Article title: Subclavian Steal Syndrome
Author: Samia Sulaimain
Editors: Miramar Haddad, Haneen A. Banihani
Reviewer: Dr. Hala Qaryouti
Subclavian steal syndrome (SSS) is a condition characterized by abnormal blood flow in arteries due to occlusion or stenosis of the subclavian artery, causing retrograde flow in the ipsilateral vertebral artery and ensuing symptoms of cerebral ischemia. While many patients with SSS may be asymptomatic, some patients present with manifestations of arterial insufficiency, which impact the brain and upper extremities of the body supplied by the subclavian artery. SSS may manifest as syncope or presyncope related to cerebral hypoperfusion, which can occur while using the arm, often during exercise.
According to the Joint Study of Extracranial Arterial Occlusion by Fields et al., only 168 out of 6534 patients (2.5%) who have arteriographic evidence of subclavian or innominate stenosis with >30% occlusion show symptoms of subclavian steal syndrome. Of which, only 5.3% experienced neurological symptoms . However, the exact prevalence remains unknown .
The left subclavian artery, which supplies the left upper limb via the axillary artery, is commonly the most affected by atherosclerosis. As a result, it is involved in subclavian steal syndrome three times more frequently than the right subclavian artery. On the right side, disease or occlusion of the innominate, also known as brachiocephalic, artery may lead to occlusion in the subclavian artery origin.
Subclavian steal syndrome occurs most commonly in people over the age of 50 years. There is a greater male predilection with a 2:1 male-to-female ratio. The reasoning behind this being the increasing likelihood of atherosclerotic disease in older males[3,4].
Given the fact that atherosclerosis is the main etiology of SSS, risk factors involved in the development of fatty plaques are considered a direct contributor to subclavian steal. These risk factors are divided into modifiable and non-modifiable.
Modifiable risk factors are related to cigarette smoking, diabetes mellitus, hypertension, hypercholesterolemia and hyperhomocysteinemia. Whereas non-modifiable risks include aging, male gender and a positive family history of SSS.
Other relatively rare risk factors involved with subclavian steal syndrome include:[1-3]
- Takayasu arteritis, a type of large vessel granulomatous vasculitis, found classically in middle-aged women of Asian descent.
- Presence of a cervical rib, an “extra” rib originating from the seventh cervical vertebra
- Neuromuscular compression of the subclavian artery in the thoracic outlet, a presentation more common in athletes
- Surgical repair of coarctation of the aorta
- Rare anatomical factors, including aortic dissection, external vertebral artery compression, and vertebral artery congenital malformations
- Congenital abnormalities include right aortic arch with isolated left subclavian, preductal aortic arch, or an interrupted aortic arch.
Etiology and Pathogenesis
Significant subclavian artery occlusion results in a decrease in arterial pressure distal to the occlusion site. If the lesion is proximal to the origin of the vertebral artery, the pronounced pressure drop may reverse or ‘steal’ the blood flow in the ipsilateral vertebral artery to ensure adequate flow to the ipsilateral arm.
SSS is classified into three grades based upon the hemodynamic alteration of the vertebral artery:
- Grade 1: decreased (antegrade) mid-systolic blood flow in the vertebral artery.
- Grade 2: retrograde blood flow in the vertebral artery by ipsilateral arm hyperemia test.
- Grade 3: permanently retrograde vertebral artery blood flow
While identifying the grade can help grasp better knowledge regarding the prognosis, no linear relationship relating to the prognosis and the extent to which symptoms are seen has been noticed, making it unclear whether higher grade is related to more extreme manifestations. It has been noted, however, that patients with grade 2 tend to present with more symptoms than the two other groups.
SSS is often asymptomatic, and diagnosis may be incidental in cases necessitating ultrasound testing for coronary or carotid artery disease, or when a blood pressure difference is detected between the arms during physical examination.
When symptomatic, patients may complain from arm pain or claudication, numbness, paresthesia, or fatigue secondary to upper extremity ischemia exacerbated during activity or vigorous exercise . Patients may also present with neurological symptoms as a result of vertebrobasilar insufficiency, including dizziness, hearing loss, tinnitus, ataxia, vertigo, syncope, and disequilibrium . In addition, visual symptoms include blurry vision, inability to focus and monocular and binocular visual loss .
