Subclavian Steal Syndrome

Vertebral steal phenomenon is the flow reversal of ipsilateral vertebral artery due subclavian artery steno-occlusive disease proximal to the origin of vertebral artery. Combination of retrograde flow in the vertebral artery and neurological symptoms are referred as subclavian steal syndrome [1]. Classification of subclavian steal can be defined either morphologically from which the artery steals its blood supply from or according to hemodynamic states of the vertebral artery. Morphologically it is categorized as vertibro-vertibral, carotid- basilar, external carotid- vertebral, carotid - subclavian. According to hemodynamic abnormalities, Grade 1 defined as reduced antegrade flow [1]. Grade 2 as reversal of flow due to reactive hyperemia and Grade 3 as permanent retrograde flow [1]. Patients are commonly asymptomatic. They can present with vertebro basilar symptoms and arm ischemia [1]. Subclavian steal syndrome is most commonly caused by atherosclerotic disease and can be due to traumatic, embolic, surgical, congenital and neoplastic lesions [2]. Vertebral artery Doppler studies have shown to be sensitive in detecting subclavian steel phenomena even when the flow is entirely antegrade [3]. Visualization of vertebral artery and analysis of Doppler flow can be obtained in 92% to 98% the time [2].the normal vertebral artery diameter measures 4mm and the left vertebral artery can be larger than the right [3]. They have a low resistance waveform with a continuous forward flow in systole and diastole similar to common carotid artery. Vertebral arteries demonstrate a broader spectrum due to its small size. Normal peak systolic velocity can range from 20 to 60cm/S. The velocity can be higher at the origin and on the left side due to the difference in the diameter [3]. Several types of waveforms are identified in subclavian steal phenomina. A diameter narrowing of 45% causes a sharp mid systolic deceleration in mid systole with a sharp systolic peak and a more rounded, lesser second peak while preserving the antegrade flow [2,4]. This is known as the pre Bunny waveform. Bunny waveform is described as having two systolic peaks with a nadir reaching almost to the level of end diastole. Bunny waveform is demonstrated in 55% stenosis of subclavian artery. To and fro or the bidirectional wave indicate a significant stenosis (88% or greater), this shows an antegrade flow initially and subsequent retrograde flow in each cardiac cycle. Complete retrograde flow is seen in occlusion or high grade pre vertebral subclavian artery [4]. This is 100% sensitive to high grade stenosis or occlusion. Exercising the arm for 5 minutes of inflating the sphygmomanometer for 5 minutes to induce rebound hyperemia on the ipsilateral side can enhance the sonographic finding and may convert the incomplete steal to a complete steal [2]. Flow sensitive, low resolution 2D time of flight(TOF) images demonstrate flow reversal as signal void within the vertebral artery and confirms a patent vertebral artery in 3D contrast enhanced MR images. This sign is known as the “localizer sign” [5]. Catheter angiography is considered the gold standard method for evaluation of diseases of great vessels including subclavian steal. It demonstrate the stenosis or the occlusion with the antegrade flow on normal vertebral artery, retrograde filling of abnormal vertebral artery and late filling of subclavian artery on the abnormal side [1].