Anatomy, Thorax, Mediastinum

The thoracic mediastinum is the compartment that runs the length of the thoracic cavity between the pleural sacs of the lungs. This compartment extends longitudinally from the thoracic inlet to the superior surface of the diaphragm. Although there are no physical barriers between compartments other than the pericardium, the mediastinum is typically discussed based on subdivisions. The four-compartment model divides the mediastinum into the superior, anterior, middle, and posterior portions. The mediastinum houses many vital structures including the heart, great vessels, trachea, and essential nerves. It also functions as a protected pathway for structures traversing from the neck, superiorly, and into the abdomen, inferiorly. The mediastinum is also clinically significant due to the variety of physical anomalies and pathologies that can occur in this region.

The mediastinum is divided compartmentally and consists of subdivisions that house and support vital structures within the thorax. Below are the borders of each region of the mediastinum.

• Superior Mediastinum: bordered by the thoracic outlet superiorly, transverse thoracic plane (the plane of Ludwig) or sternal angle inferiorly, medial border of the pleural sacs laterally, dorsal surface of the sternum anteriorly, and ventral surface of the first four thoracic vertebral bodies posteriorly[1][2][3]
• Anterior Mediastinum: bordered by the pericardium posteriorly, medial border of the pleural sacs laterally, and the sternum, transversus thoracis muscles, and fifth, sixth, and seventh left costal cartilages anteriorly[1][2][4][5]
• Middle Mediastinum: formed by the borders of the pericardial sac anteriorly and posteriorly, reflected to the medial borders of the pleural sacs bilaterally, transverse thoracic plane superiorly, and thoracic surface of the diaphragm inferiorly[1][2][5][6][7]
• Posterior Mediastinum: bordered by the pericardium anteriorly, the thoracic surface of the diaphragm inferiorly, the transverse thoracic plane superiorly, the bodies of the fifth to the twelfth thoracic vertebrae posteriorly, and the pleural sacs laterally[1][2][5]

Each region of the thoracic mediastinum contains unique structures. Listed below are the different regions of the thoracic mediastinum and a high-level overview of their components.

• Superior Mediastinum:
• Organs: thymus, trachea, esophagus
• Arteries: aortic arch, brachiocephalic trunk, left common carotid artery, left subclavian artery
• Veins and lymphatics: superior vena cava, brachiocephalic veins, the arch of the azygos, thoracic duct
• Nerves: left and right vagus, recurrent laryngeal, cardiac, left and right phrenic nerves[1][2][8][3]
• Anterior Mediastinum:
• Organs: thymus
• Arteries: internal thoracic branches
• Veins and lymphatics: internal thoracic branches, parasternal lymph nodes
• Nerves: none[1][2][8][4][5]
• Middle Mediastinum:
• Organs: the heart and its great vessel roots, trachea and main bronchi
• Arteries: ascending aorta, pulmonary trunk, pericardiacophrenic arteries
• Veins and lymphatics: superior vena cava, pulmonary veins, pericardiacophrenic veins
• Nerves: phrenic, vagus, sympathetics[1][2][8][9][5][6][7]
• Posterior Mediastinum:
• Organs: esophagus
• Arteries: descending thoracic aorta
• Veins and lymphatics: azygos hemiazygos veins, thoracic duct
• Nerves: vagus, splanchnic, sympathetic chain[1][2][8][5]

Each region of the mediastinum can be considered to serve a primary function.

The superior mediastinum is essentially a conduit space allowing structures to pass between the head, neck, and thorax.[1][2][3] The anterior mediastinum is protective in nature and filled with connective and fatty tissue that cushions and supports the thymus as well as the vital cardiac structures just posterior to it.[2][5] The middle mediastinum houses the heart and the roots of the great vessels.[2][5][6][7] The posterior mediastinum, which can be thought of as a continuation of the superior mediastinum, also serves as a conduit. It provides space for the passage of structures between the thoracic and abdominal cavities.[2][5]

The thoracic mediastinum houses the heart and the great vessels. Due to this anatomical configuration, the thoracic mediastinum has many blood vessels traveling through it.[1][2][5][6][7] Since the lymphatic system closely integrates into the cardiovascular system, there is also a significant presence of lymphatics in this area.[1][2][5][10]

The superior mediastinum contains the arch of the aorta and its three associated major branches: the brachiocephalic trunk, the left common carotid, and the left subclavian arteries.[1][2][5] In addition to these large arteries, some smaller branches of the aorta are present within the superior mediastinum including the thymic branches of the internal thoracic arteries, the proximal portions of the pericardiophrenic arteries, and the third and fourth posterior intercostal arteries.[11][9][3] The majority of lymphatic drainage in this region coalesces at the thoracic duct, which empties into the bloodstream at the left subclavian vein.[1][4][5][10]

The internal thoracic arteries descend inferiorly and deep to the lateral borders of the sternum. These arteries exit the anterior thoracic mediastinum inferiorly and branch into the musculophrenic arteries and superior epigastric arteries.[11] Lymphatics in this region include the parasternal, pericardial and superior diaphragmatic lymph nodes.[12][5][6][10]

The middle thoracic mediastinum, bounded by the pericardium, contains the coronary arteries and its associated branches. The lymphatics within the pericardium are a complex network of vessels that penetrate all layers of the cardiac tissue. These vessels ultimately drain posteriorly towards the pre-tracheal lymph nodes that lie between the aorta and the trachea.[12][6][10]

The posterior thoracic mediastinum contains the descending aorta as it courses slightly left of midline down towards the diaphragm. The most notable arterial branches in this region are the proximal portions of the intercostal arteries. The lymphatics of this region include the thoracic duct, retrocardiac lymph nodes, diaphragmatic lymph nodes, posterior mediastinal lymph nodes, and prevertebral lymph nodes.[1][2][5][10]

In the superior mediastinum, there are two broad categories of nerves passing within the region, nerves that originate superior to the thorax and nerves that originate within the thorax.

The superior thoracic mediastinum contains the left and right phrenic nerves (C3 - C5), the left and right vagus nerves, and other associated vagal branches including the esophageal plexus, the inferior cervical cardiac branches, the thoracic cardiac branches, and the recurrent laryngeal nerves. As for the nerves originating from within the thorax, there is the superior portion of the sympathetic trunk (T1 through T4) and its associated sympathetic branches such as the thoracic cardiac and pulmonary branches.[8][13]

The anterior mediastinum does not contain any major named nerves.[2][8][5][8]

The middle mediastinum is densely innervated by the autonomic nervous system and somatically innervated by the phrenic nerves. The sympathetic innervation arises from branches of the sympathetic trunk around the T2 through T4 levels while parasympathetic innervation derives from branches of the left and right vagus nerves. Branches of the left and right phrenic nerves provide somatic innervation to the fibrous and parietal pericardial layers.[14][1][2][8][9][6][13] Because the boundaries of the middle mediastinum are the borders of the pericardium itself, only a small portion of the nerves mentioned truly reside within the middle mediastinum.

The posterior mediastinum contains many autonomic and somatic nerves. Sympathetic nerves arise from the sympathetic chain at the T5 to T12 levels and parasympathetic innervation is provided by the left and right vagus nerves which form of the esophageal plexus. The somatic nerves in the posterior mediastinum are the intercostal nerves.[1][2][8][5]