Variant of subclavian steal syndrome: unusual anatomical relationship between left subclavian artery and left vertebral artery

Discussion

SSS is a compromise of the circulation of the subclavian artery resulting in a variety of symptoms that can be broadly separated into two categories. First, VBI and second, ischaemia of the upper extremity.1 Some patients can be asymptomatic, termed subclavian steal phenomenon, and they do not require invasive evaluation or treatment as the phenomenon is an appropriate physiological response to the occlusion.1 5

In classic SSS, the occlusion occurs proximal to the origin of the vertebral artery, which results in low pressure in the distal subclavian.1 The blood flow then compensates by flowing through the contralateral vertebral artery, through the basilar artery and then flowing retrograde down the ipsilateral vertebral artery to perfuse the distal upper limb.1 The resulting VBI can potentially result in syncope, vertigo, ataxia, seizures and confusion.1 5 7 There are numerous differentials for VBI including intracranial vascular disease, carotid artery disease, vertebral artery disease, brain tumour and subdural haematoma.7 Interestingly, in our patient, she demonstrated no VBI symptomatology. Her anatomy seen on CT angiography showed her left vertebral artery originating from the aortic arch rather than the left subclavian, which would explain why she experienced no VBI symptoms. This is a relatively undocumented phenomenon in the literature. A case report by Holder et al 8 in 1981 reviewed five cases of subclavian steal with anomalous origin of the vertebral artery from the aortic arch. Interestingly, four of these cases had neurological symptoms with antegrade blood flow.8 The authors suggested that rather than the retrograde nature of blood flow determining the subclavian steal, it is the amount of blood left to supply the basilar artery that causes the pathology to occur.8

Although not the case in our patient, there are various other arteries that can supply the distal upper limb. There are cases of coronary subclavian steal, describing patients who have previously had a coronary artery bypass surgery (CABG) using the IMA.9 In a significant subclavian stenosis proximal to the origin of the ipsilateral IMA, the distal subclavian can ‘steal’ the flow from the coronary supply.9 This can result in angina-like symptoms in the post-CABG patient.9

Symptoms can be provoked by a variety of factors. Her variant anatomy may have exacerbated her upper arm symptoms. Her occlusion occurred proximal to both the thyrocervical trunk and the left IMA (figure 2). Therefore, the left subclavian artery can possibly steal from these two arteries, rendering her symptom-free at rest. There have been numerous cases of ‘thyrocervical steal’, where extensive collateral vessels from the vertebral artery supply the thyrocervical trunk and therefore the upper limb.8 10 In the case of our patient, her CT angiography did not demonstrate collateral flow to the thyrocervical trunk and distal subclavian. Exercise can also exacerbate symptoms.5 This was seen in our patient, who found her symptoms were provoked while using her left arm in her activities of daily living, mainly when raising her arm above head height. The use of the arm and the increased demand in the distal upper limb may have compromised the left subclavian steal inducing her symptoms.

The aetiology of the stenosis varies between age and ethnic groups.1 5 8 11 12 Atherosclerosis is the most common cause and affects men greater than women at a ratio of 2:1, and more commonly affects patients over 50 years old.13 Atherosclerosis was the cause of our patient’s stenosis. Her CT angiography demonstrated a 28 mm irregular calcified plaque at the proximal left subclavian artery causing a severe focal stenosis of at least 90% (figure 2). Other less common risk factors include Takayasu arteritis, which affects women in 80%–90% of cases and has an onset between 10 and 40 years old,11 12 with the greatest prevalence in Asian populations.14 Our patient was of Asian heritage, so this was considered as an aetiology until the CT angiography revealed the plaque formation. Other less common aetiologies of occlusion include dissecting aortic arch aneurysm, fibromuscular dysplasia, external compression of the subclavian artery by masses, radiation-induced vascular disease, anatomical anomalies and embolism.1 5 13

BP difference between arms has shown to be a sensitive threshold for detecting SSS.6 Labropoulos et al 6 found a difference greater than 20 mm Hg between arms is highly correlated with SSS. In their study, only 16% of patients with a difference greater than 20 mm Hg did not have SSS. They also found a positive correlation between increasing arm BP difference and the occurrence of symptoms.6 Only 1.38% of patients were symptomatic with a BP difference between 20 and 30 mm Hg, but the prevalence increased to 38.5% when BP difference was greater than 50 mm Hg.6 A meta-analysis of association between difference in systolic BP with vascular disease and mortality by Clark et al 15 found a difference of just 15 mm Hg between arms may help to identify patients who need further vascular assessment. Their meta-analysis saw a difference of 15 mm Hg or more was associated with increased peripheral vascular disease, pre-existing cerebrovascular disease and increased cardiovascular mortality.15 The authors did note a negative result (ie, BP difference less than 10 mm Hg) does not rule out SSS as both Labropoulos et al 6 and Clark et al 15 were limited by data derived from patient referred for coronary angiography and therefore an analysis with unselected populations would make findings more applicable to the general population.

The initial investigation into suspected SSS and its variants should be duplex ultrasound.5 It can assess the haemodynamic alterations in the vertebral artery; however, it cannot assess the entire vertebral artery or the anatomy of the branches of the aortic arch.5 It is also unable to identify the pathology behind the stenosis, which is crucial in the management of the condition.5 The gold standard to evaluate the anatomy and identify the pathology affecting the aortic arch and its branches is either CT angiography or magnetic resonance angiography.5

The primary choice of treatment for SSS is endovascular intervention.5 It was first described by Bachman and Kim16 in 1980 and popularised due to its minimally invasive nature and its comparable outcomes with surgical carotid–subclavian bypass or transposition.16 17 Endovascular treatment with balloon angioplasty with or without stenting has excellent success rates with acceptable short-term and intermediate-term primary patency.5 A recent study of 553 patients found primary stenting to have primary patency rates of 90.6%, 83.4% and 80.5% at 1, 3 and 5 years, respectively.18 Balloon angioplasty without primary or secondary stenting has shown slightly lower intermediate to long-term success rates.19 Hebrang et al defined success as normal bilateral upper extremity BP differential, which was found to be 86.5% and 78.8% at 1 and 3 years, respectively.19 Given the difference in definition of successful patency, more randomised controlled trials comparing the outcomes of angioplasty alone with angioplasty with stenting are still warranted. A 2016 retrospective meta-analysis of angioplasty alone to angioplasty plus stent found comparable outcomes in follow-up.20 Immediate technical rates were more successful in the stenting group (92.8%), whereas the angioplasty group was 86.9%.20 Primary patency in the angioplasty plus stenting was slightly lower than angioplasty alone at both short-term (88.7% vs 89.9%) and long-term follow-up (76.9% and 79.6%).20 Based on the current evidence, a definitive recommendation to stent versus angioplasty alone cannot be drawn.5 Further research into treatment options and their efficacy is needed to guide future practice.

Complications for endovascular treatment for SSS include transient ischaemic attacks (TIAs), stroke, pseudoaneurysm formation, access site haematoma, vessel rupture and dissection.5 TIA and stroke have an incidence of 2.2%–4.9% post-endovascular treatment.5 18 In angioplasty alone, the major complication is restenosis or reobstruction, which can be treated with a second angioplasty or secondary stent placement.5

Management also includes aggressive treatment of underlying risk factors such as tobacco use, hypertension, hypercholesterolaemia and diabetes.21 Our patient had a thorough review of all relevant risk factors including further BPs, HbA1c, blood sugar levels, cholesterol and lipid profiles, and screening for tobacco use. All investigations were normal in our patient. She was counselled on diet and lifestyle factors that could contribute to further disease process and was happy to keep monitoring these every 6 months.