Subclavian Steal Phenomenon

Synonym: Harrison and Smyth's syndrome, Subclavian Steal Syndrome, SSS

The subclavian steal phenomenon (SSP) occurs when there is stenosis or occlusion of the subclavian artery proximal to the origin of the vertebral artery. This may cause flow reversal in the ipsilateral vertebral artery as blood is 'stolen' from the circular vertebro-basilar 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 US examination of the cerebral supply.

The term 'subclavian steal syndrome' should strictly be applied only to cases where this aberrant blood flow causes neurological symptoms. These are related to reduced cerebral perfusion when the arm ipsilateral to the subclavian stenosis is exercised.

See image of normal anatomy of aortic arch and its branches.¹
Subclavian steal phenomenon affects the left side much more commonly than the right, with relative incidence about 3-4:1.^2,3 This is due to anatomical differences that means the left subclavian artery is much more commonly affected by atherosclerosis.
See diagrams of aberrant flow patterns in left and right subclavian steal phenomena.¹

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

Prevalence

This is a relatively uncommon disorder: subclavian stenosis affects about 2% of the general population and 7% of the clinical population.⁴ SSP is seen as incidental finding on sonography of vertebral system in about 2–5% of those examined. Of these, only about 5% of these patients suffer symptoms.²

Risk factors⁴

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

• Family history of vascular disease
• Smoking
• Hypertension
• Diabetes
• Hyperlipidaemia

In Asia, a significant proportion of SSP (36%) is caused by Takayasu's arteritis. These tend to present at a much earlier age (< 30 years) and have a female predominance.² Takayasu's arteritis is a very rare disease in Europe.⁵

Symptoms

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 body or transient hemiparesis
• Hand ischaemia causing arm claudication and rest pain (not a usual feature, consider atheroembolism as a cause)
• Drop attacks (syncope) - fall to the ground without warning, ± temporary loss of consciousness with immediate recovery⁶
• Pulsatile tinnitus⁷

Neck movements may also cause symptoms.

Signs

• BP is decreased (>20 mm Hg) in the affected arm distal to the steno-occlusive disease. 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.
• Look for a subclavian bruit.

• Aortic dissection
• Giant cell (temporal) arteritis
• Takayasu's arteritis
• Atherosclerosis/thromboembolism
• Cerebral/cerebellar pathology e.g. MS, tumour

• Doppler ultrasound or angiography shows retrograde flow down vertebral artery (often an asymptomatic, incidental finding).
• CT or MRI to exclude intracerebral lesion and show any infarcts.

Angioplasty or surgery can both be used to bypass the stenosis of the subclavian artery. Both carry a risk of stroke and death - angioplasty 3.6%, surgery 0.4-2.4% (death only) and recurrent attacks follow surgical treatment in 10-24% of patients. Where symptoms are not severe, conservative management is usually recommended.²

Symptoms may spontaneously resolve due to the establishment of extracranial collaterals to the subclavian circulation. This makes the decision to treat a symptomatic patient not clear cut: treatment is usually reserved for patients with debilitating vertebrobasilar transient ischaemic attacks.
More generally, subclavian stenosis is significantly associated with increased total and CVD-related death, independent of CVD risk factors and existent CVD at diagnosis.⁸

1. Coronary-subclavian steal syndrome: usually iatrogenic and follows coronary artery bypass grafting utilising the internal mammary artery.⁹ Subclavian stenosis causes 'stealing' of coronary blood flow via the arterial anastomosis, causing angina.
2. 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.

• Whilst Luigi Contorni is credited with describing the collateral circulation in 1960, subsequently described with the neurological symptoms associated with arm exercise by Reivich in 1961, and later termed "subclavian steal syndrome", Contorni himself proposed that the syndrome be named Harrison and Smyth's syndrome in honour of the two 19th century surgeons who originally identified the importance of the collateral circulation and its clinical effects.¹⁰
• Robert Harrison (1796-1858) a student of Colles, was Professor of Anatomy and Physiology in Dublin, and succeeded Colles as Professor of Anatomy in 1837. His textbook for students "Surgical Anatomy of Arteries" in 1829, clearly describes the collateral circulation after occlusion of the first part of the subclavian artery.
• Andrew Smyth (1833-1916) was an Irishman who attended medical school in Louisiana and worked in New Orleans, whose case description in the "New Orleans Record" of 1866 correctly identified the effects of inverted vertebral flow on the brain.