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.
The thoracic outlet has many anatomic variations that can cause narrowing of the thoracic outlet, often worse with arm motion. This narrowing compresses the vessels or nerves, causing blood clot, pain, tingling, numbness, weakness or muscle wasting. Clinical diagnosis of TOS is limited, but MRI can show anatomic abnormalities, compression, and blood clots.