The thoracic outlet is the region where neurovascular structures exit the chest. It is composed of the first rib, the subclavian artery and vein, the brachial plexus and the upper part of the lung. Enlargement or movement in these structures can cause compression of the nerve or blood vessels. Occasionally patients will have an extra rib that causes the syndrome. This compression causes the symptoms experienced in the neck and arm. It occurs more often in women between ages 35 and 55.