What Is Subclavian Steal?

Subclavian steal is a vascular condition in which blood flow is redirected away from the brain because of a narrowing in the subclavian artery. The subclavian artery is one of the major arteries supplying each arm. When the first portion of this artery becomes narrowed or blocked, blood pressure within that vessel drops significantly.

The vertebral artery, which normally carries blood upward to the brainstem and cerebellum, branches off the subclavian artery. When the subclavian artery is narrowed before this branch point, the pressure difference causes blood to flow backward down the vertebral artery and into the arm instead of traveling upward to the brain. This reversal of flow effectively steals blood from the posterior circulation of the brain, producing symptoms of vertebrobasilar insufficiency.

How the Steal Causes Dizziness

Dizziness is one of the most common symptoms of subclavian steal syndrome. The dizziness arises because the brainstem and cerebellum are momentarily deprived of adequate blood supply. These regions rely on the vertebral and basilar arteries for oxygen and nutrients. When blood is diverted to the arm, the posterior brain structures receive less flow, leading to lightheadedness, vertigo, imbalance, or a sense of impending fainting.

In many people, the subclavian steal is silent at rest. The brain compensates through collateral circulation from other arteries, such as the carotid system and the opposite vertebral artery. However, when demand increases, the compensation fails and dizziness appears or worsens.

Exercise Induced Dizziness

A hallmark feature of subclavian steal is exercise induced dizziness. During repetitive or vigorous use of the affected arm, the muscles in that limb require a dramatic increase in blood flow. Because the subclavian artery is already narrowed, the extra demand cannot be met through normal antegrade flow. Instead, the arm pulls blood from the vertebral artery, intensifying the retrograde steal and reducing cerebral perfusion further.

Activities that commonly provoke exercise induced dizziness in this condition include arm cycling, swimming, rowing, sweeping, painting overhead, or any motion that involves sustained or repeated use of the shoulder girdle. Patients often report that symptoms begin within seconds to minutes of starting the activity and subside shortly after the arm is rested. Associated features may include arm fatigue, a sensation of coolness in the limb, and a measurable difference in blood pressure between the two arms.

Recognising the Condition

Clinicians typically suspect subclavian steal when a patient describes reproducible dizziness during arm exertion. The examination may reveal a difference in systolic blood pressure between the left and right arms of more than twenty millimetres of mercury. A bruit may be audible over the supraclavicular area, and the radial pulse on the affected side may be weak or delayed.

Imaging of the arteries confirms the diagnosis. Ultrasound with colour Doppler can visualise reversed flow in the vertebral artery, particularly during reactive hyperaemia after transiently occluding blood flow to the arm. More detailed anatomical images can be obtained with magnetic resonance angiography or computed tomography angiography, which show the location and severity of the subclavian stenosis.

Management and Outlook

Treatment depends on the severity of symptoms and the underlying cause of the arterial narrowing. Atherosclerosis is the most frequent cause, and risk factor modification is central to care. Smoking cessation, blood pressure control, lipid lowering therapy, and antiplatelet medication may slow progression of disease in the arteries.

For patients with disabling exercise induced dizziness, revascularisation may be offered. Percutaneous angioplasty with stenting of the subclavian artery is a minimally invasive option that restores antegrade flow and abolishes the steal. Surgical bypass procedures are reserved for complex or recurrent cases.

After successful revascularisation, the backward flow in the vertebral artery normalises and dizziness generally resolves. Most patients are able to resume physical activity without recurrence of the steal phenomenon.