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36 m a s s a g e & b o d y wo r k s e p te m b e r/o c to b e r 2 0 2 1 Thoracic Outlet By Christy Cael education | FUNCTIONAL ANATOMY The thoracic outlet generally describes the pathway of the brachial plexus and subclavian artery and vein through the lateral neck, anterior shoulder girdle, and into the upper extremity. It is comprised of three specific regions: the interscalene triangle, located proximally; the costoclavicular triangle, located more anterior and distal; and the subcoracoid space, which is the most distal. Each region has specific anatomical features and common dysfunctions that create high potential for compression of the underlying nervous or circulatory structures. ANATOMY The interscalene triangle is a narrow gap between the anterior and middle scalene muscles at the lateral neck. Both scalene muscles have attachments on the first rib, which forms the base or inferior border of the triangle. Both the brachial plexus and subclavian artery pass through this gap and are vulnerable to compression at this point. Hypertrophy or hypertonicity of the scalenes may cause significant narrowing of space between the two muscles or elevate the first rib enough to compromise the neurovascular structures contained within. The costoclavicular triangle is located distal to the interscalene triangle and is the next area of the thoracic outlet that may become pathologically narrow. Here the brachial plexus, subclavian artery, and subclavian vein descend inferiorly, anteriorly, and laterally and must pass between the first rib and clavicle. Muscles that attach to the clavicle, such as the subclavius, pectoralis major, anterior deltoid, and trapezius, all influence the position of the clavicle relative to the first rib, as do the previously mentioned scalenes. This relative position dictates the amount of space within the costoclavicular triangle. As they continue distally into the upper extremity, the neurovascular structures must pass deep to the anterior deltoid, pectoralis major, and pectoralis minor tendon into the subcoracoid space. This channel formed between the coracoid process superiorly, pectoralis minor tendon anteriorly, and the second through fourth ribs posteriorly is most narrow when the shoulder is fully abducted. Excessive muscle development or shortening of either the pectoralis major or minor may contribute to neurovascular compression in this region. THORACIC OUTLET Regions Interscalene Triangle • Anterior Border: Anterior scalene muscle • Posterior Border: Middle scalene muscle • Inferior Border: Medial surface of first rib Costoclavicular Triangle • Anterior Border: Middle third of clavicle • Posteromedial Border: First rib • Posterolateral Border: Upper border of scapula Subcoracoid Space • Superior Border: Coracoid process of scapula • Anterior Border: Pectoralis minor muscle • Posterior Border: Ribs 2–4 Contents • Nerve: Brachial plexus • Blood Supply: Subclavian artery and vein Brachial plexus Subclavian artery Subclavian vein Clavicle Acromion process First rib Pectoralis major Manubrium of sternum Coracoid process Brachial plexus Pectoralis minor