Subclavian Steal Syndrome

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CHAPTER 96 Subclavian Steal Syndrome

The aortic arch extends from approximately the level of the upper border of the second sternocostal articulation on the right, runs superiorly, posteriorly, and to the left in front of the trachea, where it then turns posteriorly left of the trachea and finally inferiorly on the left side of the body. The branches that conventionally arise from the arch are the right brachiocephalic artery, the left common carotid artery, and the left subclavian artery (see Chapter 71).

When stenosis or occlusion involves the subclavian artery, the vertebral artery can reverse direction to serve as a collateral pathway to reconstitute the subclavian artery, a phenomenon known as subclavian steal phenomenon, which can lead to brainstem ischemia and neurologic symptoms. This chapter will discuss the clinical manifestations, imaging evaluation, and treatment of subclavian steal.

DEFINITION

Subclavian steal syndrome is defined as stenosis or occlusion of the subclavian artery proximal to the origin of the vertebral artery, with consequent reversal of blood flow in the vertebral artery to supply the distal subclavian artery, resulting in neurologic symptoms. This reversal of flow is said to “steal” blood from the intracranial circulation, which can cause ischemia or stroke.¹ In the absence of neurologic symptoms, the constellation of findings can be called subclavian steal phenomenon.

PREVALENCE AND EPIDEMIOLOGY

Most patients with the syndrome are older than 50 years because of its association with atherosclerosis. If subclavian steal is found in young adults or children, Takayasu arteritis should be suspected.

ETIOLOGY AND PATHOPHYSIOLOGY

Atherosclerosis is the most common cause of subclavian steal syndrome. Although Takayasu arteritis is an infrequent cause of the syndrome, it occurs more commonly in people of Asian descent.²

Subclavian artery stenosis or occlusion proximal to the vertebral artery origin results in the alteration of vertebral artery hemodynamics. The low pressure in the subclavian system eventually results in reversal of flow in the ipsilateral vertebral artery, which serves as a source of collateral flow to the distal subclavian artery.

Vollmar and colleagues ³ have described four categories of subclavian steal phenomenon based on the vascular territories that provide and receive flow—vertebrovertebral, carotid-basilar, external carotid-vertebral, and carotid-subclavian. Another classification scheme⁴ is based on the degree of hemodynamic disturbances of the vertebral artery: stage I (occult steal, decreased blood flow), stage II (partial steal, transient or partial reversal of flow), and stage III (complete steal, permanent reversal of flow).

When the collateral flow directs a large amount of flow into the subclavian artery and away from the brain, neurologic symptoms can occur. Such symptoms have been reported to occur in approximately 36% of patients.⁵ Often, the circle of Willis can provide enough collateral circulation to the brainstem to avert neurologic symptoms. If the posterior communicating artery is stenotic, occluded, or absent, or if there is concomitant carotid disease, then neurologic symptoms are more likely to occur.

MANIFESTATIONS OF DISEASE

Clinical Presentation

Most patients with reversal of flow in the vertebral artery have neurologic symptoms.⁵ In the absence of neurologic symptoms, patients are said to have subclavian steal phenomenon.

Neurologic symptoms of subclavian steal syndrome include lightheadedness, dizziness, vertigo, upper extremity numbness, visual alterations, transient ischemic attacks and, rarely, seizures or stroke.⁶^–⁸ Ipsilateral upper extremity exercise increases the blood flow demand in the subclavian artery and can cause transient neurologic symptoms caused by an increase in diversion of intracranial blood flow.⁹

On physical examination, subclavian steal may manifest with a subclavian artery bruit or reduced blood pressure in the ipsilateral upper extremity, along with weakness of the radial pulse.¹⁰

Imaging Indications and Algorithm

Doppler sonography is the imaging modality of choice for the diagnosis of subclavian steal phenomenon. When sonography is limited, such as for the evaluation of subclavian disease, CT or MRI can be used to evaluate the anatomy. Unlike CT, MRI has the ability unequivocally to determine direction of blood flow in the vertebral artery, and it does not require ionizing radiation or iodinated contrast agent. Because of its invasiveness, angiography has a limited role in current practice, and is used primarily when percutaneous intervention is contemplated.

Imaging Techniques and Findings

Ultrasound

Sonography is the initial imaging examination when subclavian steal syndrome is suspected. Ultrasound with pulsed Doppler spectral analysis can demonstrate patency and flow direction in the vertebral artery and can establish the presence of subclavian steal.¹¹^,¹²

Subclavian steal phenomenon is associated with specific vertebral artery waveforms. There are four types of waveforms that indicate the degree of abnormal hemodynamics ¹¹:

• Type 1: Flow velocity at the nadir of the mid-systolic notch greater than that of end-diastole

