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.

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 with 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 systemic syndromes, 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, the initial diagnostic test is the chest radiograph, which generally shows the mass and allows determination of its location within the mediastinum (Fig. 16-2). The mass can be further characterized by a variety of other techniques, but CT is generally the most valuable. The 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. With magnetic resonance imaging (MRI), blood vessels can be distinguished from other mediastinal structures without the use of radiographic contrast. Unlike the traditional presentation of CT images as axial views, MRI can display images in coronal and sagittal planes as well as cross-sectional axial views. ¹⁸F-Fluorodeoxyglucose positron emission tomography yields information on tissue metabolism, which is generally increased in active neoplastic or infectious processes.

The definitive diagnosis of the type of mediastinal mass generally depends on examination of tissue by histopathologic techniques. Tissue is frequently obtained either by mediastinoscopy, in which a rigid scope is inserted into the mediastinum via an incision at the suprasternal notch, or by exploration of the mediastinum by a surgical approach that is anterior and adjacent to the sternum (parasternal mediastinotomy). The technique of video-assisted thoracic surgery also has been used to obtain tissue from the mediastinum. In some patients, aspiration or biopsy of the mass by a needle inserted percutaneously may provide sufficient tissue to make a diagnosis. In many cases, the patient undergoes a more extensive procedure that allows biopsy and removal of the mass at the same time.

Treatment

Treatment of the various mediastinal masses depends to a large extent on the nature of the lesion. In many cases, complete removal of the mass by surgery is the preferred procedure if technically feasible. Because benign lesions may enlarge and compress vital mediastinal structures, excision of the mass is frequently indicated. In addition, there may be complicating hemorrhage or infection of a benign lesion and eventually even malignant transformation of an initially benign tumor; these factors also favor removal, if possible, following initial diagnosis.

Treatment of malignant tumors depends on the type of tumor and the presence or absence of invasion of other mediastinal structures. Because surgical removal of malignant lesions often is neither indicated nor possible, chemotherapy and radiotherapy are frequently the primary forms of treatment.

Etiology and Pathogenesis

The three major sources of air entry to the mediastinum are (1) through the skin and chest wall, as occurs commonly in the setting of penetrating trauma; (2) from a tear or defect in the esophagus or the trachea, allowing air to enter the mediastinum directly; and (3) from the alveoli. In the last circumstance, an increase in intraalveolar pressure may induce air entry into interstitial tissues of the alveolar wall. This interstitial air may dissect alongside the wall of blood vessels coursing through the interstitium. After air tracks back proximally, it eventually may enter the mediastinum at the site of origin of the vessels in the mediastinum.

Proximal dissection of extraalveolar air is probably the most common cause of a pneumomediastinum. In some cases, the reason for the increase in intraalveolar pressure is obvious—for example, severe coughing, vomiting, or straining. In patients receiving mechanical ventilation, the positive pressure produced by the ventilator may result in alveolar rupture and a pneumomediastinum. A pneumomediastinum may develop in asthmatic persons, presumably because of the development of high intraalveolar pressure in a lung unit behind an obstructed bronchus. In other circumstances, the immediate cause of the pneumomediastinum is not apparent, and the patient truly has a “spontaneous pneumomediastinum.”

Pathophysiology

With accumulation of air in the mediastinum, an increase in pressure would be expected to cause a decrease in venous return to the great veins, with resulting cardiovascular compromise. However, when pressure builds up within the mediastinum, air usually dissects further along fascial planes into the neck, allowing release of the pressure and preventing disastrous cardiovascular complications. In addition, an increase in mediastinal pressure sometimes results in rupture of the mediastinal pleura and escape of air into the pleural space, with consequent development of a pneumothorax (see Chapter 15).

After air has entered the soft tissues of the neck, the patient is said to have subcutaneous emphysema. With continued entry of air from the mediastinum into the neck, the air dissects further over soft tissues of the chest and abdominal walls, producing more extensive subcutaneous emphysema.

Because of the escape route available for mediastinal air and the opportunity for decompression, major cardiovascular complications are quite uncommon. The development of subcutaneous emphysema, although unsightly and frequently uncomfortable, usually is not associated with major clinical sequelae.

Clinical Features

At the onset, patients with a pneumomediastinum often experience relatively sudden substernal chest pain. They may have dyspnea and (rarely) cardiovascular compromise and hypotension. In some cases, the pneumomediastinum causes no symptoms, and the problem is detected on chest radiograph (e.g., a film obtained during an asthmatic attack).

Physical examination may reveal a crunching or clicking sound synchronous with the heartbeat on cardiac auscultation (Hamman sign). If the patient has subcutaneous emphysema associated with the pneumomediastinum, popping and crackling sounds (crepitations) may be heard and felt on palpation of the affected skin and subcutaneous tissue.

Diagnostic Approach

The chest radiograph and chest CT scan are the most important studies for documenting a pneumomediastinum. Gas may be seen within the mediastinal tissues and usually is visible as one or more radiolucent stripes alongside and parallel to the heart border or aorta (Fig. 16-3).

Treatment

Generally, no treatment is necessary for a pneumomediastinum, even when accompanied by subcutaneous emphysema. The air is usually resorbed spontaneously over time. When a pneumomediastinum is a consequence of tracheobronchial or esophageal rupture, surgery may be necessary to repair the underlying tear. Esophageal rupture can result in a rapidly progressive and fatal bacterial infection of the mediastinum, and urgent surgical intervention must be considered if esophageal perforation is confirmed. In the rare circumstance when pressure builds up within the mediastinum, an incision or placement of a catheter or chest tube may be necessary to allow escape of air from the mediastinum and release of positive pressure.

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