Thoracic Outlet Syndrome

Thoracic outlet syndrome (TOS) is the name given for a spectrum of symptoms caused by compression of the brachial nerve plexus, which innervates the arm, and the subclavian artery and vein that provide blood circulation [Page 1659]to the arm. TOS is thought to be caused primarily by anatomical variations by the way the upper portion of the rib cage is constructed. This syndrome is typically diagnosed in people between 20 and 40 years of age, and is much more common in women.

Normally, the first rib attaches to the first thoracic vertebra. The brachial plexus is derived from spinal nerves in the neck and cascades between the clavicle and first rib before entering the upper arm. Similarly, the subclavian artery and vein exit the chest cavity by looping over the first rib and following the plexus. Muscles, especially the anterior and middle scalenes and the trapezius, normally protect these structures without compressing them. However, incidences occur when individuals are born with an extra rib originating from the seventh cervical vertebra, which compresses the nerves or the vessels running over the first thoracic rib. Similarly, a fibrous band can originate from a cervical rib or vertebra and connect to the rib cage, causing compression.

TOS can also be an acquired condition. Risk factors include occupations that require large amounts of overhead work and athletic activities that involve repetitive shoulder movement (especially swimming, weight lifting, and rowing). Musicians such as violinists and flutists may also be more susceptible. Poor posture can worsen symptoms as well.

TOS is further classified into two main categories: neurogenic and vascular. Neurogenic TOS is more common and results when the brachial nerve plexus is compressed. Pain in the arm, especially while doing work with the arms or shoulders, is often the presenting symptom.

Pain can also occur if patients turn their head or breathe deeply (both of which narrow the passage for the nerves and vessels). Muscle weakness in the shoulder, arm, and hand also result from plexus compression; atrophy of the hand muscles can be quite pronounced. In addition, patients may experience tingling or impaired sensation.

In vascular TOS, symptoms are caused by compression of the subclavian artery or vein. Arterial compression starves the arm for oxygen, resulting in it becoming pale and cold. Venous compression causes accumulation of fluid and decreased flow into the arm as blood backs up. Occlusion can cause blood clots to form at the compressed locations; these clots can worsen the blockage or embolize downstream and lodge in smaller vessels. Arterial compression can also lead to aneurysm formation.

Diagnosis of TOS is often very difficult due to the spectrum of symptoms and the lack of a definitive and accurate test. Physical examination is crucial; motor and sensory function of the arm and hand are carefully checked, and there are several positional tests that can reduce pulses at the wrist or cause a murmur over the neck if TOS is present. Radiographs are useful to identify cervical ribs, but they do not detect fibrous bands. Doppler ultrasound and magnetic resonance imaging (MRI) angiography are used to evaluate blood flow in suspected vascular TOS. Conduction tests of nerves in the arm and electromyography of the hand muscles can detect many cases of neurogenic TOS, although these tests have high rates of false-negative results and should not be used to rule out TOS. Complicating diagnosis is the fact that TOS closely mimics the symptoms of several other conditions, notably cervical vertebral disc disease and carpal tunnel syndrome.

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