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The vertebral and basilar arteries supply blood to several structures in the brain including: the occipital cortex, the brainstem consisting of the midbrain, pons and medulla, the cerebellum and the thalamus. Atherosclerosis or hardening of the arteries is the primary cause of vertebrobasilar disease. The narrowing of the vertebral or basilar arteries caused by atherosclerosis creates vertebrobasilar insufficiency (VBI), or an insufficient delivery of blood flow to the posterior structures of the brain.
The phenomenon of vertebral steal occurs when a severe proximal subclavian artery stenosis or occlusion is present and blood reaches the affected arm by passing down the ipsilateral vertebral artery from the posterior fossa circulation, which is being supplied from the other vertebral artery or the circle of Willis (see Fig. 48.3B).