Subclavian Steal Syndrome

Etiology

The most common etiology of subclavian steal syndrome is atherosclerosis. Subclavian steal syndrome is more commonly seen on the left side, possibly due to the more acute origin of the left subclavian artery, leading to increased turbulence, causing accelerated atherosclerosis [2].

Some of the other risk factors for SSS include[3][4][5]:

• Takayasu arteritis, which is a form of large vessel granulomatous vasculitis, commonly seen in young or middle-aged women of Asian descent
• Subclavian artery compression in the thoracic outlet. This presentation usually presents in athletes like cricket bowlers and baseball pitchers, due to neuromuscular compression, as the subclavian artery crosses over the first rib.
• Presence of cervical rib, which is an extra rib that originates from the seventh cervical vertebra
• Following surgical repair of coarctation of the aorta
• Congenital abnormalities like right aortic arch
• Rare anatomical factors include aortic dissection, vertebral artery congenital malformations, and even external vertebral artery compression

Epidemiology

The exact incidence or prevalence of subclavian steal syndrome is not known. Most literature reports the prevalence of SSS as between 0.6% to 6.4%.[6] The Joint Study of Extracranial Arterial Occlusion by Fields et al., showed a 2.5% incidence (168/6534), with only 5.3% of these patients experiencing neurological symptoms.[7] Males are more affected compared to females, due to atherosclerotic causes, by a ratio of about 2 to 1.

Pathophysiology

The pathophysiology involves blood flow diversion from the brain territories to the arm, causing symptoms of vertebrobasilar insufficiency, especially during the vigorous exercise of the arm or sudden sharp turning of the head in the direction of the affected side. The symptoms arise due to two types of mechanisms by which the arm "steals" blood flow from the vertebrobasilar territory; 1) a lack of blood supply because of subclavian artery stenosis or 2) rarely malformation disease, that may include an arteriovenous distal arm shunt.

The severity of subclavian steal is classified into three grades[8]:

• Grade I (pre-subclavian steal) - reduced antegrade vertebral flow.
• Grade II (intermittent/partial) - alternating flow - antegrade flow in the diastolic phase and retrograde flow in the systolic phase.
• Grade III (permanent/advanced) - permanent retrograde vertebral flow.

Coronary subclavian steal syndrome is another type of subclavian steal syndrome, described in patients who have undergone coronary artery bypass graft surgery using internal mammary artery (IMA) graft.[6] It is defined as the reversal of flow in a previously constructed IMA leading to myocardial ischemia, due to the presence of subclavian artery stenosis, proximal to the origin of the ipsilateral IMA.

History and Physical

Subclavian artery stenosis is asymptomatic in most patients. It is sometimes incidentally found when there is a blood pressure difference between the arms or on ultrasound testing of patients with coronary or carotid artery disease.

• It can be symptomatic in some patients and can present as arm pain, fatigue, numbness, or paresthesias. These symptoms are secondary to upper extremity ischemia during vigorous exercise.
• It can also present as a variety of neurological symptoms secondary to vertebrobasilar insufficiency. These symptoms include dizziness, blurring of vision, syncope, vertigo, disequilibrium, ataxia, tinnitus, or hearing loss. The neurological symptoms can be precipitated by upper extremity exercise as well as with head movements, with the rotation of face toward the opposite side. A detailed history can help in diagnosing subclavian steal.
• On examination, a difference of at least 15 mmHg presents between the affected and the normal arm.
• Simultaneous palpation of radial pulses in both arms will show a decreased amplitude and delayed arrival on the affected side.
• Examination of bilateral carotid arteries using palpation and auscultation can also help in detecting occlusive disease.
• Subclavian arteries should be examined in the supraclavicular fossa using palpation and auscultation for paraclavicular bruits.

