The thoracic inlet: Normal anatomy

The Thoracic Inlet: Normal Anatomy Deborah L. Reede, MD The thoracic inlet is the junction between the neck and the chest. A number of neural structures traverse this region. A knowledge of the location of these various neural structures and their relationship to one another is important when interpreting cross-sectional images of this region, This article will review the normal anatomy of the major neural structures that are found in this region.

Copyright © 1996by W.B. Saunders Company

HE T H O R A C I C inlet is a narrow space that serves as the junction between the neck and the thorax. This space is wider in its transverse than anterior-posterior diameter. Its boundaries are the manubrium anteriorly, the first thoracic vertebra posteriorly, and the first rib laterally. This area is further delineated by Sibson's fascia, which extends on each side of the neck from the transverse process of the seventh cervical vertebra posteriorly to the medial border of the first rib. Because this fascia's posterior attachment to the rib is more cranially located than its anterior attachment, the plane of the thoracic inlet is tilted downward anteriorly, and on each side it is higher medially that it is laterally. Because of this orientation, when viewed in the sagittal plane, the lung apices are seen posteriorly and the structures at the base of the neck are located anteriorly. On axial images the apices of the lungs are seen in the posterior lateral aspect of the thoracic inlet and the major structures passing from the neck to the thoracic inlet are located anteriorly between the lungs. The major structures found in the thoracic inlet are as follows:

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NEURAL STRUCTURES Vagus nerve (located in the carotid sheath) Recurrent laryngeal nerve Sympathetic chain Phrenic nerve Brachial plexus

VASCULAR STRUCTURES 1. The subclavian artery and vein

2. The brachiocephalic artery and vein 3. The common carotid arteries and internal jugular veins (located in the carotid sheath) LYMPHATICS 1. Thoracic and right lymphatic ducts

ESOPHAGUS The anterior scalene muscle can be used as a reference point to locate major neural and vascular structures in this region. This muscle consists of numerous muscle fibers that originate from the posterior tubercles of the fourth through sixth cervical vertebra. This muscle has an oblique orientation. It extends inferiorly from a superior medial to an inferior lateral position to attach to the superior aspect of the first rib posterior to the subclavian groove. The subclavian vein is located anterior to the anterior scalene muscle and the second portion of the subclavian artery posterior to it. The subclavian artery is divided into three parts based on its location in reference to the anterior scalene muscle. The first portion of the artery crosses the apex of the lung superficial to the subpleural membrane and is found medial to the anterior scalene muscle. Posterior to the anterior scalene muscle is the second portion and lateral to it the third portion. Components of the brachial plexus are found posterior or superior to all portions of the subclavian artery. Some of the components of the cervical sympathetic chain are also located posterior to the subclavian artery. The phrenic nerve, vagus nerve, and ansa subclavia cross the thoracic inlet adjacent to the anterior borders of the first portion of the subclavian artery (Fig 1). The phrenic nerve is located laterally and the vagus nerve medially as they cross the thoracic inlet. 1-5 A knowledge of the normal anatomy in this From the The Long Island College Hospital, SUNY Heahh Science Center at Brooklyn, and New York University School of Medicine, Brooklyn, IVY. Address reprint requests to Deborah L. Reede, MD, Department of Radiology, The Long Island College Hospital, 340 Henry St, Brooklyn, N Y l l201. Copyright © 1996 by W..B. Saunders Company 0887-2171 / 97/1706-000255. 00/0

Seminars in Ultrasound, CT, and MRI, Vol 17, No 6 (December), 1996: pp 509-518

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Fig 1. Thoracic inlet. The major neural structures crossing the thoracic inlet are shown.

