Thoracic Outlet Syndrome Overview
Thoracic outlet syndrome is an uncomfortable condition characterized by numbness and pain radiating down the arm associated with shoulder position. Severe cases can also block the flow of the main artery that supplies the arm (very rare).
Women tend to have more incidence of thoracic outlet syndrome than men do. Individuals that become more involved in overhead activity (swimmers, stocking clerks, carpenters) tend to have a higher incidence of this disorder as well as weight lifters. Injuries to the clavicle (collar bone) can also produce thoracic outlet syndrome.
Thoracic outlet syndrome develops due to abnormal pressure on the nerves called the brachial plexus that travel through the tunnels of the shoulder. The “thoracic outlet” consists of multiple tunnels that these nerves (and blood vessels) travel through.
Anatomy of the Thoracic Outlet
The origin of the nerves in the shoulder stem from the spinal nerves. These spinal nerves exit through the holes in the bones of the neck vertebra (foramen). The spinal nerves can become individually compressed in these bony holes. This condition is called foraminal stenosis or radiculopathy and can occur from a bone spur or compression from a herniated disc. This type of compression can be mistaken for thoracic outlet syndrome and is discussed in another section on this website under cervical radiculopathy and cervical herniated disc.
The nerves that travel down into the arm (brachial plexus) originate from the spinal nerves C5 through T1. The C5 nerve exits between the C4 and C5 vertebrae, the C6 nerve exits between the C5 and C6 vertebrae and so on until the T1 nerve where this nerve exits between the T1 and T2 vertebrae. Even though there are only seven cervical vertebrae, there is a C8 nerve, which exits between C7 and T1. When these nerves finally exit from the spinal canal, they travel through different structures to get to their respective targets. The targets of these nerves are muscle, joint, tendon, bone and skin.
When these individual nerves exit from the spinal foramen, they join together, separate and then join again in a peculiar “dance” called the brachial plexus. This nerve plexus has three tunnels that it passes through. The first tunnel is the path between the scalene muscles. The second tunnel is the area between the clavicle (collar bone) and the first rib. The last tunnel is located between the pectoralis minor muscle tendon and the acromion (part of the bony shoulder blade that protrudes in front to provide a lever arm for this muscle).
The first tunnel is located in between two muscles; the scalenus anticus and scalenus medius. These muscles originate from the first rib and insert on the transverse processes of the side of the cervical vertebra. It just so happens that the spinal nerves exit the foramen in the spine right between these two muscles. A very tight band of muscle or a more rigid tendon or fibrous band within the muscle can impinge upon these nerves causing symptoms.
The brachial artery joins these nerves low in this interval. As an interesting point of anatomy, the brachial vein is not present in this first tunnel. The brachial artery comes in very handy in the case of testing for thoracic outlet syndrome, which will be covered later in this topic.
The second tunnel is located between the first rib and the clavicle (collar bone). There are three structures that descend into the arm through this tunnel; the aforementioned brachial plexus, the brachial artery and the brachial vein. Compression can occur to the nerves if there is abnormal motion of the clavicle, as the clavicle is designed to move and rotate when the shoulder moves. The clavicle acts like a strut when the shoulder is overhead and can impinge upon these nerves. Thickening of the clavicle (in the case of a healed fracture) can also impinge the brachial plexus).
The last tunnel the brachial plexus has to negotiate through before this nerve plexus finally splits up into the respective peripheral nerves is the tunnel between the tendon of the pectoralis minor and the acromion of the shoulder blade. The nerves the brachial plexus finally separates into are the musculocutaneous, axillary, median, ulnar and radial nerves.
Pathology (what can go wrong)
Any narrowing of these tunnels can compress the nerves with certain shoulder positions and produce symptoms. Shoulder movements that normally cause symptoms involve bringing the arm overhead, reaching at shoulder height or behind the body. Symptoms are characterized as numbness, pain, paresthesias (pins and needles) and a feeling of “deadness” in the arm that radiates into the hand. The outside of the hand (opposite the thumb side) is typically affected.
Neck motion can also cause impingement if the nerves are caught in the first tunnel (scalenus anticus and medius). Typically, with this entrapment, turning the head to the symptomatic side can bring on the symptoms. If the symptoms are brought about by looking up (cervical extension) without lifting the shoulder, the disorder more likely than not is from compression of the nerves in the neck (radiculopathy) and not in the shoulder (thoracic outlet syndrome).
Cervical ribs are extra ribs that have formed congenitally (in the womb) and occur in less than one percent of the population. These ribs are typically identified on an X-ray. If cervical ribs are present, these ribs can crowd out the nerves and artery in the first or second tunnels. The presence of these extra ribs does not mean that one will have thoracic outlet syndrome as less than ten percent of patients with these ribs have thoracic outlet symptoms.
It is very rare that motor strength is diminished by thoracic outlet syndrome as the sufferer will stop whatever activity is causing the symptoms and the compression injury to the nerves is short lived. In very severe cases however, muscle weakness can occur.
The symptoms can become so severe that simply holding the arms on a steering wheel can produce pain and numbness. Some patients complain that the arm “goes dead” in this position. Sleeping can be a challenge to some as inadvertently, these individuals sleep with their arm overhead or the shoulder “scrunched up” under the pillow, which then compresses the brachial plexus. These nerve symptoms can cause the patient to awaken in the middle of the night.
The symptoms that occur when the brachial plexus nerves are compressed are very similar to the “funny bone” (ulnar) nerve at the elbow becoming compressed. Everyone has had the experience that resting the inside of their elbow on a hard surface for a long period of time will cause the typical numbness and pins and needles to radiate into the outside of the hand as the hand “falls asleep”. These symptoms occur due to pressure on the ulnar nerve at the elbow. Pressure prevents nerve conduction.
An interesting fact is that this same ulnar nerve (which originates in the shoulder) is also the typical nerve that is most affected with compression in the thoracic outlet. This is due to the fact that the ulnar nerve originates from the lower roots of the brachial plexus. These lower roots are stretched a longer distance with overhead activity (like a cable that runs over a pulley). This is similar to a track star that runs the race on the outside of the track. He or she has to run a longer distance than the runner on the inside of the track if this individual stays in the outside lane.