The thoracic outlet is a space between the rib cage (thorax), and the collarbone (clavicle) through which the main blood vessels and nerves pass from the neck and thorax into the arm. Thoracic outlet syndrome is a combination of pain, numbness, tingling, weakness, or coldness in the upper extremity caused by pressure on the nerves and/or blood vessels in the thoracic outlet. In some cases, the cause of compression is evident such as an extra first rib or an old fracture of the clavicle, which reduces the space of the outlet. Sometimes, if the shoulder muscles in your chest are not strong enough to hold the collarbone in place, it can slip down and forward, putting pressure on the nerves and blood vessels that lie under it. Poor posture or obesity also may contribute to the cause. Compression may also occur with repetitive activities that require the arms to be held overhead. In other cases, the cause is not clear.