area is imperative when interpreting crosssectional images in this area. The neural structures that are found in the lower neck and thoracic inlet are not routinely visualized on cross-sectional imaging. A basic knowledge of the normal location of these structures as outlined in this article will enable one to predict which nerves could be involved by lesions in this area. VAGUS NERVE An understanding of the course of the vagus nerve is essential when evaluating patients with multiple lower cranial nerve palsies and vocal cord paralysis. This nerve has the longest course of all of the cranial nerves. Portions of this nerve can be found from its point of origin in the brain stem to the abdomen. The motor component of the nerve originates from four nuclei in the medulla. Nerve fibers from three of these nuclei, the dorsal vagal nucleus, nucleus soli-

tarius, and the spinal nucleus of the trigeminal nerve, emerge as rootlets from the medulla, below the glossopharyngeal nerve in the retroolivary sulcus between the olive and the inferior cerebellar peduncle. These rootlets join in the basal cistern to form a single nerve that exists from the skull through the jugular foramen. Motor fibers from the nucleus ambiguus join the vagus nerve below the jugular foramen. The fibers from the nucleus ambiguus travel with the spinal accessory nerve for a short distance before they join the vagus nerve. The two sensory ganglia of the vagus nerve, the superior (jugular) and inferior (nodosum), are found in the jugular foramen. A portion of the inferior (nodosum) ganglia extends below the jugular foramen. Once it enters the neck, the vagus nerve travels in the posterior aspect of the carotid sheath between the carotid artery (medially) and the internal jugular vein (laterally) (Fig 2).

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Recurrent laryngeal nerve

Fig 2. Cross-sectional diagram showing the location of the major neural structures in the neck, (Reprinted with permission. s)

There are three major branches of the vagus nerve in the neck, the pharyngeal branch, superior laryngeal nerve, and the recurrent laryngeal nerve. The pharyngeal branch is formed on the superior aspect of the inferior sensory ganglion of the vagus nerve. This nerve consists primarily of fibers from the spinal accessory nerve. It travels between the internal and external carotid arteries to the superior border of the

middle constrictor muscle. Here it divides into numerous branches that are joined by branches of the sympathetic trunk, glossopharyngeal nerve, and external laryngeal nerves to form the pharyngeal plexus. This plexus is the primary motor supply to the muscles of the pharynx and soft palate (except for the tensor veli palatine and the stylopharyngeus muscle). The superior laryngeal nerve originates from the inferior aspect of the inferior sensory gan-

Fig 3. Recurrent laryngeal nerve--tracheoesophageal sulcus. Axial magnetic resonance T1-weighted image shows the left recurrent laryngeal nerve in the tracheoesophageal sulcus (arrow).

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Fig 4. Phrenic nerve. Axial CT (A) and Tl-weighted MRI (B) demonstrate the phrenic nerve (arrow) anterior to the anterior scalene muscle (A}.

glion of the vagus nerve. This nerve contains sympathetic fibers from the superior cervical ganglion of the sympathetic trunk. It descends adjacent to the pharynx and at the level of the hyoid bone divides into the external laryngeal (motor) and internal laryngeal (sensory) nerves. Both nerves traverse the thyrohyoid membrane of the larynx. The external laryngeal nerve sends motor fibers to the inferior constrictor and cricothyroid muscles. This nerve also has branches that join the pharyngeal plexus and the superior cardiac nerves. The internal laryn-

geal nerve is the sensory supply to the mucous membranes of the hypopharynx and larynx. The recurrent laryngeal nerve is a portion of the vagus nerve that can be found in the neck, thoracic inlet, and superior mediastinum. The point of origin of the recurrent laryngeal nerves (right and left) differ. On the right, the recurrent laryngeal nerve arises from the vagus nerve after it crosses the anterior aspect of subclavian artery at the level of the thoracic inlet. The right recurrent laryngeal nerve loops around the artery and then travels superiorly in the trache-

