A 60-year-old man reports dizziness and visual blurring when using his right arm for overhead work. Right radial pulse is weaker than the left, and blood pressure in the right arm is 30 mmHg lower than the left. Doppler shows reversed flow in the right vertebral artery during arm exercise. What is the mechanism of his symptoms?

• A Emboli from a right subclavian artery plaque to the posterior circulation
• B Compression of the vertebral artery by a cervical rib during abduction
• C Retrograde flow down the right vertebral artery stealing blood from the basilar circulation ✓
• D Hypoperfusion of the anterior circulation through an incomplete circle of Willis

Explanation

Proximal subclavian artery stenosis lowers pressure distal to the lesion, so during increased arm demand blood flows retrograde down the ipsilateral vertebral artery, diverting flow from the basilar system and producing posterior circulation symptoms. This is subclavian steal syndrome. Embolism causes focal ischaemic events rather than exercise-provoked dizziness, cervical rib compression is a cause of thoracic outlet syndrome rather than this haemodynamic pattern, and the circle of Willis is not the relevant pathway here.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.