Thoracic Outlet Syndrome (TOS)

Thoracic outlet syndrome is a condition whereby symptoms are produced from compression of nerves or blood vessels, or both, because of an inadequate passageway through an area (thoracic outlet) between the base of the neck and the armpit. The thoracic outlet is surrounded by muscle, bone, and other tissues. Any condition that results in enlargement or movement of the tissues of or near the thoracic outlet can cause the thoracic outlet syndrome. These conditions include muscle enlargement (such as from weight lifting), injuries, an extra rib from the neck at birth (cervical rib), weight gain, and tumors at the top of the lung (rare). Often no specific cause is found.

It is felt by some researchers that the evolution of the torso of primates from a four-legged to a two-legged position may predispose humans to the development of thoracic outlet syndrome. The resulting vertical posture produced flattening of the chest cage and a shift of the shoulder joint backward, both of which narrowed the thoracic outlet.

Symptoms include neck, shoulder, and arm pain, numbness, or impaired circulation to the extremities (causing discoloration). Often symptoms are reproduced when the arm is positioned above the shoulder or extended. Patients can have a wide spectrum of symptoms from mild and intermittent, to severe and constant. Pains can extend to the fingers and hands, causing weakness.

Source: MedicineNet.com
http://www.medicinenet.com/thoracic_outlet_syndrome/article.htm