Evaluation

Duplex ultrasound is one of the less invasive bedside investigations to start with if the symptoms are consistent with subclavian steal syndrome, and it is always indicated as the first-line of tests. It can readily diagnose and also quantify proximal subclavian artery stenoses. Significant subclavian artery stenosis is predicted by a subclavian artery peak systolic velocity greater than 240 cm/second.[9] However, a Doppler ultrasound cannot properly evaluate the origin of the vertebral artery. It helps in identifying the extracranial occlusive disease.

Magnetic resonance angiography is an accurate diagnostic modality for patients with suspected SSS.[10] It can also aid in the evaluation of intracranial cerebrovascular circulation as well as the extracranial vessels.

CT angiography is another diagnostic modality that can help in the diagnosis and grading of subclavian artery stenosis. It is indicated in patients with abnormal findings on duplex ultrasound.

A confirmatory test is usually needed to decide on the intervention strategy, and color doppler ultrasound must always be complemented with either contrast-enhanced magnetic resonance angiography or computed tomography angiography before making any decisions about treatment. Digital subtraction angiography (DSA) is generally the choice if stenting has been decided as the treatment of choice and helps better in depicting the details of the anatomical hurdle.

Treatment / Management

Many patients do not require any intervention as either they are asymptomatic or their mild and non-disabling symptoms improve with time. 

Subclavian artery stenosis is a marker of atherosclerotic disease in many patients and hence indicates the risk of adverse cardiovascular events in such patients. These patients benefit from secondary preventive measures, including control of blood pressure, treatment of dyslipidemia, smoking cessation, glycemic control in diabetes mellitus, and lifestyle changes. 

An open surgical bypass is one of the options for symptomatic patients. The most common choice for surgical correction is extra-anatomic revascularization (e.g., carotid transposition, carotid-subclavian bypass).

For patients with short proximal stenosis or occlusion, an endovascular intervention can be a consideration. About 10% of patients can present with less than 70% recurrent stenosis. Such patients can benefit from repeat angioplasty. About 5% of those patients might require surgery.[11]

Antiplatelet therapy and oral anticoagulation can be tried in patients with high surgical risk or with unfavorable anatomy for surgical intervention. However, the effectiveness of this option has not been studied.

Differential Diagnosis

Peripheral Arterial Disease (PAD) of the Upper Extremity

This can be secondary to thromboembolism, arteritis, or fibrodysplasia. It can present with exercise-induced pain, pain at rest, or digit ulceration. Sometimes, asymmetric arm blood pressures are the only finding. It can be differentiated from SSS due to the absence of neurological manifestations in the former.  

Posterior Circulation Stroke

This can present with neurological manifestations like syncope, dizziness, blurring of vision, or ataxia. MR angiography or CT angiography can help in differentiating it from subclavian steal syndrome.

Aortic Stenosis

In a patient presenting with syncope, though SSS merit considered in the differentials, it is essential to rule out cardiac causes, including aortic stenosis. An ejection systolic murmur in the aortic area on examination can be suggestive. An echocardiogram is definitive in diagnosing aortic stenosis.

Diabetic Neuropathy

The presence of 'glove and stock' pattern of symptoms suggest peripheral neuropathy. Electromyography may help in diagnosing peripheral neuropathy and thereby distinguishing it from SSS.

Prognosis

Subclavian steal syndrome is a relatively benign condition. As it is a marker of atherosclerosis, it can indicate the risk for future events like myocardial ischemia or stroke. Hence, secondary preventive measures in patients can help in a good prognosis. Symptomatic patients who undergo surgical intervention with angioplasty and stenting or open surgical bypass, also have a good prognosis. Most patients (over 95%) have sustained resolution of ischemic symptoms and do not require reintervention of the target vessel.[11]

Complications

There are no long term complications reported from subclavian steal syndrome by itself. But, as it can lead to vertebrobasilar insufficiency, patients who present with syncope can experience falls, leading to the risk of head injury.

In coronary-subclavian steal, the reversal of blood flow through the internal mammary graft from coronary to subclavian circulation can result in myocardial ischemia.