Subclavian steal phenomenon

Synonym: Harrison and Smyth's syndrome, subclavian steal syndrome, SSS

What is subclavian steal phenomenon?^{1 2 3}

The subclavian steal phenomenon (SSP) occurs when there is stenosis or occlusion of the subclavian artery proximal to the vertebral artery origin, causing reversed flow in the ipsilateral vertebral artery. Blood is 'stolen' from the circular vertebrobasilar system to supply the distal territory of the occluded or stenosed artery. Retrograde flow in the vertebral artery, associated with a subclavian or innominate (brachiocephalic) artery stenosis, can be an incidental finding during Doppler ultrasound examination of the cerebral supply.

Subclavian stenoses are most often asymptomatic and therefore do not need any treatment. The term 'subclavian steal syndrome' should only be used in cases where this aberrant blood flow causes symptoms which affect the brain, upper limb or the heart. These are related to reduced cerebral perfusion when the arm ipsilateral to the subclavian stenosis is exercised.

Anatomy and blood flow in subclavian steal phenomenon^{1 2 3}

Subclavian steal phenomenon affects the left side much more commonly than the right, with relative incidence about 3-4:1.

NB: if the left vertebral artery arises directly from the aortic arch (as it does in 2% of the population), stenosis of the proximal left subclavian artery cannot cause the syndrome because there is no communication between the vertebral and subclavian arteries.

How common is subclavian steal phenomenon? (Epidemiology)³

Prevalence

This is a relatively uncommon disorder. Its incidence has been reported as between 0.1% and 3.4% of adults.⁴

Approximately 30% of patients who have been confirmed with a peripheral arterial disease have stenosis of the subclavian artery, but only a tiny percentage become symptomatic, primarily because of the formation of collateral blood vessels.

Risk factors

As the majority of cases are due to atherosclerosis, risk factors for subclavian steal phenomenon are as for cardiovascular disease (CVD) risk factors in general:

• Family history of vascular disease.

• Smoking.

• Hypertension.

• Diabetes.

• Hyperlipidaemia.

Thoracic outlet syndrome and stenosis after surgical repair of aortic coarctation or Fallot's tetralogy (with a Blalock-Taussig shunt) are other possible causes.

In Asia, a significant proportion of subclavian steal phenomenon (36%) is caused by Takayasu's arteritis. These tend to present at a much earlier age (<30 years) and have a female predominance. Takayasu's arteritis is a very rare disease in Europe.

Subclavian steal phenomenon symptoms^{1 2 3}

Only a minority of patients with subclavian steel stenosis have symptoms. Arm claudication is the most common complaint; pain or fatigue in the arm occurs following exercise.

Seek a history of a provoking event that is clearly linked to symptoms. These may be reproducible. Symptoms are usually related to vertebrobasilar and posterior cerebral circulation ischaemia.

On exercising the upper limb on the affected side, the patient may experience any of the following:

• Vertigo.

• Visual loss, ranging from unilateral visual field loss (amaurosis fugax) to bilateral total blindness.

• Transient periods of ataxia, diplopia, dysphagia and dysarthria.

• Tingling or numbness of the face, sensory hemianaesthesia affecting the body or transient hemiparesis.

• Intermittent arm claudication. (NB: rest pain is not a usual feature; consider atheroembolism as a cause.)

• Coolness or paraesthesia of the arm may occur either at rest of with exertion.

• Drop attacks (syncope) - fall to the ground without warning, ± temporary loss of consciousness with immediate recovery.

• Dizziness, diplopia, nystagmus, tinnitus or even hearing loss may occur.⁵

Signs

• Blood pressure is decreased (>15 mm Hg) in the affected arm distal to the steno-occlusive disease.

• However, even a difference in systolic blood pressure of 10 mm Hg or more between arms has been shown to be strongly associated with subclavian stenosis.⁶

• Check radial and ulnar pulses and elevate the arm, where they may be felt to diminish. It is unusual for a case of genuine subclavian steal syndrome to have no difference in blood pressure between the two arms.

• A subclavian bruit may be audible.

Differential diagnosis

• Aortic dissection.

• Giant cell arteritis (temporal arteritis).

• Takayasu's arteritis.

• Atherosclerosis/thromboembolism.

• Cerebral/cerebellar pathology - eg, multiple sclerosis, brain tumour.

Investigations^{1 2}

• Doppler ultrasound or angiography shows retrograde flow down the vertebral artery (often an asymptomatic, incidental finding).

• Scanning with duplex ultrasonography and transcranial Doppler is more sensitive than conventional angiography for detecting flow reversal.⁵

• Magnetic resonance angiography offers comparable resolution to computed tomography angiography but is more expensive and less widely available.

• CT or MRI scanning may be undertaken to exclude intracerebral lesion and show any infarcts.

Other investigations may include:

• Colour Doppler ultrasound.

• CXR - to exclude external compression by, for example, cervical rib.

• ECG.

Subclavian steal phenomenon treatment^{1 2}

Conservative treatment is usually the best initial therapy, with surgery reserved for refractory symptomatic cases. Percutaneous angioplasty and stenting, rather than bypass grafts of the subclavian artery, is the usual favoured surgical approach.

Even asymptomatic subclavian artery stenosis is associated with increased risk of cardiovascular disease.⁷ Therefore, medical treatment that includes beta-blockers, angiotensin-converting enzyme (ACE) inhibition, and a statin is usually recommended.⁵

Percutaneous transluminal angioplasty (+/- stenting) or surgery (carotid-subclavian bypass with either synthetic graft or saphenous vein graft or carotid-subclavian transposition) can both be used to bypass the stenosis of the subclavian artery. Technical success of the percutaneous approach can be achieved in over 90% of cases, with five-year patency rates of 85%.⁸ Longer or more distal occlusions are usually managed by surgery.

Endovascular methods are increasingly popular, particularly in high-risk patients, due to their minimally invasive approach under local anaesthetic.⁹

Prognosis^{1 2}

Symptoms may spontaneously resolve due to the establishment of extracranial collaterals to the subclavian circulation.

Treatment is therefore usually reserved for patients with debilitating vertebrobasilar transient ischaemic attacks. The outcome for patients who have antegrade vertebral blood flow re-established, either by surgical revascularisation or endovascular stenting of the diseased subclavian artery, is now excellent.

More generally, subclavian stenosis is significantly associated with increased total and CVD-related death, independent of other cardiovascular risk factors.

Associated steal syndromes

Coronary-subclavian steal syndrome

Usually iatrogenic and follows coronary artery bypass grafting utilising the internal mammary artery.¹⁰ Subclavian stenosis causes 'stealing' of coronary blood flow via the arterial anastomosis, causing angina.

Spinal artery steal syndrome

This very rare condition occurs due to vertebral artery flow reversal, to supply blood to the spinal cord, caused by proximal vertebral artery occlusion.¹¹