Subclavian Steal Syndrome

Subclavian steal is retrograde flow in the ipsilateral vertebral artery caused by a haemodynamically significant stenosis or occlusion of the subclavian artery (or innominate artery on the right) proximal to the vertebral artery origin. Flow is diverted from the posterior circulation to supply the arm. Subclavian steal syndrome refers to the symptomatic state, whereas steal phenomenon alone is common and usually silent.

Epidemiology

• Subclavian artery stenosis of 50% or more is found in 2 to 4% of the general population, and in up to 11% of patients with peripheral arterial disease.
• Flow reversal is seen in about 3 to 6% of patients undergoing carotid ultrasound, but only a small minority are symptomatic.
• The left subclavian artery is affected in about 70% of cases, because of its direct origin from the aortic arch.

Causes

• Atherosclerosis (most common): smoking, hypertension, diabetes and dyslipidaemia.
• Large vessel vasculitis: Takayasu arteritis (a leading cause in young women, especially in Asia and Africa) and giant cell arteritis.
• Others: radiation arteritis, fibromuscular dysplasia, congenital arch anomalies and coarctation, aortic dissection, thoracic outlet compression, trauma, and previous Blalock-Taussig shunt or subclavian flap repair.

Stages of vertebral artery haemodynamics on Doppler

• Stage 1 (pre-steal): mid-systolic deceleration with a bunny-ear waveform, and antegrade flow preserved.
• Stage 2 (partial steal): alternating flow, retrograde in systole and antegrade in diastole.
• Stage 3 (complete steal): persistent retrograde flow.

Related patterns

• Coronary-subclavian steal: proximal subclavian stenosis reduces flow in a left internal mammary artery graft, with angina after bypass.
• Dialysis access steal: arteriovenous fistula in a limb with proximal arterial stenosis.

Vertebrobasilar symptoms are more likely when posterior circulation reserve is limited by contralateral vertebral hypoplasia or occlusion, basilar stenosis or an incomplete circle of Willis. Stroke is uncommon, and most posterior circulation events in such patients reflect coexisting intracranial atherosclerosis.

Most patients are asymptomatic and found by a blood pressure difference between arms.

Upper limb features

• Arm fatigue, claudication, coolness and paraesthesia on exertion.
• Reduced or delayed radial pulse and systolic blood pressure difference of more than 15 to 20 mmHg between arms.
• Supraclavicular bruit, and rarely rest pain, digital ulcers or embolic phenomena.

Neurological features, often precipitated by arm exercise or sudden arm movement

• Vertigo, presyncope, syncope, diplopia, visual blurring, ataxia, dysarthria, drop attacks, tinnitus and perioral numbness.
• Brainstem or cerebellar infarction in a minority.

Coronary-subclavian steal: exertional angina, usually after bypass with a left internal mammary graft, with exercise provoking arm use.

Features suggesting Takayasu arteritis: age under 40, constitutional symptoms, carotid or other bruits, absent pulses, claudication, and hypertension from renal artery involvement.

Differential diagnosis: carotid stenosis, vertebral artery origin stenosis or dissection, cardiac arrhythmia, orthostatic hypotension, cervical spondylosis, vestibular disease, thoracic outlet syndrome and aortic arch syndromes.

Measure blood pressure in both arms in every patient with vascular disease, with the higher reading used for management. Both arms may be affected, so apparently equal pressures do not exclude bilateral disease.

Bedside

• Bilateral arm blood pressures after resting for 5 minutes, and palpation of brachial, radial and ulnar pulses.
• A difference of 15 mmHg or more identifies subclavian stenosis with high sensitivity. Radial pulse delay compared with the opposite side supports the diagnosis.

Duplex ultrasound

• High velocity turbulent flow at the stenosis (peak systolic velocity above 240 cm/s, or ratio to the pre-stenotic segment above 2) and damped post-stenotic waveform.
• Retrograde vertebral flow, which can be unmasked by reactive hyperaemia: inflate a cuff above systolic pressure on the affected arm for 3 to 5 minutes, then release it to increase reversal.
• Also assess carotid arteries and the contralateral vertebral artery.

