The thoracic outlet is the space between the thorax and axilla through which the subclavian vein, subclavian artery, and brachial plexus travel from their central origins to their peripheral termini. Its bounds include the clavicle, first thoracic rib, insertion of the pectoralis minor muscle onto the coracoid process of the humerus, and the sternum. It contains three areas: the scalene triangle, the costoclavicular space, and the subcoracoid or pectoralis minor space. Aberrant anatomy is common in the thoracic outlet and may predispose patients to compression of the neurovascular bundle and development of clinical thoracic outlet syndrome (TOS). Much of this aberrancy is explained by the embryologic origins of the structures that comprise the thoracic outlet. A thorough understanding of this anatomy and embryology is therefore critical to the understanding of TOS.
Key points
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The subclavian artery and brachial plexus travel together through the costoclavicular space posterior to the anterior scalene muscle, whereas the subclavian vein travels anterior to the anterior scalene muscle.
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The brachial plexus is composed of C5-T1 nerve roots, trunks, divisions, cords, and branches, and variability in development may influence anomalies of the surrounding structures.
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The first rib normally develops from the T1 costal process to the manubrium. Anomalous first ribs originate from T1 and may fuse to the second rib, whereas cervical ribs arise from cervical vertebral bodies (usually C7).
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The scalene muscles may have significant variability, including several anomalies associated with thoracic outlet syndrome.
Introduction
The thoracic outlet is defined as the space in the lower neck between the thorax and axilla through which the subclavian vein, subclavian artery, and brachial plexus travel from their central origins to their peripheral termini. It is bounded by the clavicle anteriorly, the first thoracic rib posteriorly, the insertion of the pectoralis minor muscle onto the coracoid process of the humerus laterally, and the sternum medially. It is subdivided into three areas: the scalene triangle above the clavicle, the costoclavicular space or cervicoaxillary canal between the clavicle and first rib, and the subcoracoid or pectoralis minor space below the clavicle , ( Fig. 1 ).
The subclavian vasculature and brachial plexus—together termed the neurovascular bundle—pass from the scalene triangle into the costoclavicular space before exiting through the subcoracoid space. The thoracic outlet is both a confined and a dynamic space: compression of the neurovascular bundle resulting in the clinical syndrome generally termed thoracic outlet syndrome (TOS) may occur constantly or intermittently with movement of the neck, thorax, and arm ( Fig. 2 ).
Aberrant anatomy is common in the thoracic outlet. Such anatomy predisposes patients to compression of the neurovascular bundle and development of clinical TOS. Much of this aberrancy is explained by the embryologic origins of the structures that comprise the thoracic outlet. A thorough understanding of this anatomy and embryology is therefore critical to the understanding of TOS.
Neurovascular anatomy and embryology
Subclavian Artery
The subclavian arteries are the primary blood supply to the upper extremities. The left subclavian artery branches directly from the aorta, whereas the right subclavian artery arises from the brachiocephalic artery. Each subclavian artery ascends superiorly into the neck before arching laterally and traveling posterior to the anterior scalene muscle through the scalene triangle and exiting the thoracic outlet via the costoclavicular space (above the first rib, below the clavicle) to become the axillary artery. Because of the close proximity of the clavicle, first rib, and anterior and middle scalene muscles, the costoclavicular space is the most frequent site of arterial compression.
The left subclavian artery develops from the left seventh intersegmental artery, whereas the right subclavian artery arises from the fourth aortic arch, right dorsal aorta, and right seventh intersegmental artery. Anomalies of the distal subclavian artery are rare, although anomalies of the aortic arch frequently encompass the origin of the either artery. In the thoracic outlet, the subclavian artery occasionally penetrates the anterior scalene muscle and rarely may pass entirely anterior to it.
Subclavian Vein
The subclavian veins provide the venous drainage of the upper extremities. The left subclavian vein also receives chyle from the thoracic duct drainage. On either side, the vein ascends superiorly with the subclavian artery into the neck. It then travels anterior to the anterior scalene muscle (outside of the technical scalene triangle) and continues in parallel to the artery with the anterior scalene muscle separating the two structures. It then exits through the costoclavicular space as the axillary vein. The subclavian vein is bound by the anterior scalene muscle laterally, the costoclavicular ligament medially, the subclavius tendon cranially, and first rib caudally ( Fig. 3 ). This course explains the compression of the subclavian vein against the subclavius tendon found in patients with an abnormally anterior insertion of the anterior scalene muscle. ,
The subclavian vein develops in the fourth gestational week and is formed by the fusion of venous tributaries from the upper limb bud. The vein can be found in an anomalous location posterior to the anterior scalene muscle, immediately adjacent to the subclavian artery. More rarely, the subclavian vein splits to form a “clavicular loop” or travels between the clavicle and the subclavius muscle.
