If left untreated, TOS can lead to neurological deficits as a result of the hypoperfusion and hypometabolism of certain areas of the brain and cerebellum.TOS has similar symptoms to pectoralis minor syndrome (PMS), which usually results from compression of the braxial plexus beneath the pectoralis minor muscle (while neurogenic TOS is caused by compression of the same nerves above the clavicle). Unlike TOS there is typically few headaches or neck pain in patients with PMS only, instead with pain in the chest area. Initially, it was believed that 95 percent of patients with TOS had nerve compression in the scalene area, but in the twenty-first century it is now recognized that the majority have nerve compression under the pectoralis minor, either by itself or in addition to the scalene area. One study of 100 patients diagnosed with neurogenic TOS found that 75 percent had neurogenic PMS and 30 percent in fact had PMS without TOS. Causes
TOS can be attributed to one or more of the following factors:
Congenital abnormalities are frequently found in persons with TOS. These include cervical rib, prolonged transverse process, and muscular abnormalities (e.g., in the scalenus anterior muscle, a sickle-shaped scalenus medius) or fibrous connective tissue anomalies. Trauma (e.g., whiplash injuries) or repetitive strain is frequently implicated. Rarer acquired causes include tumors (especially pancoast tumor), hyperostosis, and osteomyelitis

Diagnosis
Adsons sign and the costoclavicular maneuver lack specificity and sensitivity and should make up only a small part of the mandatory comprehensive history and physical examination undertaken with a patient suspected of having TOS.
By event
There are many causes of TOS. The most frequent cause is trauma, either sudden (as in a clavicle fracture caused by a car accident), or repetitive (as in a legal secretary who works with his/her hands, wrists, and arms at a fast-paced desk station with non-ergonomic posture for many years). TOS is also found in certain occupations involving much lifting of the arms and repetitive use of the wrists and arms. One cause of arterial compression is trauma, and a recent case involving fracture of the clavicle has been reported.The two groups of people most likely to develop TOS are those with neck injuries due to traffic accidents and those who use computers in non-ergonomic postures for extended periods of time. TOS is frequently a repetitive stress injury (RSI) caused by certain types of work environments. By structure causing constriction
It is also possible to classify TOS by the location of the obstruction:
Anterior scalene syndrome (compression on brachial plexus and/or subclavian artery caused by muscle growth). Cervical rib syndrome (compression on brachial plexus and/or subclavian artery caused by bone growth). Costoclavicular syndrome (narrowing between the clavicle and the first rib) – diagnosed with the costoclavicular maneuver.Some people are born with an extra incomplete and very small rib above their first rib, which protrudes out into the superior thoracic outlet space. This rudimentary rib causes fibrous changes around the brachial plexus nerves, inducing compression and causing the symptoms and signs of TOS.