Thoracic Outlet Syndrome (TOS)

Thoracic Outlet Syndrome (TOS) is a disorder of the upper limb that causes pain, and/or swelling. The thoracic outlet is an area on both sides of the body at the root of the neck where the main artery, vein and nerves cross over the first rib to continue down into the arm. Problems occur when this area is compressed or made smaller. 
(Resource: Vascular Society, Thoracic Outlet Specialist Interest Group)

Thoracic Outlet Syndrome can be the result of anatomical problems. Some people are born with an extra rib called a "cervical rib" or fibrous bands. These can narrow the space in the thoracic outlet area and cause compression.

Other people may be born with a smaller opening. It is not uncommon for symptoms to begin after an injury or accident such as a fractured collar bone or whiplash.

Activities that cause enlargement of the scalene muscles, such as weightlifting or repetitive movements such as computer work or swimming, can also cause symptoms. The brachial plexus can also be compressed beneath the pectoralis minor muscle further out towards the shoulder. This is a condition called neurogenic pectoralis minor syndrome (NPMS). NPMS can occur in conjunction with TOS or may occur on its own.

(Resource: Vascular Society, Thoracic Outlet Specialist Interest Group)