16: Mediastinal disease

OUTLINE

Anatomic Features, 207

Mediastinal Masses, 208

Etiology, 208

Clinical Features, 210

Diagnostic Approach, 210

Treatment, 211

Pneumomediastinum, 212

Etiology and Pathogenesis, 212

Pathophysiology, 212

Clinical Features, 212

Diagnostic Approach, 213

Treatment, 213

The mediastinum is the region of the thoracic cavity located between the two lungs. Included within the mediastinum are numerous structures, ranging from the heart and great vessels (aorta, superior and inferior venae cavae) to lymph nodes and nerves. The physician dealing with diseases of the lung is confronted with mediastinal disease in two main ways: (1) an imaging study (chest radiograph or computed tomography [CT]) shows an abnormal mediastinum or (2) the patient has symptoms similar to those originating from primary pulmonary disease. This chapter describes some of the anatomic features of the mediastinum and discusses two of its most common clinical problems: mediastinal masses and pneumomediastinum.

Anatomic features

The mediastinum is bounded superiorly by bony structures of the thoracic inlet (specifically the manubrium, first ribs, and first thoracic vertebra) 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

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 provided 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 specific morphologic features. Thymomas are diagnosed most commonly in patients between 40 and 60 years of age and have a similar incidence among men and women. 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 abnormally rapid muscle fatigue and weakness and pathophysiologically by a decrease in functional acetylcholine receptors at neuromuscular junctions caused by autoantibodies against the acetylcholine receptor. Other systemic syndromes associated with thymoma include pure red blood cell aplasia, hypogammaglobulinemia, and thymoma-associated multiorgan autoimmunity, which is similar to graft-versus-host disease and characterized by skin rash, enterocolitis, and hepatitis.

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

Germ cell tumors are believed to originate from primitive germ cells that 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 but more reliably malignant mediastinal 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. Lymphoma involving the mediastinum is most common in either the anterior or the middle mediastinal compartment.

Lymphoma and carcinoma commonly affect anterior or middle mediastinal compartments. Malignant mediastinal disease may be isolated or part of more widespread involvement.

The thyroid gland may be the origin of a mediastinal mass as a result of extension of thyroid tissue from its normal location in the neck into the mediastinum. Because these masses are typically not functional, patients do not have clinical or laboratory evidence of hyperthyroidism. Only rarely do these masses of thyroid origin prove to be malignant.

Other tumors, including carcinomas, may produce a mediastinal mass. In many cases, mediastinal involvement is secondary to a primary neoplasm found elsewhere, particularly in the lung. In occasional

cases, no other tumor is apparent, and patients are believed to have a primary carcinoma originating in the

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mediastinum. Carcinomatous involvement of the mediastinum is not limited to the anterior mediastinum but is also common in the middle mediastinal compartment.

A variety of less common neoplasms may occur in the anterior mediastinum, including parathyroid tumors and tumors of fatty or connective tissue origin. Given the infrequency of these tumors, they are not discussed in this book.

Middle mediastinal masses

Carcinomas and lymphomas may be found in the middle mediastinum, as mentioned in the discussion of anterior mediastinal masses. In addition, the middle mediastinum is frequently the location of benign cysts originating from structures found within this region. For example, fluid-filled pericardial and bronchogenic cysts originate from the embryonic formation of the pericardium and tracheobronchial tree, respectively. However, these cysts are generally self-contained and usually do not directly communicate with either the pericardium or airways. Nonmalignant enlargement of lymph nodes in the middle mediastinum, often in the hilar regions, is commonly found in granulomatous diseases such as sarcoidosis, histoplasmosis, and tuberculosis.

Posterior mediastinal masses

The posterior mediastinum is characteristically the location of tumors of neurogenic origin. These tumors may arise from a variety of nerve elements found in peripheral nerves, the sympathetic nervous system chain, or paraganglionic tissue. Examples include neurilemomas (arising from the Schwann sheath), ganglioneuromas and neuroblastomas (benign and malignant lesions arising from the sympathetic nervous system, respectively), and pheochromocytomas. Diaphragmatic hernias, either congenital or acquired, frequently are posterior, with the herniated intraabdominal organ appearing as a posterior mediastinal mass.

Clinical features

Almost one-half of patients with a mediastinal mass have no symptoms, and the mass is first detected on incidentally performed chest imaging. In patients who develop symptoms, the most common are chest pain, cough, and dyspnea. Occasionally, evidence is seen of esophageal or superior vena caval compression, leading to difficulty swallowing (dysphagia) or to facial and upper extremity edema attributable to impairment of venous return (superior vena cava syndrome). Thymic tumors may manifest with one of the associated paraneoplastic syndromes described previously such as muscle weakness (from myasthenia gravis) or anemia (from pure red cell aplasia). A variety of systemic symptoms may be related to the presence of a lymphoma or other malignancy or to hormone production by hormonally active mediastinal tumors.

Diagnostic approach

In almost all cases, a mediastinal mass is initially identified by either a posteroanterior and lateral chest radiograph or a chest CT scan. In addition to showing the mass, these studies allow determination of its location within the mediastinum (Fig. 16.2). When the mass is first detected on chest radiograph, a chest CT scan is then performed for further characterization (Fig. 16.3). A contrast-enhanced CT scan is particularly useful for defining the cross-sectional appearance of the lesion, its density, and its relationship to other structures within the mediastinum (Fig. 16.4).

FIGURE 16.3 Chest computed tomography scan of a patient with a bronchogenic

cyst appearing as a mass in the middle and posterior mediastinum (arrow).

Source: (Courtesy of Dr. Paul Stark.)