The boundaries defining the mediastinum, as well as its subdivisions or compartments, and important anatomical considerations regarding mediastinal lymphatics are discussed below.
The mediastinum is defined as the potential space between the two pleural cavities bounded by the sternum anteriorly, the vertebral column posteriorly, the thoracic inlet superiorly, and the diaphragm inferiorly (Fig. 80-1).1 The major mediastinal structures are the heart and great vessels, the trachea and main bronchi, and the esophagus, all closely related to one another and connected by loose connective tissue. Also present are the thymus, lymph nodes, and fat. Hence, air or infection can disseminate widely throughout the mediastinal space, contained laterally only by the mediastinal pleural reflections. The mediastinum communicates with both the neck and the retroperitoneum, and these portals can also serve as routes of egress from the mediastinum. Fascial planes connect the neck, mediastinum, and retroperitoneum and thus facilitate movement of air or infection from one location to another.
A. Lateral view of the mediastinum as seen through a right thoracotomy. B. Lateral view of the mediastinum as seen through a left thoracotomy. (Reproduced with permission from LoCicero J. Median sternotomy and thoracotomy. In: Shields TW, ed. Mediastinal Surgery. Philadelphia, PA. Lea & Febiger; 1991.)
Several subdivisions of the mediastinum have been emphasized in the surgical and radiologic literature but there is no consensus. Most often, three compartments are proposed: anterior, middle (visceral), and posterior (paravertebral sulcus) (Fig. 80-2).2 The boundaries of these divisions are not agreed upon, further emphasizing their nonanatomic origins. Shields proposed a simple three-compartment subdivision in 1972 that makes both anatomic and surgical sense. The anterior compartment is bounded by the sternum and the anterior surface of the pericardium and great vessels. The middle (visceral) compartment extends from the posterior limit of the anterior compartment to the anterior surface of the vertebral columns and then to the thoracic inlet. The posterior compartment (paravertebral sulcus) extends from the anterior surface of the vertebral column to the anterior surface of the paravertebral ribs. The structures in these compartments are listed in (Table 80-1). The pericardial sac is the only true compartment of the mediastinum and it provides a strong barrier to infection. Subdividing the mediastinum into compartments proves most helpful when one is interpreting a plain radiograph that shows a mediastinal mass. Knowledge of the contents of the involved compartment facilitates arriving at a proper diagnosis.
Mediastinal tumors are divided for the convenience of diagnosis and surgical approach into three compartments: anterior, middle, and posterior. (Used with permission of Marcia Williams.)
Table 80-1Contents of Mediastinal Compartments