A patient with severe atherosclerosis of the left subclavian artery proximal to the origin of the vertebral artery develops left arm claudication and transient brainstem symptoms (dizziness, diplopia) triggered by left arm exercise. This phenomenon is called the subclavian steal syndrome. The direction of blood flow in the LEFT vertebral artery during arm exercise is:
- A Reversed (downward from the basilar artery toward the arm) to supply the ischaemic left subclavian territory ✓
- B Normal antegrade (upward toward the basilar artery)
- C Completely absent — the vertebral artery is occluded
- D Bidirectional, with alternating flow depending on the cardiac cycle phase
Correct answer: A. Reversed (downward from the basilar artery toward the arm) to supply the ischaemic left subclavian territory
Explanation
In subclavian steal syndrome, proximal subclavian stenosis/occlusion creates a pressure gradient during left arm exercise; blood is 'stolen' from the basilar artery by retrograde flow down the ipsilateral vertebral artery to supply the arm. This robs the posterior circulation of flow, causing vertebrobasilar ischaemia (dizziness, diplopia, ataxia, syncope). Duplex ultrasound or MR angiography demonstrates the retrograde vertebral artery flow. The right vertebral artery flow remains antegrade.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.