoplastic VA (missing part of its fourth segment). The embolus in the latter case would have had to travel through the opposite, normal VA.

The ICA and the VA occlude with different patterns. The ICA usually occludes its full length from its origin to the siphon because it has no significant branches between these two points. The VA tends to occlude in a segmental manner involving part or most of its cervical portion, but then, it reconstitutes at the level of C2-C1 by collaterals supplied by the occipital or cervical ascending arteries. This allows us to reconstruct an occluded VA at this distal level, something we cannot do for the ICA.

Our understanding of the carotid and vertebral systems is dissimilar. The protagonism that each system has in the development of a stroke was proven for the carotid territory
in 1951 by Fisher¹, ² and for the vertebral territory in 1956 by Hutchinson and Yates.³ Both studies showed that embolization could occur from plaques that had degenerative features such as ulceration, intraplaque hemorrhage, or superimposed thrombus. It has always been possible to inspect directly the carotid lesions in the operating room and at autopsy and this has allowed the correlation of specific features of the carotid plaque, such as hemorrhage, surface thrombus, and ulceration, with the incidence of clinical symptoms. In the VA, on the contrary, because the operations we do are transpositions or bypasses, neither surgeons nor pathologists have a chance to study the offending specimen.