Dizziness and vertigo are relatively more common symptoms seen in over 50% of patients, followed by diplopia in 19% of cases, syncope (18%) and dysarthria (12.5%).
Workup and Diagnosis
In case of clinical suspicion of SSS, physical examination findings include a difference in pulses and brachial systolic blood pressure between the upper extremities, where a difference of at least 15 mmHg is noticed. In addition, recurrent angina pectoris after successful grafting using LIMA may also indicate stenosis in the left subclavian artery.
Imaging techniques considered for the diagnosis of SSS include duplex ultrasonography (DUS), computed tomography (CT) and computed tomography angiography (CTA), magnetic resonance angiography (MRI), digital subtraction angiography (DSA), four-vessel cerebral arteriography, and chest radiography. Electrocardiography could also be taken into consideration. [1,3,8]
Duplex ultrasound is the most important test of the subclavian, extracranial carotid and vertebral arteries. DUS is used to show retrograde blood flow and any significant occlusive lesions.
CTA is considered the first-line test and has high sensitivity and specificity. It identifies subclavian artery stenosis and/or occlusion, delayed enhancement of the ipsilateral vertebral artery, and other lesions in arch vessels, yet cannot determine the direction of flow in the ipsilateral vertebral artery.
MRA identifies subclavian artery stenosis and/or occlusion, delayed enhancement of the ipsilateral vertebral artery, other lesions in arch vessels, and flow direction in the ipsilateral vertebral artery.
Digital subtraction angiography is performed during the endovascular intervention to identify subclavian artery stenosis, delayed filling of the ipsilateral vertebral artery, and other extracranial and intracranial cerebral vascular lesions. DSA is a confirmatory test of choice if stenting is chosen as the appropriate treatment.
Differential Diagnosis 
Posterior Circulation Stroke: Although infarction of the posterior cerebral artery can present with symptoms similar to SSS (i.e., syncope, dizziness, ataxia, or blurring of vision), imaging such as MR or CT angiography can differentiate between the disease.
Peripheral Artery Disease (PAD) of upper extremities: this may only present with asymmetric blood pressure, but can also cause exercise-induced pain or pain at rest and digit ulceration. PAD could be differentiated from SSS by its lack of neurological manifestations.
Aortic Stenosis: Ruling out cardiac causes is an essential step in patients with syncope. A systolic ejection murmur in the aortic region is highly suggestive, but the definitive diagnosis for aortic stenosis is established using an echocardiogram.
Peripheral Neuropathy: Peripheral neuropathy may be suggested by the “glove-and-stocking” pattern and diagnosed by electromyography.
No intervention is required in many asymptomatic or mild cases that improve over time. However, since subclavian steal syndrome is a marker for atherosclerotic disease, patients must follow secondary preventive measures to control their blood pressure, cut off smoking, achieve glycemic control in diabetes mellitus and treat dyslipidemia to avoid any possible complications. 
Limiting physical activity in the involved extremity is also helpful in preventing provoked episodes [1,2]. Moreover, routine evaluation of blood pressure and detailed physical examination of patients who are at higher vascular risk, such as in cases of dyslipidemia, diabetes, and hypertension, could assist in the early detection of SSS.
A treatment option for symptomatic SSS is open surgical bypass, and the most frequent choice is extra-anatomic revascularization. As for patients with short proximal stenosis or occlusion, endovascular intervention may be the more appropriate consideration. Around 10% of patients with <70% recurrent stenosis and only 5% of those require surgery and may benefit from repeat angioplasty.
Some patients may have high surgical risk or unfavorable anatomy for surgery; therefore, antiplatelet therapy and oral anticoagulation may be chosen despite the lack of studies on their effectiveness.
SSS is considered a benign condition, and no long-term complications are reported. However, reversing blood flow through the internal mammary graft from coronary to subclavian circulation may cause myocardial ischemia. Vertebrobasilar insufficiency may lead to patients suffering from syncope and experiencing falls, leading to serious head injuries. [5,9]
Secondary preventive and early detection measures significantly help improve prognosis and avoid the risk atherosclerosis imposes on the individual and which may lead to myocardial ischemia or stroke.
Patients who undergo angioplasty, stenting, or open surgical bypass, usually have a good prognosis and over 95% sustain resolution of ischemic symptoms, and the target vessel does not require further reintervention .
Recent Updates and Further Reading
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