Figure 1.

Illustration of 5 types of posterior tandem occlusions with distal embolic occlusion of the basilar artery. (A) A proximal stenosis/occlusion of the dominant right vertebral artery, with patency of left non-dominant vertebral artery. (B) A proximal stenosis/occlusion of the dominant right vertebral artery, with stenosis of the left non-dominant vertebral artery. (C) A proximal stenosis/occlusion of the dominant right vertebral artery, with chronic occlusion of left non-dominant vertebral artery (ostium or V2-V4 segments). (D) Equivalent-sized vertebral arteries, with a proximal stenosis/occlusion of the right vertebral artery. (E) Equivalent-sized vertebral arteries, with a proximal stenosis/occlusion of the right vertebral artery and stenosis of the left vertebral artery.