Thoracic outlet syndrome - Symptoms and causes - Mayo Clinic Neurogenic (neurologic) thoracic outlet syndrome. This most common type of thoracic outlet syndrome is characterized by compression of the brachial plexus. The brachial plexus is a network of nerves that come from your spinal cord and control muscle movements and sensation in your shoulder, arm and hand. Venous thoracic outlet syndrome. Thoracic outlet syndrome (TOS) is a general term used to describe three conditions which occurs in the thoracic outlet, an area formed by the top ribs and the collarbone. The syndrome occurs when a nerve or blood vessel is compressed by the rib, collarbone, or muscle. Thoracic outlet syndrome (TOS) is a term used to describe a group of disorders that occur when there is compression, injury, or irritation of the nerves and/or blood vessels (arteries and veins) in the lower neck and upper chest area.