Subclavian Steal Syndrome

How subclavian steal works

The two subclavian arteries carry blood to the arms. Each gives off a vertebral artery that runs up the neck to supply the back of the brain: the brainstem, cerebellum and the areas for vision and balance. The two vertebral arteries join inside the skull to form the basilar artery.

If the subclavian artery is narrowed before the vertebral artery branches off, pressure in the arm beyond the blockage falls. Blood then takes a detour: it flows up one vertebral artery, across the junction at the base of the brain, and backward down the other vertebral artery to the arm. The arm is supplied, but at the brain's expense.

Doctors distinguish:

• Subclavian steal phenomenon: reversed flow seen on scans, without symptoms. This is common in people with atherosclerosis.
• Subclavian steal syndrome: reversed flow with symptoms of reduced blood flow to the brain or arm.

Causes

• Atherosclerosis: by far the most common cause, in older adults with smoking, high blood pressure, high cholesterol or diabetes. The left subclavian artery is affected about 3 to 4 times as often as the right.
• Takayasu arteritis: inflammation of large arteries, typically in young women.
• Giant cell arteritis in older adults.
• Thoracic outlet syndrome with arterial compression.
• Radiation, trauma, or congenital abnormalities of the aortic arch.

Symptoms

Most people with reversed flow have no symptoms, because the brain has other routes of blood supply. Symptoms appear more often when other neck arteries are also narrowed.

Brain symptoms (vertebrobasilar)

Usually triggered by exercising the affected arm, which increases its demand for blood:

• dizziness or vertigo
• blurred vision, double vision or brief loss of vision
• unsteadiness or poor coordination
• fainting or near-fainting
• slurred speech or headache at the back of the head

See vertebrobasilar insufficiency for other causes of reduced blood flow to the back of the brain.

Arm symptoms

• aching, cramping or tiredness in the arm with use (arm claudication), relieved by rest
• coldness, pallor, numbness or tingling in the hand
• in severe cases, painful fingertips or small ulcers from tiny clots

Coronary-subclavian steal

People who have had heart bypass surgery using the left internal mammary artery (which branches from the left subclavian) can develop chest pain or a heart attack if the left subclavian narrows, because blood is stolen from the heart graft. This form needs prompt treatment.

How it is diagnosed

• Blood pressure in both arms: a systolic difference of more than about 15 mm Hg suggests subclavian narrowing. The affected side has a weaker or delayed pulse.
• Listening with a stethoscope above the collarbone for a whooshing sound (bruit).
• Duplex ultrasound of the neck arteries shows reversed flow in the vertebral artery and narrowing of the subclavian artery.
• CT angiography or MR angiography maps the blockage and other arteries before treatment.
• Catheter angiography is used mainly during treatment.

Because subclavian disease is a marker of widespread atherosclerosis, doctors also check for coronary artery disease, carotid narrowing and peripheral artery disease in the legs.

Treatment

For everyone

• Stop smoking, the most important single step.
• Antiplatelet medicine such as aspirin or clopidogrel, as prescribed.
• Statin to lower cholesterol and stabilize plaque.
• Control blood pressure and diabetes.
• Always measure blood pressure in the arm with the higher reading, since the affected arm underestimates true blood pressure.

People with the phenomenon but no symptoms usually need only these measures.

Procedures for symptomatic disease

Treatment to restore flow is recommended for persistent brain or arm symptoms, coronary-subclavian steal, or before heart bypass using the internal mammary artery:

• Angioplasty and stenting: a balloon and metal mesh tube are passed through an artery in the groin or arm to open the narrowing. It is minimally invasive with high success rates, though some narrowings recur.
• Surgical bypass: most commonly a carotid-subclavian bypass, or moving the subclavian artery onto the carotid artery (transposition). It is durable and preferred for long blockages or failed stents.

Inflammatory causes such as Takayasu arteritis are treated with immune-suppressing medicines first.

Outlook

Most people with subclavian steal never develop serious brain symptoms, and it rarely causes a stroke by itself. Its main importance is as a sign of atherosclerosis elsewhere, which raises the risk of heart attack and stroke. With risk factor control, and stenting or bypass when needed, the outlook is good.

Frequently asked questions

What are the symptoms of subclavian steal syndrome?

Many people have none. When present, symptoms include dizziness, vertigo, blurred or double vision, unsteadiness or fainting, often triggered by exercising the affected arm, plus aching, tiredness, coldness or numbness in that arm. Chest pain can occur in people with certain heart bypass grafts.

Is subclavian steal syndrome serious?

It is rarely dangerous on its own and seldom causes a stroke directly. It matters because it signals atherosclerosis that may also affect the heart and other arteries. Symptomatic cases and those affecting heart bypass grafts need treatment to restore blood flow.

What does a big blood pressure difference between arms mean?

A difference of more than about 15 mm Hg in the top number between arms can indicate narrowing of the subclavian artery on the lower side. It is linked to higher cardiovascular risk. Repeat the readings, and if the difference persists, ask your doctor about further tests such as an ultrasound.

How is subclavian steal syndrome treated?

Everyone needs risk factor treatment: stopping smoking, antiplatelet medicine, a statin and blood pressure control. People with ongoing symptoms usually have angioplasty with stenting, or a surgical bypass such as a carotid-subclavian bypass, to restore normal flow to the arm and brain.