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Fig 5. Left phrenic nerve-superior mediastinum. Axial CT (A) and Tl-weighted MRI (B) show the left phrenic nerve (straight arrow) between the left common carotid artery (C) and the left subclavian artery (S). Note the location of the vagus nerve (small curved arrow) and recurrent laryngeal nerves (large curved arrow) at this level,

oesophageal sulcus (Figs 2 and 3) en route to the larynx where it terminates at the level of the cricoarytenoid junction. On the left side the recurrent laryngeal nerve arises from the vagus nerve in the mediastinum after it crosses the aortic arch. It loops under the aortic arch where it travels between the aorta and the left pulmonary artery (aortic pulmonic window). The nerve then travels posteriorly to the tracheoesophageal sulcus and once there follows the same

course as the right recurrent laryngeal nerve. Because the left recurrent laryngeal nerve has a longer intrathoracic course, it is more susceptible to damage from mediastinal pathology than the right recurrent laryngeal nerve. 2-6 PHRENIC NERVE

This nerve is formed primarily from the C4 nerve root with minor contributions from C3 and C5. Superiorly it is found adjacent to the

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artery superior lateral border of the anterior scalene muscle. It has an oblique vertical course that crosses the anterior scalene muscle deep to the cervical fascia, from a superolateral to an inferior medial position (Fig 1). Inferiorly the phrenic nerve passes medial to the muscle and thus comes to lie on the anterior surface of the first portion of the subclavian artery at the level of the thoracic inlet (Figs 4A and B). Once in the mediastinum, both phrenic nerves are in contact with the mediastinal pleura laterally. After they enter the mediastinum, the length, vertical orientation, and medial relationships of these two nerves differ. The right phrenic nerve is shorter and more vertically oriented than the left. Medially throughout its course, this nerve is found adjacent to venous structures (right brachiocephalic vein, superior vena cava, right atrium, and inferior vena cava). This nerve crosses the diaphragm either with the inferior vena cava or lateral to it. The course of the left phrenic nerve is not as vertically oriented as the right, therefore its course is more complicated. The left phrenic nerve is found between the left common carotid and the subclavian arteries in the superior mediastinum (Figs 5A and B). It then passes medially and anteriorly superficial to the left vagus nerve above the level of the aortic arch. It crosses the aortic arch superficial to the superior intercostal vein and then passes laterally and inferiorly over the pericardium and left ventricle. This nerve passes through the muscular portion of the diaphragm anterior to the

Fig 6. Branchial plexus. Diagram shows the brachial plexus and its relationship to the anterior scalene muscle, Subclavian artery and axillary artery. (Reprinted with permission. 8)

central tendon adjacent to the left side of the heart.2-4,7 The phrenic nerve is usually the only motor supply to the diaphragm. Occasionally there may be an accessory phrenic nerve that is derived from C4, C5, or the ansa subclavia. Clinically this is important because patients with accessory phrenic nerves may not develop a complete paralysis of the diaphragm. BRACHIAL PLEXUS

The brachial plexus is formed by the anterior nerve roots of C5-T1. Occasionally there are minor contributions from C4 and T2. These roots join to form trunks, divisions, cords, and subsequently peripheral nerves (Fig 6). Direct visualization of the individual components of the brachial plexus is achieved more often with MR than with CT. However, because the components of the brachial plexus are not routinely visualized on either of these imaging modalities, one needs to be familiar with the location of these nerves when cross-sectional imaging is used to evaluate patients with brachial plexopathies. The superior nerve roots that form the brachial plexus are located anterior to the inferior nerve roots because of the lordotic curvature of the cervical spine. All of the nerve roots that form the brachial plexus are found posterior to the anterior scalene muscle (Figs 7A and B). The C5 and C6 nerve roots travel between the anterior and middle scalene muscles and unite

