The mediastinum is anatomically divided into two parts by an imaginary line that runs from the sternal angle (formed by the junction of the sternal body and manubrium) to the T4 vertebrae. The superior mediastinum extends upwards, terminating at the superior thoracic aperture, and the inferior mediastinum extends downwards, terminating at the diaphragm. The inferior mediastinum is further subdivided into the anterior mediastinum, middle mediastinum and posterior mediastinum. In this article, we shall look at the anatomy of the middle mediastinum – its borders, contents and clinical correlations.
Borders
The middle mediastinum is bordered by the following structures-
Contents
The middle mediastinum is the largest subdivision of the inferior mediastinum, and contains a number of important organs, vessels, nerves, and lymphatic structures.
Organs
The middle mediastinum contains the heart, and its protective sheath, the pericardium. It also houses the tracheal bifurcation and the left and right main bronchi.
Vessels
The middle mediastinum is associated with the origins of the great vessels that run to and from the heart. The ascending aorta - the first part of the aorta which arises from the aortic orifice - moves upwards, exiting the fibrous pericardium and entering the superior mediastinum. From there, the ascending aorta becomes the aortic arch and gives rise to two branches; the left and right coronary arteries. The pulmonary trunk - giving rise to the left and right pulmonary arteries - is also present, and is relatively short and wide, allowing a large volume of blood to pass through it. The superior vena cava - returning deoxygenated blood from the upper half of the body - is formed by the right and left brachiocephalic veins.
Nerves
The middle mediastinum also contains the cardiac plexus and the phrenic nerves. The cardiac plexus is a network of nerves located at the base of the heart, containing sympathetic and parasympathetic fibres. The sympathetic nerves are derived from the T1-T4 segments of the spinal cord, and the parasympathetic innervation is supplied by the vagus nerve. This plexus can be subdivided into superficial and deep components. The phrenic nerves (left and right) are mixed nerves that provide motor innervation to the diaphragm. They arise in the neck and descend through the middle mediastinum to reach the diaphragm.
Lymphatics
The tracheobronchial lymph nodes are located within the middle mediastinum. They form a group of nodes associated with the trachea and bronchi of the respiratory tract, and can be characteristically enlarged in certain lung pathologies. This group of nodes is formed from the gathering of bronchial nodes within the hila of the lungs, and are connected by fine lymphatic channels.
The Middle Mediastinum is bounded laterally by the pleura and posteriorly by the spine. Anteriorly it is bounded by the sternum and the diaphragm. This section of the body is an essential part of understanding and managing a variety of clinical conditions, and its contents can be divided into three groups – organs, vessels, and nerves and lymphatics.
The organs of the middle mediastinum include the thymus, trachea, esophagus, and the heart and its great vessels, and lymph nodes. The trachea in particular also contains the tracheal bifurcation and the left and right main bronchi.
Vascular structures which traverse the middle mediastinum include the main pulmonary artery and vein, superior and inferior vena cava, and azygos vein. Of particular importance is the ascending aorta, which is the first part of the aorta. This ascends upwards, exiting the fibrous pericardium and entering the superior mediastinum, where it then becomes the aortic arch. The main pulmonary trunk, meanwhile, gives rise to the left and right pulmonary arteries. The superior vena cava, which is formed by the right and left brachiocephalic veins, returns deoxygenated blood from the upper half of the body.
Nerves are also located in the middle mediastinum. The cardiac plexus is a network of nerves located at the base of the heart, containing sympathetic and parasympathetic fibres. These nerves are derived from the T1-T4 segments of the spinal cord, and the parasympathetic innervation is supplied by the vagus nerve. The plexus can be subdivided into superficial and deep components. The phrenic nerves, meanwhile, are mixed nerves that provide motor innervation to the diaphragm. They arise in the neck and then descend through the middle mediastinum to reach the diaphragm.
The lymphatics of the middle mediastinum can be divided into those associated with the trachea, thymus, esophagus, heart, and great vessels. Of particular importance are the tracheobronchial lymph nodes, which are a group of nodes associated with the trachea and bronchi of the respiratory tract, and are characteristically enlarged in certain lung pathologies. They form from the gathering of bronchial nodes within the hila of the lungs. Individual groups of nodes are connected via fine lymphatic channels.
The contents of the middle mediastinum are essential for proper understanding and management of a variety of medical conditions, and thus this region should be given special consideration. Differentiating between the myriad organs, vessels, nerves and lymphatics within the mediastinum can be a daunting task, but it is necessary for providing the best possible care.
