ABSTRACT
Thoracic outlet syndrome (TOS) is a collection of several conditions in which the neurovascular structures in the root of the neck are compressed, thereby causing a variety of symptoms and signs in the shoulder and upper limb. It is four times more common in women than men and affects those in the second to fifth decades of life. Its aetiology is unclear but is probably related to the tortuous path taken by the first thoracic nerve root and the subclavian artery and vein as they curve up over the first rib before descending into the arm. Any condition which causes relative hypotonia of the muscles which elevate the pectoral girdle will cause the upper limb to droop in relation to the thorax and increase compression of the neurovascular bundle over the first rib. The greater angle of inclination of the first rib in men is probably protective. Compression of the neurovascular elements may also be caused by a cervical rib or a fibrous band arising from it, congenital anomalies of the shoulder girdle, post-fixing of the brachial plexus, hypertrophy or spasm of scalenus anterior and prolonged carrying of a heavy rucksack, although this more often causes compression of the upper trunk. Cervical ribs which are 5 cm or more long will impinge on the brachial plexus, usually lifting it upwards and forwards.