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Fig 7. Brachial plexus nerve roots. Axial CT (A) and T1weighted MRI (B) obtained at the level of the thyroid shows the cervical nerve roots (arrow) passing between the anterior (A) and middle (M) scalene muscles,

adjacent to the lateral border of the anterior scalene muscles to form the upper trunk of the brachial plexus. The C7 nerve root forms the middle trunk. In the lower neck, the C8 and T1 nerves join behind the subclavian artery to form the lower trunk (Figs 8A and B). Each trunk has an anterior and posterior division that is formed in the supraclavicular region. These nerves are found slightly above (superior and middle trunk) and directly posterior (inferior trunk) to the subclavian artery. The divisions of the superior

and middle trunk are formed lateral to the anterior scalene muscle. The division of the lower trunk is formed at or after it crosses the first rib (behind the clavicle or in the axilla). There are three cords that are formed from the divisions. These cords travel between the clavicle and the first rib to enter the axilla. Here they are located in the axillary sheath adjacent to the axillary artery and are named based on their location in reference to this vessel (lateral, median, and posterior) (Fig 9). The musculocu-

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Fig 8. Lower trunk of the brachial plexus. Axial CT (A) and Tl-weighted MRI (B) obtained at the level of the thoracic inlet demonstrate the T1 nerve root (arrow) passing medial to the lung apex and joining the C8 nerve root posterior to the subclavian artery (SA). Note the anterior scalene muscle (A) anterior to the subclavian artery,

taneous, median, and ulnar nerves are the peripheral nerves formed from the cords in the axilla. 14 CERVICAL SYMPATHETIC TRUNK

The cervical sympathetic trunk traverses the neck from the base of the skull to just below the first rib, where it is continuous with the thoracic portion of the sympathetic trunk (Fig 1). Therefore, components of the cervical sympathetic trunk can be found above, below, and at the

level of the thoracic inlet. The cervical sympathetic trunk consists of three ganglion (superior, middle, and inferior) that are interconnected by sympathetic fibers. As with other nerves in the neck, they are usually not visualized on crosssectional images. However, a knowledge of their typical location will enable one to predict possible involvement. Components of the cervical sympathetic trunk are located posterior to the carotid sheath and anterior to the prevertebral muscles (Fig 2).

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Fig 9. Brachial plexus--axillary region. Tl-weighted sagittal MRI shows multiple nerve roots adjacent to the axillary artery

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stellate ganglion. This ganglion is found between the transverse process of C7 and the first rib posterior to the vertebral artery. Directly below this ganglion is the apex of the lung. Two or more sympathetic branches connect the middle and inferior (or stellate) ganglion. The middle cervical ganglion is located anterior to the vertebral artery and the inferior or stellate ganglion is located posterior to the artery (Fig 10). Sympathetic branches pass medial and lateral to the vertebral artery to connect these ganglion. The ansa subclavia, a branch

The superior cervical ganglion is the largest of the cervical ganglion. It is found at the level of C2 and C3 vertebral body posterior to the carotid sheath at the level of the carotid bifurcation. It connects via sympathetic fibers traveling anterior to the prevertebral muscles with the middle cervical ganglion. This ganglion is the smallest of the cervical ganglion and is located at the level of the C6 vertebra anterior or superior to the inferior thyroid artery. The inferior cervical ganglion in 80% of patients is fused with the first thoracic ganglion to form the

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of the sympathetic trunk that originates in the inferior cervical ganglion, loops under the subclavian artery from a posterior to an anterior position. It then travels superiorly to connect

with the middle cervical ganglion. Below the thoracic inlet the cervical sympathetic trunk communicates with the thoracic portion of the sympathetic system.3,4,7

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5. Montgomery RL (ed): Head and Neck Atlas with Clinical Correlations. New York, NY, McGraw-Hill, 1981 6. Carpenter MB, Sutin J: Human Neuroanatomy (ed 8). Baltimore, MD, Williams & Wilkins, 1983, pp 342-350 7. Berkovitz BKB, Moxham BJ: A Textbook of Head and Neck Anatomy. London, Yearbook/Wolfe, 1988 8. Sore PM, Curtin HD (eds): Head and Neck Imaging (ed 3). St. Louis, MO, Mosby-Yearbook, 1996