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Compression of the neurovascular structures can occur with any factor that increases angulation of the nerves or vessels around features in the thoracic outlet or that causes narrowing of the outlet.

Possible causes of compression of the brachial plexus and/or the subclavian vasculature can be related to the scalene muscles, a cervical rib, a clavicle fracture (nonunion, malunion, or abundant callus formation), and accessory neck muscles and fibrous bands at the interscalene region.

Hyperabduction syndrome has been described as compression or traction of the neurovascular structures under the pectoral tendon and coracoid when the arm is abducted.

Abduction of the arm to an overhead position, excessive shoulder girdle depression, and overly developed trapezius and neck musculature have been associated with TOS. Affected athletes often perform repetitive motions involving abduction of the arm to 180 degrees or pulling the shoulders down and back, such as with the cocking phase of serving in tennis.

Muscle swelling from trauma, exercise, or hypertrophy is thought to initiate TOS, as seen in some tennis players and baseball pitchers, especially those with greater muscle development of the dominant arm, increased scapular depression, and failure to maintain adequate scapular stabilization. Muscle weakness, trauma, arteriosclerosis, abnormal anatomy, and poor posture may play a role as well

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