Stenosis of the<unk>  artery, proximal to the origin of the vertebral vessel, results in decreased perfusion pressure to the distal<unk>  artery, leading to retrograde flow in the ipsilateral vertebral artery with exercise of the ipsilateral arm.  In short, the arm<unk> s blood flow from the vertebrobasilar system resulting in neurologic and upper extremity symptoms due to arterial insufficiency. 
Increased incidence of left-sided<unk>  stenosis
The presence of collateral blood supply is the main determinant of which patients develop neurologic symptoms
Atherosclerosis
Thoracic outlet obstruction
Congenital anomalies
Takayasu arteritis
Subclavian steal syndrome
Thoracic outlet syndrome
Axillary artery thrombosis
Vertebrobasilar insufficiency
Superior vena cava syndrome
Syncope
Aortocaval fistula
Vertigo