what is a tos?
(November 2009). 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.horacic 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.