Anatomic Features

The mediastinum is bounded superiorly by bony structures of the thoracic inlet and inferiorly by the diaphragm. Laterally, the mediastinal pleura on each side serves as a membrane separating the medial aspect of the lung (with its visceral pleura) from the structures contained within the mediastinum. The mediastinum most frequently is divided into three anatomic compartments: anterior, middle, and posterior (Table 16-1). This division is particularly useful for characterizing mediastinal masses because specific etiologic factors often have a predilection for a particular compartment. Normal structures located within or coursing through each of the compartments may serve as the origin of a mediastinal mass. Consequently, knowledge of the structures contained in each of the three compartments is important for the clinician in evaluating a patient with a mediastinal mass.

The borders of the three mediastinal compartments are visualized most easily on a lateral chest radiograph (Fig. 16-1). Several descriptions exist for the limits defining each compartment. According to the scheme used here, the anterior mediastinum extends from the sternum to the anterior border of the pericardium. Included within this region are the thymus, lymph nodes, and loose connective tissue.

The borders of the middle mediastinum are the anterior and posterior pericardium. This region includes the heart, pericardium, great vessels, trachea, lymph nodes, and phrenic nerves. The upper portion of the vagus nerve also courses through the middle mediastinum.

The posterior mediastinum extends from the posterior pericardium to the posterior chest wall. This compartment normally includes the vertebral column, neural structures (including the chain of sympathetic nerves and lower portion of the vagus nerves), esophagus, and descending aorta. Some lymph nodes and loose connective tissue may also be found in the posterior mediastinum.

Mediastinal Masses