Brachial Plexus
The brachial plexus is composed of nerves from the C5 to T1 roots that innervate most of the shoulder and arm. As the nerve roots exit the spinal cord and form the brachial plexus trunks, they travel through the scalene triangle, posterior to the subclavian artery and anterior to the middle scalene muscle, split into anterior and posterior divisions, and exit through the costoclavicular space alongside the subclavian artery. In the axilla, they transition from trunks to divisions and then cords ( Fig. 4 ).
The brachial plexus develops before bone maturation and influences bony development. At the end of the first gestational month, the ventral rami begin budding from the neural tube and begin to form the brachial plexus as they grow toward their sclerotomes and myotomes. The developing brachial plexus splits the scalene muscles into anterior and middle components. This gives rise to considerable variability in the relationship between the components of the plexus and the anterior scalene muscle. Common variations include a C5 root anterior to the anterior scalene muscle, C5 and C6 anterior to the anterior scalene muscle, C5 and C6 through the anterior scalene muscle, C5 and C6 through a double anterior scalene muscle, and C5 anterior with C6 through the anterior scalene muscle. , ,
The contribution of nerve roots to the brachial plexus may also vary. A prefixed plexus occurs when a branch of C4 contributes to the brachial plexus with or without contribution from T1. This has a tendency to pull the plexus in a cephalad direction. Alternatively, a postfixed plexus is drawn closer to the thorax by contribution from T2 with little or no contribution from C5. It is postulated that the prefixed and postfixed plexi may be associated with anomalies of the cervical or first rib.
Phrenic Nerve
The phrenic nerve innervates the diaphragm and forms from branches of C3-5, of which C3 and C4 normally combine cephalad to the thoracic outlet. The combined branches and the C5 branch descend along the anterior surface of the anterior scalene muscle posterior to the subclavian vein. The C5 branch typically joins as the combined branches cross the anterior scalene muscle from lateral to medial. If the C5 branch continues along a separate course, it is termed an “accessory phrenic nerve.” Rarely, the phrenic nerve may travel anterior to the subclavian vein and may even obstruct it. This is termed a “prevenous” nerve , ( Fig. 5 ).
Long Thoracic Nerve
The long thoracic nerve innervates the serratus anterior muscle and forms from branches of C5-7. The C5 and C6 branches travel through the belly of the middle scalene muscle where they combine and exit the muscle as a single nerve which crosses the lateral edge of the first rib. The location of the C7 contribution to the nerve is highly variable and occasionally occurs within the belly of the middle scalene muscle. It can be easily injured during dissection of the middle scalene muscle.
Dorsal Scapular Nerve
The dorsal scapular nerve innervates the rhomboid muscles and part of the levator scapulae muscle. It arises from C5, travels briefly with the C5 branch of the long thoracic nerve, and then separates at the top of the middle scalene muscle to descend through its lateral edge.
Cervical Sympathetic Nerve Chain
The cervical sympathetic nerve chain is part of the autonomic nervous system and travels along the anterior surface of the cervical transverse processes. Although not technically located in the thoracic outlet, operations to decompress the thoracic outlet place the sympathetic chain at risk for injury, particularly when the middle scalene muscle is released from the proximal rib in the costotransverse space. Injury to the sympathetic chain results in Horner’s Syndrome, although this is typically self-limited.
Bony anatomy and embryology
First Rib
The first rib develops from the T1 costal process and joins the manubrium. Together with the T1 vertebral body and manubrium, the first rib forms the opening of the superior thoracic cage, also known as the thoracic inlet or superior thoracic aperture. Normal ribs have a costotransverse ligament posteriorly and are joined to the manubrium anteriorly. The anterior portion of the rib may form a symphysis with the sternal head of the clavicle. Abnormal first ribs have a tendency to fuse to with the second rib and are often thinner and more cephalad in location. They often look like cervical ribs on the radiograph but may be distinguished by their T1 origin. Abnormal first ribs result in similar pathology as do cervical ribs. They are primarily associated with arterial or neurogenic TOS. It should be noted, however, that most patients with bony abnormalities are asymptomatic.
Cervical Rib
A cervical rib arises anomalously from a cervical vertebral body, mostly commonly C7. It was first described in dissections by Galen and Vesalius ( Fig. 6 ). A complete cervical rib attaches to the normal first rib by fusion or with a true joint. The incidence is reported in 0.1% to 6% of the adult population and is more frequent in women. ,
