Fig. 1.

Classic type of vessel wall disruption during dissection. A normal vertebral arterial wall consists of three layers-intima (the innermost layer), media (the middle muscular layer), and adventitia (the outermost layer) (A). When a tear occurs in the arterial wall (arrow in A) and allows blood to enter the wall of the artery and split its layers, the result is the intrusion of blood within the layers of an arterial wall (intramural hematoma). This causes stenosis of the lumen when blood collects between the intima and media (B) or an aneurysmal dilatation of the artery when the hematoma predominantly involves the media and adventitia (C).