Doctors know thoracic outlet syndrome as TOS. Specifically, the thoracic outlet is the hollow space above your collarbone, between your shoulder and your neck. If you want to find your thoracic outlet, place your fingers on the side of your neck and slide down to the space above your collarbone.
Many anatomic variations appear in the thoracic outlet that can cause narrowing of this important space, often made worse with arm motion. This narrowing compresses the vessels or nerves, causing blood clots, pain, tingling, numbness, weakness or muscle wasting. Clinical diagnosis of TOS is limited, but MRI proves anatomic abnormalities, compression, and blood clots.