Subclavian steal syndrome (SSS) is a condition in which the subclavian artery proximal to the origin of the vertebral artery narrows or becomes occluded, usually due to atherosclerosis. This blockage results in a lack of blood reaching the ipsilateral arm through the subclavian artery, which can lead to a reversal of blood flow in the affected vertebral artery so that blood from the contralateral side can flow through the circle of Willis to supply the affected arm. The condition is mostly asymptomatic. If symptoms occur, they are mainly caused by ischemia of the affected arm and include limb pain, fatigue, paresthesia, and cold skin. Neurological symptoms such as dizziness or even syncope are rare and usually only occur in the presence of cerebrovascular lesions. A characteristic diagnostic sign is a discrepancy in blood pressure between the arms of > 15 mm Hg. Imaging (e.g., ultrasonography) can identify a reversal of blood flow and/or atherosclerosis that confirms the diagnosis. Endovascular intervention or surgery are used to treat symptomatic SSS.
Stenosis of the subclavian artery proximal to the origin of the vertebral artery; → hypoperfusion distal to the stenosis → reversal of blood flow in ipsilateral vertebral artery → compensation through collateral arteries; → reduced blood flow in the basilar artery → reduced cerebral perfusion upon exertion involving the affected arm 
Subtypes and variants
- Coronary-subclavian steal syndrome