Etiology
In medicine, etiology refers to the causes of diseases or pathologies.
Anatomic Relationships
The subclavian artery leaves the thorax by arching over the first rib behind the scalenus anticus muscle and in front of the scalenus medius muscle. It then passes under the clavicle and finally enters the axilla beneath the pectoralis minor muscle. Except that it passes anteriorly rather than posteriorly to the scalenus anticus muscle, the subclavian vein has an identical course. The brachial plexus follows the route of the subclavian artery, but it lies a little more posteriorly and laterally.
Contributing Factors
In addition to the anatomic relationships, which provide potential areas of pressure leading to the production of neurovascular-compression syndromes, there are five contributing factors to consider: dynamic, static, congenital, traumatic, and arteriosclerotic.
Dynamic
There is an unusually wide latitude of motion in the components of the shoulder joint. A moderate degree of motion takes place at the sternoclavicular articulation, this being one of the few universal joints in the body. The acromioclavicular articulation permits the inferior angle of the scapula to move laterally, approximately 45 degrees, during elevation of the arm. Finally, the articulation between the humerus and scapula permits the widest range of motion of any joint in the body. These movements, involving changes in relative position of regional structures, may result in compression or impingement upon either vessels and/or nerves. For example, when the arm is in full hyperabduction above the head, the axillary artery is bent 180 degrees from its position when the arm is at the side. This motion pulls the vessel across the coracoid process and head of the humerus, as across a pulley.
Static
Vigorous work or muscular exercise may result in an increase of muscular bulk, thereby reducing the space through which the artery, vein, and nerves must pass. On the other hand, a reduction in muscle mass and tone may cause middle-aged sagging of local structures. That the latter is the more important factor is indicated by the greater frequency of these syndromes in middle-adult life.
Congenital
The presence of a cervical rib, a bifid clavicle, or a bony protuberance on the first rib may cause pressure on the vessels or nerves when the arm is in certain positions. Also there may be a fascial band behind the scalenus anticus or an abnormal insertion of the scalenus medius on the first rib.
Traumatic
The most common traumatic causes of these syndromes are fractures of the clavicle and subacromial dislocation of the humeral head. Occasionally, a crushing injury of the upper thorax may unduly stretch parts of the brachial plexus and/or thrombose the artery or vein.
Arteriosclerotic
The degree of activity and effort that is well tolerated by a healthy, flexible artery may cause thrombosis in a vessel that is narrowed and sclerotic. This situation has been observed in several individuals in their sixth and seventh decades of life, whose shoulder girdles were anatomically normal for their age, but whose arteries were hardened and relatively inflexible.