Thoracic outlet syndrome (TOS), a term coined by Rob and Standeven,1 refers to compression of the subclavian vessels and brachial plexus at the superior aperture of the chest. It was previously designated as scalenus anticus, costoclavicular, hyperabduction, cervical rib, and first thoracic rib syndromes according to presumed etiologies. The various syndromes are similar, and the compression mechanism is often difficult to identify. Most compressive factors operate against the first rib.2,3
At the superior aperture of the thorax, the subclavian vessels and the brachial plexus traverse the cervicoaxillary canal to reach the upper extremity. The cervicoaxillary canal is divided into two sections by the first rib: the proximal division, composed of the scalene triangle and the costoclavicular space, and the distal division, composed of the axilla (Fig. 124-1). The proximal division is important to achieve acceptable neurovascular decompression. It is bounded superiorly by the clavicle, inferiorly by the first rib, anteromedially by the costoclavicular ligament, and posterolaterally by the scalenus medius (middle scalene) muscle and the long thoracic nerve. The scalenus anticus (anterior scalene) muscle, which inserts on the scalene tubercle of the first rib, divides the costoclavicular space into two compartments: The anteromedial compartment contains the subclavian vein, and the posterolateral compartment contains the subclavian artery and the brachial plexus. The latter compartment, which is bounded by the scalenus anticus (anterior scalene) muscle anteriorly, the scalenus medius (middle scalene) muscle posteriorly, and the first rib inferiorly, is called the scalene triangle.
The cervicoaxillary canal has a proximal division consisting of the scalene triangle and costoclavicular space and a distal division composed of the axilla. The proximal division is more susceptible to neurovascular compression.
The cervicoaxillary canal, particularly its proximal division, also termed the costoclavicular area, normally has ample space for the passage of the neurovascular bundle without compression. Narrowing of this space occurs during functional maneuvers. It narrows during abduction of the arm because the clavicle rotates backward toward the first rib and the insertion of the scalenus anticus (anterior scalene) muscle. In hyperabduction, the neurovascular bundle is pulled around the pectoralis minor tendon, the coracoid process, and the head of the humerus. During this maneuver, the coracoid process tilts downward and thus exaggerates the tension on the bundle. The sternoclavicular joint, which ordinarily forms an angle of 15–20 degrees, forms a smaller angle when the outer end of the clavicle descends (as in drooping of the shoulders in poor posture), and narrowing of the costoclavicular space may occur. Normally, during inspiration, the scalenus anticus muscle raises the first rib and thus narrows the costoclavicular space. This muscle may cause an abnormal lift of the first rib, as in cases of severe emphysema or excessive muscular development, which is seen in young adults.
The scalene triangle, which normally occurs between the scalenus anticus anteriorly, ...