Definition and boundaries of the mediastinum
Mediastinum is a complex of organs and anatomical structures located in the thoracic cavity between the right and left pleural cavities surrounding the right and left lungs. Laterally, the mediastinum is bounded by the mediastinal pleura, anteriorly by the sternum, posteriorly by the thoracic vertebral column, inferiorly by the diaphragm, and superiorly it communicates with the neck region through the superior thoracic aperture.
The mediastinum includes: the trachea and main bronchi, thymus (in children) or its replacement adipose tissue (in adults), azygos and hemiazygous veins draining into the superior vena cava system, the thoracic aorta, esophagus, thoracic spine, sympathetic trunks, vagus and phrenic nerves, thoracic duct and regional lymph nodes, as well as the heart with the pericardium and great vessels.
Division into anterior and posterior mediastinum
A conventional coronal plane is drawn along the posterior surface of the pericardium and the anterior surface of the trachea and main bronchi. This plane divides the mediastinum into two major compartments: anterior and posterior.
- Anterior mediastinum contains the heart with the pericardium, thymus (or retrosternal adipose tissue), phrenic nerves, internal thoracic arteries and veins, parasternal and anterior mediastinal lymph nodes.
- Posterior mediastinum contains the trachea and main bronchi, thoracic aorta, esophagus, thoracic duct, azygos and hemiazygous veins, vagus nerves, sympathetic trunks and splanchnic nerves, adjacent part of the vertebral column, and posterior mediastinal lymph nodes.
Superior mediastinum
A horizontal plane drawn through the manubriosternal junction (Angle of Louis) and the intervertebral disc between the Th4–Th5 bodies separates the superior mediastinum from the inferior.
The superior mediastinum contains: the thymus (or its replacement tissue), the superior part of the esophagus and trachea, the aortic arch with its branches (brachiocephalic trunk, left common carotid and left subclavian arteries), brachiocephalic veins and the initial part of the superior vena cava, thoracic duct, vagus, phrenic, and recurrent laryngeal nerves, upper tracheobronchial and paratracheal lymph nodes.
Inferior mediastinum
The inferior mediastinum is divided into three compartments: anterior, middle, and posterior.
- Anterior inferior mediastinum is bounded anteriorly by the body of the sternum and posteriorly by the anterior surface of the pericardium. It contains internal thoracic vessels, parasternal lymph nodes, and retrosternal tissue.
- Middle mediastinum corresponds to the heart with the pericardium and great vessels: ascending aorta, pulmonary trunk with the right and left pulmonary arteries, pulmonary veins, and the terminal part of the superior vena cava. Phrenic nerves and pericardiacophrenic vessels also pass here, and bifurcation lymph nodes are located here.
- Posterior inferior mediastinum is bounded anteriorly by the posterior wall of the pericardium and posteriorly by the anterior surface of the thoracic vertebral bodies. It contains the thoracic descending aorta, esophagus with vagus nerves, thoracic duct, azygos and hemiazygous veins, sympathetic trunks, and greater and lesser splanchnic nerves.
Clinical and topographic significance
The fascial spaces of the mediastinum communicate with the tissue spaces of the neck and retroperitoneal space, creating pathways for the spread of purulent processes (descending mediastinitis). Knowledge of the projections of major vessels and nerves is essential for pericardiocentesis, central venous catheterization, and surgeries on the esophagus, aorta, trachea, and mediastinal lymph nodes. Compartmental division is used in diagnostic imaging for the topical localization of tumors and lymphadenopathies.
Key points
- The mediastinum is a complex of organs between the pleural cavities, bounded by the sternum, spine, and diaphragm.
- The coronal plane along the anterior surface of the trachea divides the mediastinum into anterior and posterior parts.
- The horizontal line through the sternal angle and the Th4–Th5 disc separates the superior mediastinum from the inferior.
- The superior mediastinum contains the aortic arch with its branches, brachiocephalic veins, thymus, trachea, esophagus, and thoracic duct.
- The inferior mediastinum is divided into anterior, middle, and posterior compartments.
- The middle mediastinum contains the heart with the pericardium, ascending aorta, pulmonary trunk, pulmonary veins, and phrenic nerves.
- The posterior mediastinum contains the descending aorta, esophagus, thoracic duct, azygos and hemiazygous veins, vagus nerves, and sympathetic trunks.
- The fascial spaces of the mediastinum serve as pathways for the spread of infection and hold major surgical significance.