The most common form of thoracic outlet syndrome (TOS) involves the nerves and appears to be caused by hyperextension (excessive backward bending)  of the neck, a type of injury usually referred to as “whiplash.” Swollen and scarred neck muscles can compress or irritate the brachial plexus nerves (running between the neck and the shoulder). This type of injury is also common in contact sports such as football.

TOS may also be caused by weak musculature related to aging, obesity, large breasts and/or lack of exercise. In this case, the muscles that normally support the shoulder become weak (e.g., droopy shoulder syndrome), allowing the thoracic outlet to become compressed.

Although external trauma and weakened musculature are the most common types of TOS, congenital abnormalities play a part as well. A patient may have an abnormal bony growth in the neck that, undisturbed, may not produce any symptoms. Trauma or certain repetitive arm motions (e.g., typing, assembly line work) may cause the abnormal bone to press against the nerves, resulting in symptoms.

An extra rib, called the cervical rib in the neck, appears in less than 1 percent of the population. People born with a cervical rib are 10 times more likely to develop TOS, and some type of injury usually triggers symptoms.

It should be noted that other injuries related to repetitive motion  can be caused by other conditions, such as carpal tunnel syndrome and tendinitis. These conditions can coexist with TOS, but are considered separate and are treated separately.

Other risk factors include:

The risk of developing thoracic outlet syndrome (TOS) can be diminished by:

• Regular exercise and stretching to build the neck and shoulder muscles
• Maintaining a correct posture when sitting or standing
• Maintaining a healthy weight

People at risk for developing TOS should avoid carrying heavy bags over the shoulder, which can compress the collar bone and increase pressure on the thoracic outlet.