Thoracic outlet syndrome (TOS) is a syndrome involving compression at the thoracic inlet, which is known clinically as the superior thoracic outlet resulting from excess pressure placed on a neurovascular bundle passing between the anterior scalene and middle scalene muscles. It can affect one or more of the nerves that innervate the upper limb and/or blood vessels as they pass between the chest and upper extremity, specifically in the brachial plexus, the subclavian artery, and rarely, the subclavian vein.