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

Etiology

Because of the predilection for certain types of masses to occur in specific mediastinal compartments, it is easiest to separately consider masses occurring in each of the three anatomic regions. However, a fair amount of overlap occurs; that is, many types of mediastinal masses are not exclusively limited to the one compartment where they most frequently appear. A summary of the types of mediastinal masses, arranged by anatomic compartment, is given in Table 16-1.

Anterior Mediastinal Masses

The major types of anterior mediastinal mass are thymoma, germ cell tumor, lymphoma, thyroid gland enlargement, and miscellaneous other tumors.

Thymomas, or tumors of the epithelium of the thymus gland, are the most common type of neoplasm originating in the anterior compartment. They may be benign or malignant in behavior, depending more on whether they exhibit local invasion than on any particular morphologic features. Thymomas are diagnosed most commonly in patients between the ages of 40 and 60 years and are found equally in men and woman. These tumors are notable for their association with a variety of systemic paraneoplastic syndromes. The best known and most common of these is myasthenia gravis, which is found in 10% to 50% of patients with thymic tumors. Myasthenia gravis is characterized clinically by muscle fatigue and weakness and pathophysiologically by a decreased number of acetylcholine receptors at neuromuscular junctions. The latter is due to antibodies against the acetylcholine receptor. Other systemic syndromes associated with thymoma include pure red blood cell aplasia, hypogammaglobulinemia, and several conditions that appear to have an autoimmune origin, such as systemic lupus erythematosus and polymyositis.

Myasthenia gravis and other paraneoplastic syndromes occur frequently in patients with thymoma.

Germ cell tumors are believed to originate from primitive germ cells that probably underwent abnormal migration during an early developmental period. Several types of germ cell tumors have been described. The most common is the teratoma, a tumor composed of ectodermal, mesodermal, and endodermal derivatives. The types of tissue seen are clearly foreign to the area from which the tumor arose and may include elements such as skin, hair, cartilage, and bone. Like thymomas, these tumors may be benign or malignant, with approximately 80% described as benign. Other, less common, germ cell tumors include seminomas and choriocarcinomas.

Lymphomas may involve the mediastinum, either as part of a disseminated process, in which the mediastinum is only one locus of the disease, or as primary mediastinal masses without other clinically apparent areas of involvement. Hodgkin lymphoma, particularly the nodular sclerosis subtype, is well described as manifesting solely as a mediastinal mass, although non-Hodgkin lymphoma may have a similar presentation. Like carcinoma, lymphoma involving the mediastinum is most common in the anterior or middle mediastinal compartment.

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