Patients with progressive strokes often had bilateral vertebral artery occlusions. The mechanism of basilar artery occlusion is different depending on the segment of the vessel that is occluded. On one hand, most cases of distal (top of the basilar) or proximal (vertebrobasilar junction) occlusions are due to embolism (blood clot) either from a cardiac or an arterial source. On the other hand, midbasilar artery occlusion is typically the result of atherothrombosis (plaque).