Thoracic outlet syndrome is caused by compression or irritation of the nerves as they exit the neck toward the upper extremity. Often it is the compression or irritation of the brachial plexus, not from compression of the subclavian artery. In this discussion, the author notes some researchers who believe that the sacroiliac plays a large role in the etiology of this condition. Others feel an abnormal thoracic curve is the cause.
Liebenson, CS J Manipulative Physiol Ther. 1988 (Dec);11 (6): 493-499