Cross-sectional imaging

• CT angiography from the arch to the circle of Willis is preferred and delineates the lesion, calcification, length, arch anatomy, the vertebral origin and the intracranial circulation.
• MR angiography is an alternative, with time-resolved sequences to show flow reversal.
• Catheter angiography with a pressure gradient across the lesion (a peak-to-peak gradient above 10 to 15 mmHg is significant) is performed during intervention.

Further evaluation

• ESR and CRP, and imaging for Takayasu arteritis or giant cell arteritis (wall thickening on CT, MRI or PET) when inflammatory disease is suspected.
• Lipids, HbA1c and renal function.
• ECG and prolonged rhythm monitoring for syncope, and brain MRI for posterior circulation infarcts.
• Coronary assessment before bypass surgery in patients with subclavian stenosis. Screen with bilateral blood pressure measurement.

Asymptomatic steal

• No revascularisation. Treat as atherosclerotic vascular disease with risk factor control.
• Use the higher arm pressure for blood pressure targets. Avoid excessive blood pressure lowering that may precipitate posterior circulation hypoperfusion.

Medical therapy for all patients

• Antiplatelet: aspirin 75 to 100 mg daily or clopidogrel 75 mg daily.
• Statin: atorvastatin 40 to 80 mg or rosuvastatin 20 to 40 mg daily, with LDL cholesterol target below 55 mg/dL (1.4 mmol/L).
• Blood pressure below 130/80 mmHg, smoking cessation (varenicline 0.5 mg daily for 3 days, 0.5 mg twice daily for 4 days, then 1 mg twice daily for 12 weeks), diabetes control and exercise.
• Takayasu arteritis in the active phase: prednisolone 0.5 to 1 mg/kg daily with a steroid-sparing agent (methotrexate 15 to 25 mg weekly, azathioprine 2 mg/kg/day, or tocilizumab 162 mg SC weekly). Revascularisation is delayed until disease is quiescent for as long as possible.

Indications for revascularisation

• Disabling arm claudication or ischaemia, recurrent posterior circulation symptoms attributable to steal, or coronary-subclavian steal.
• Asymptomatic stenosis before coronary bypass using the ipsilateral internal mammary artery, or planned dialysis access in the affected limb.
• Embolic complications.

Endovascular treatment (first line for most)

• Angioplasty with balloon-expandable stenting for the ostial lesions, and self-expanding stents for more distal lesions. Technical success exceeds 95%, with primary patency 80 to 90% at 3 to 5 years.
• Totally occluded subclavian arteries can be recanalised in 70 to 85% by antegrade or retrograde (brachial or radial) access.
• Periprocedural care: aspirin 75 to 100 mg and clopidogrel 75 mg (300 to 600 mg load) for at least 1 to 3 months, then single antiplatelet therapy. Heparin 70 to 100 units/kg during the procedure.
• Cerebral protection is used selectively. Avoid stent coverage of the vertebral origin, and consider vertebral protection when a stent crosses it.
• Complications: stroke (under 1%), vertebral embolisation, access complications, arterial rupture and restenosis (5 to 15%).

Surgical treatment

• Carotid-subclavian bypass with prosthetic graft (PTFE or Dacron) has the best long-term patency, over 90% at 10 years, and is preferred for long occlusions, failed stenting and diffuse Takayasu disease.
• Carotid-subclavian transposition is an alternative with high patency.
• Axillo-axillary or femoral-axillary bypass when the chest approach is hazardous.
• Risks include phrenic nerve injury, chylous fistula, Horner syndrome, brachial plexus injury and graft infection. Perioperative cefazolin 2 g IV.

Follow-up

• Duplex at 1, 6 and 12 months after intervention, then annually. Measure bilateral arm blood pressures at each visit.
• Long-term antiplatelet and statin therapy, with inflammatory marker surveillance in vasculitis.