Figure 1: Axial TOF image with superior saturation band demonstrates flow signal in both common carotids and right vertebral artery. Flow signal is absent in the left vertebral artery, which could represent either occlusion or flow reversal within the artery.

Figure 2: Axial TOF image with inferior saturation band demonstrates flow signal in the jugular veins bilaterally, as well as, in the left vertebral artery (LRA). These imaging findings combined with the imaging findings from Figure 1 are consistent with flow reversal in the left vertebral artery.

Figure 3: Oblique sagittal MIP image demonstrates a stenotic lesion at the origin of the left subclavian artery. There is mild to moderate stenosis at the origin of the left common carotid artery. The right common carotid and right subclavian arteries are unremarkable.

Subclavian steal syndrome.

Subclavian steal syndrome (SSS) results from either a very severe stenosis or occlusion of the subclavian artery proximal to the origin of the vertebral artery. The blood supply to the arm is maintained by retrograde flow in the ipsilateral vertebral artery distal to the obstruction. This retrograde flow in the vertebral artery is supplied at the expense of the cerebral circulation by “stealing” blood from the ipsilateral vertebral and/or basilar artery or the innominate artery. An atherosclerotic lesion at the orifice of the subclavian artery is the most common (94%) etiology of SSS. Other etiologies include congenital (e.g. atrial septal defect, ventricular septal defect, patent ductus arteriosus, tetralogy of Fallot, and aortic coarctation), dissecting anerurysm, chest trauma, inflammatory arteritis, and vascular thrombosis.

Symptoms of SSS include signs of brachial insufficiency such as claudication, paresthesia, numbness, or weakness in the involved arm during increased activity. A lower systolic blood pressure greater than 20 mm Hg in the affected side can be seen in unilateral SSS. Neurologic symptoms, secondary to vertebrobasilar hypoperfusion, may develop in patients with proximal subclavian steal artery stenosis when the ipsilateral arm is exercised. The occurrence of neurologic events depends on additional stenosis of carotid arteries, patency of the circle of Willis, the patient’s general cardiovascular condition, and functional demand of the affected arms. It is important to note that not all patients with diagnosed “steal” phenomena are clinically symptomatic, because reversal of vertebral artery flow represents a normal collateral pathway in response to a proximal subclavian lesion.

The diagnosis of SSS can be suggested on imaging studies by reversed flow in the vertebral artery ipsilateral to the subclavian stenosis or occlusion. This imaging finding has traditionally been demonstrated via Doppler ultrasound and X-ray angiography . Magnetic resonance (MR) imaging represents an alternative non-invasive tool that can demonstrate not only the anatomy of the cervicothoracic vessels but can also quantify the amount and direction of flow through the vessels. Gadolinium-enhanced 3D MR angiography can readily demonstrate stenosis/occlusion in the proximal subclavian artery. However, phase-contrast (PC) or time-of-flight (TOF) MR techniques with saturation bands are needed to demonstrate the physiologic reversal of flow. The 2D TOF technique consists of two imaging sequences, one with the saturation band superior to the image acquisition and one inferior to the image acquisition, allowing visualization of the flow in the affected vertebral artery. Coronal 2D PC MRA techniques with superior to inferior flow encoding allow for quantification of the steal. The reverse flow in the vertebral arteries appears as a high signal intensity, indicating flow is in the craniocaudad direction.

The treatment for SSS has generally been surgical bypass (e.g. carotid-subclavian bypass, axilloaxillary bypass) to restore permanent antegrade blood flow in the vertebral artery and blood flow to the affected arm. Percutaneous transluminal angioplasty (PTA) with or without intravascular stent placement has become an increasingly attractive and successful option used to dilate stenoses or occlusions at many sites in the peripheral arterial tree and subsequently to treat patients with SSS.

References:

1. Higgins C.B and A. De Roos. Cardiovascular MRI & MRA. Philadelphia: Lippincott Williams & Wilkins, 2003. pp. 418-419.
2. Krinsky G. and N.M. Rofsky. MR Angiography of the Aortic Arch Vessels and Upper Extremities. MRI Clinics of North America: Body MR Angiography. May 1998; 6(2): 269-292.
3. Van Grimberge F, Dymarkowski S, et al. Role of Magnetic Resonance in the Diagnosis of Subclavian Steal Syndrome. Journal of Magnetic Resonance Imaging. 2000; 12: 339-342.
4. Cosottini M, Sampa V, et al. Contrast-Enhanced Three-Dimensional MR Angiography in the Assessment of Subclavian Artery Diseases. European Radiology. 2000; 10: 1737-1744.