Clinical meaning
Subclavian steal syndrome occurs when severe stenosis or occlusion of the proximal subclavian artery (proximal to the vertebral artery origin) causes reversal of blood flow in the ipsilateral vertebral artery. Normally, the vertebral arteries carry blood from the subclavian arteries upward to supply the posterior cerebral circulation (brainstem, cerebellum, occipital lobes) via the basilar artery. When the proximal subclavian artery is severely stenosed, pressure distal to the stenosis drops below vertebral artery pressure. Blood flow reverses in the ipsilateral vertebral artery, flowing retrograde from the basilar artery down into the subclavian artery to supply the arm — effectively 'stealing' blood from the posterior cerebral circulation. This steal becomes clinically significant during ipsilateral arm exercise, which increases metabolic demand and further reduces distal subclavian pressure, accentuating the retrograde vertebral flow and producing posterior circulation ischemic symptoms. The left subclavian artery is affected more commonly (3:1) because its origin from the aortic arch is anatomically more susceptible to atherosclerosis. Subclavian steal is most commonly caused by atherosclerosis (same risk factors as coronary and carotid artery disease), but can also result from Takayasu arteritis (young women, large vessel vasculitis), thoracic outlet syndrome, or previous surgery (coronary artery bypass using internal mammary artery — 'coronary-subclavian steal' may steal from both the coronary and vertebral circulations).