Subclavian steal

History: Elderly woman undergoing vascular evaluation. She has undergone left carotid endarterectomy. Now with evidence of swelling overlying the left carotid artery. Exam performed to rule out aneurysm.


Findings

Hard and soft plaque within the left and right common carotid arteries without clinically significant stenosis (normal flow velocities). Reversal of flow in left vertebral artery. Increased flow velocity in the right vertebral artery.

Differential diagnosis:
- Subclavian steal
- Arteriovenous fistula
- Complete occlusion of the left vertebral artery
- Severe vertebral artery hypoplasia


Diagnosis: Subclavian steal


Key points

Etiology: Severe subclavian artery (SCA) stenosis or occlusion with collateral flow of blood to the arm via the vertebral artery (VA)
With proximal SCA occlusion, blood flow is "stolen" from the ipsilateral VA with hyperdynamic flow through the contralateral VA as it is supplying the vertebrobasilar system and collateral blood flow to the contralateral arm.
Causes: By far the most common is atherosclerotic disease (>70% stenosis). Other less likely causes are vasculitis, neoplasm with mass effect, or dissection.
Clinically often asymptomatic. May cause arm claudication during exercise. Decreased upper extremity blood pressure on ipsilateral side of occlusion/stenosis
Very rarely causes vertebrobasilar infarct of the brainstem or cerebellum due to collateralization of the anterior circulation.
Predominantly occurs on the left side (85%).

Treatment:
- Agioplasty or stenting
- Bypass graft such as common carotid to subclavian bypass
- Address atherosclerotic risk factors


Radiology

CT findings: Occlusion or severe stenosis of the proximal SCA on contrast-enhanced study

MR findings:
- T2WI – absence of flow voids if severely stenotic SCA or occluded
- MRA – can determine degree of SCA stenosis. Phase contrast MRA can confirm vessel patency and direction of flow

Angiography: reversal of flow in the ipsilateral vertebral artery with high grade proximal SCA stenosis

Ultrasound: Gold standard
- Reversal of flow in the VA ipsilateral to the proximal SCA occlusion/severe stenosis
- Moderate steal can appear as to-and-fro flow in the ipsilateral VA
- Increased blood flow velocity in the contralateral VA
- Doppler evidence of SCA stenosis such as turbulence or high velocity
- Early subclavian steal physiology identified on Doppler waveforms include transient sharp deceleration of blood flow after the first systolic peak. As disease state progresses, the second systolic peak broadens and diminishes until reversal of flow during systole with return of antegrade flow during diastole.