• Type 2: Velocity in mid-systolic notch equal to end-diastole

• Type 3: Velocity in mid-systolic notch at baseline

• Type 4: Velocity in mid-systolic notch below baseline

In occult steal (Branchereau stage I),⁴ the pulsed wave Doppler spectrum typically demonstrates antegrade vertebral flow with midsystolic deceleration.¹¹^,¹² The waveform may show a reversed late systolic flow as a response to reactive hyperemia in the upper extremity after arm exercise (see Fig. 96-1B). In partial subclavian steal (stage II), pulsed wave Doppler depicts partial flow reversal. The Doppler waveform in occult and partial subclavian steal has been described as the bunny rabbit sign because of its resemblance to the outline of a rabbit (Fig. 96-1A).¹¹^,¹²

FIGURE 96-1 A, Occult steal. Pulsed wave Doppler spectral ultrasound image of the vertebral artery depicts mid-systolic deceleration with antegrade late systolic velocities (white outline, bunny rabbit sign). B, Partial steal. Pulsed wave Doppler spectral image (obtained after arm exercise) demonstrates mid-systolic deceleration with retrograde late systolic velocities.

(From Gosselin C, Walker PM. Subclavian steal syndrome: existence, clinical features, diagnosis and management. Semin Vasc Surg 1996; 9:93-97.)

In complete (stage III) subclavian steal, the pulsed wave Doppler spectrum shows complete reversal of flow in the vertebral artery (Fig. 96-2).¹²

FIGURE 96-2 Complete subclavian steal. Pulsed wave Doppler spectral ultrasound of the left vertebral artery shows complete flow reversal.

(From Gosselin C, Walker PM. Subclavian steal syndrome: existence, clinical features, diagnosis and management. Semin Vasc Surg 1996; 9:93-97.)

Recently, transcranial Doppler ultrasound has been studied for evaluation of flow reversal in the basilar artery in the presence of retrograde vertebral artery flow.¹³ Transcranial Doppler ultrasonography can be used to determine if there is reversal of basilar artery flow, which occurs in fewer than 25% of patients with retrograde vertebral flow. This is especially useful before intervention in patients with neurologic symptoms.

Computed Tomography

Contrast-enhanced computed tomography (CT) can demonstrate subclavian artery stenosis or occlusion with patency of the ipsilateral vertebral artery (Fig. 96-3).¹⁴ The obstructive lesion is often associated with calcification. Coronal reformations are often useful to depict the site of narrowing. CT can also be used to identify indirect signs of subclavian steal phenomenon, such as poor internal mammary opacification or decreased unilateral brain perfusion.¹⁵^,¹⁶

FIGURE 96-3 Subclavian steal phenomenon. Contrast-enhanced CT scan shows that the left subclavian artery (D, white arrow) is occluded beyond its origin (C) and reconstitutes (B) via the vertebral artery (A, black arrow).

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) can accurately and noninvasively depict the features of subclavian steal phenomenon.¹⁷ Contrast-enhanced magnetic resonance angiography (MRA) or noncontrast MRA can reveal the site of stenosis or occlusion (Figs. 96-4 and 96-5).¹⁸ Time-resolved three-dimensional MRA has been described as a means of identifying reversal of vertebral artery flow.¹⁹^,²⁰ Time-of-flight MRA has also been used to evaluate vertebral flow direction, but this technique may be less reliable (Fig. 96-6).¹⁸ Phase contrast MRI can confirm the reversal of blood flow in the vertebral artery, and velocity-encoded cine phase contrast MRI can be used to quantify velocity and flow in the vertebral and subclavian arteries (Fig. 96-7).²¹ The carotid and vertebral arteries, and even the circle of Willis, can be evaluated in the same examination.

FIGURE 96-4 Subclavian steal. Contrast-enhanced MRA scan shows an atherosclerotic occlusion (white arrowhead) of the left subclavian artery with distal reconstitution (black arrowhead) via the vertebral artery (white arrow).

FIGURE 96-5 Subclavian steal syndrome—contrast-enhanced MRA scan in a patient with Takayasu arteritis and lightheadedness induced by upper extremity exercise. The left subclavian artery is occluded approximately 1 cm beyond its origin (black arrow) and reconstitutes (black arrow) via the vertebral artery (white arrowhead). Note the bovine arch, in which the left common carotid artery (white arrow) arises from the brachiocephalic artery (asterisk). The brachiocephalic artery is also occluded and the subclavian artery reconstitutes (black arrowhead), presumably via the vertebral artery. The right common carotid artery does not reconstitute. The left common carotid artery serves as the sole source of blood flow to the intracranial circulation, leading to neurologic symptoms.

FIGURE 96-6 A, Contrast-enhanced MRA scan demonstrates a severe stenosis of the left subclavian artery (arrowhead). B, The left vertebral artery (white arrow) shows signal drop compared with the right side (gray arrow) on the time-of-flight MRA scan. The signal drop suggests reversal of flow on the left side.

FIGURE 96-7 Phase contrast MRI depicts flow reversal (white arrow) in the left vertebral artery in a patient with subclavian steal syndrome. The dark circles represent the carotid right vertebral arteries, all of which have an antegrade (cephalad) direction of flow.

(From Bitar R, Gladstone D, Sahlas D, Moody A. MR Angiography of subclavian steal syndrome: pitfalls and solutions. AJR Am J Roentgenol 2004; 183:1840-1841.)