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Background: Subclavian steal phenomenon (SSP) refers to subclavian artery steno-occlusive disease proximal to the origin of the vertebral artery and is associated with flow reversal in the vertebral artery. In 1960, Reivich recognized the association between this phenomenon and neurologic symptoms while in 1961, Contorni described retrograde flow in the vertebral artery. Fisher dubbed this combination of retrograde vertebral flow and neurologic symptoms subclavian steal syndrome (SSS), suggesting that blood is stolen by the ipsilateral vertebral artery from the contralateral vertebral artery. It was later suggested that such "steal" may cause brainstem ischemia and stroke, either continuously or secondary to arm exercise.

The term SSS may be defined for retrograde vertebral artery flow associated with transient neurologic symptoms related to cerebral ischemia. SSP refers to retrograde flow in the vertebral artery only. First diagnosed angiographically in the early 1960s, SSS is now most commonly diagnosed during Doppler ultrasound (US) examination of the neck arteries.

Pathophysiology: The primary lesion causing vertebral artery flow reversal is proximal subclavian artery stenosis or occlusion, resulting in decreased blood pressure in the arm distal to the steno-occlusive disease. This pressure reduction initially causes ipsilateral vertebral artery blood flow alteration provided the subclavian disease is proximal to the origin of the vertebral artery.

Ultimately, a flow reversal occurs in the ipsilateral vertebral artery as compensatory collateral to the compromised vascular territory beyond the subclavian steno-occlusive lesion. Other potential collateral pathways are those between the external carotid artery (ECA) and the subclavian artery, from the occipital branch of the ECA to the deep cervical branch of the costocervical trunk, and from the superior thyroid artery of the ECA to the inferior thyroid artery branch of the thyrocervical trunk.

Classification of subclavian steal can be defined by territory from which blood is stolen, as described by Vollmar et al. Vollmar recognized four types of subclavian steal: vertebro-vertebral, carotid-basilar, external carotid-vertebral and carotid-subclavian (can only occur with occlusion of brachiocephalic artery). Another classification is based on vertebral artery hemodynamics as described by Branchereau and colleagues. Hemodynamic abnormalities ranged from reduced antegrade vertebral flow (stage I), reversal of flow during reactive hyperemia testing of the arm (stage II), and permanent retrograde vertebral flow (stage III). The three stages correlate with disease severity with stage III, usually indicating subclavian artery occlusion.

Arm symptoms may be provoked by an increased blood flow requirement to the compromised upper extremity (eg. during arm exercise or after producing peripheral reactive hyperemia by arm cuff inflation), or, alternatively, by limiting vertebral compensatory flow to the subclavian artery (eg during neck movements).

Subclavian steno-occlusive disease produces neurologic symptoms when compensatory flow to the subclavian artery from the vertebral artery diverts too much flow toward the arm and away from intracranial structures. The quality of collateral blood supply and the capacity to increase collateral flow to the intracranial circulation (brainstem in particular) may be the principle determinant as to which patient develops neurologic symptomatology.