A clinically significant reduction in blood supply to the brain stem and cerebellum (i.e., vertebrobasilar insufficiency) resulting from reversal of blood flow through the vertebral artery from occlusion or stenosis of the proximal subclavian or brachiocephalic artery. Common symptoms include vertigo; syncope; and intermittent claudication of the involved upper extremity. Subclavian steal may also occur in asymptomatic individuals. (From J Cardiovasc Surg 1994;35(1):11-4; Acta Neurol Scand 1994;90(3):174-8)