THORACIC OUTLET SYNDROME (TOS) The thoracic outlet is the area between the neck and the shoulder. Thoracic Outlet Syndrome (TOS) is a collection of symptoms caused by abnormal compression of all, or parts of the brachial plexus (a group of motor and sensory nerves that exit the neck and innervate the upper extremities), the subclavian artery and vein, and various other neurovascular components that pass between the neck and shoulder. Compression can be cause by bony, ligamentous, muscular or fascial restrictions and can occur when the size or shape of the thoracic outlet changes. The outlet can be altered by trauma, pregnancy, exostosis (abnormal bone growth), congenital structural abnormalities, poor posture and postural weakness. Thoracic spine abnormalities such as decreased range of motion and asymmetries may also contribute to TOS because it houses the majority of the sympathetic nervous system. The sympathetic nervous system can alter the blood flow and nerve function of the upper extremities, as well as produce bizarre sensations in the arms in hands.