The mediastinum is bordered anteriorly by the sternum, posteriorly by the thoracic vertebral column, and laterally by the mediastinal pleura. Its contents include the heart, esophagus, trachea, thyroid gland, thymus, major nerves, and vascular structures.

Complete mediastinal displacement results in asymmetric pulmonary expansion.

Occasionally, swallowed air may be identified within the esophagus; it is recognized by visualization of the left esophageal wall separate from the azygoesophageal stripe. Megaesophagus, esophageal diverticula, hiatus hernia, and communicating esophageal duplication cysts may also appear radiographically as air-containing lesions.

Fig. 11.2 Pneumomediastinum.

Fig. 11.3 Pneumomediastinum. Air column separates the visceral from the parietal layers of the mediastinal pleura.

Frequently, there is associated deep cervical and subcutaneous chest wall emphysema. Occasionally, a large pneumomediastinum may lead to impaired systemic venous return and respiratory compromise.

Radiologic Findings

Mediastinal widening is a common radiographic finding. Anteroposterior (AP) supine views taken in shallow inspiration or expiration lead to foreshortening of mediastinal structures and to apparent widening of the mediastinum. Causes of pathologic mediastinal widening include mediastinal hemorrhage and inflammation, esophageal, aortic and vascular dilatation, and lymph node enlargement.

Acute mediastinitis is a serious though rare pathologic process with a mortality rate approaching 50%. Initial diffuse purulent inflammation progresses to abscess formation and these are frequently multiple. Mediastinitis may be secondary to esophageal perforation or postoperative infection. Less commonly it results from direct spread of infection from the retropharyngeal space, lung, or from suppurative mediastinal lymph nodes.

These patients tend to be very ill with severe retrosternal chest pain and pyrexia. When esophageal perforation is present, there is associated dysphagia and deep cervical emphysema.

Radiologic Findings

The chest radiograph shows mediastinal widening with obliteration of fat planes, an accompanying pneumomediastinum, and associated unilateral or bilateral pleural effusions.

In diffuse mediastinitis, computed tomography (CT) shows diffuse soft tissue infiltration of the mediastinum with loss of the normal fat planes. When there is progression to abscess formation, gas bubbles and air-fluid levels may be seen within discrete collections (Armstrong 1995).

Upper gastrointestinal contrast swallow using low osmolar water-soluble contrast medium will show leakage of contrast into the mediastinum from an esophageal perforation.

Chronic mediastinitis is a granulomatous inflammatory disorder which may progress to fibrosis. Known causes include tuberculosis and in the United States, histoplasmosis. However, many cases are of unknown etiology. Idiopathic mediastinal fibrosis may be of autoimmune etiology and there may be an association with retroperitoneal fibrosis (Ormond disease).

Patients may initially be asymptomatic. Symptoms usually result from stenosis or occlusion of the superior vena cava or from extrinsic narrowing of the esophagus and tracheobronchial tree.

Radiologic Findings

The chest radiograph usually shows mediastinal widening ± calcification in cases of histoplasmosis. Other findings are determined by the severity of the obstructive phenomena and include pulmonary oligemia if a pulmonary artery is involved. Narrowing of the trachea and major bronchi may also be seen.

Computed tomography will demonstrate soft tissue infiltration of the mediastinum and there maybe airway and vascular encasement. Dilated collateral vessels also may be present.

The thoracic aorta runs cranially in the anterior mediastinum (ascending aorta) from its origin at the aortic valve, curves posteriorly over the left main bronchus to form the aortic arch, and passes caudally to the diaphragm (descending aorta). The normal diameters based on CT data are:

• Aortic root: 3.7 ± 0.3 cm.
  
• Ascending aorta: 3.3 ± 0.6 cm.
  
• Descending aorta: 2.4 ± 0.3 cm.

The aortic isthmus lies just distal to the origin of the left subclavian artery at the level of the ligamentum arteriosum.

Fig. 11.4a–f Aortic and great vessel anomalies (modified from Schinz 1983). a Normal left-sided aortic arch. b Double aortic arch. c, d Left aortic arch with an aberrant right subclavian artery. c Right subclavian artery crossing the mediastinum between the trachea and esophagus. d Right subclavian artery crossing behind the esophagus.
e Right aortic arch, anterior type. f Right aortic arch, posterior type.

Fig. 11.5a–c Right aortic arch, posterior type. Note the displacement of the trachea (b) and the indentation of the esophagus (c). This patient had presented clinically with dysphagia lusoria.

Fig. 11.6a–e Persistent left superior vena cava. Note the low density widening of the superior mediastinum on the chest radiograph (a). CT defines the course of the persistent left SVC through to the coronary sinus (b–e).

Thoracic Aortic Aneurysm and Dissection

An aneurysm is a saccular or fusiform dilatation of the aortic lumen to greater than 5 cm in diameter. It may be due to atherosclerosis (especially in individuals with systemic arterial hypertension), aortitis in patients with syphilis, some vasculitides including Takayasu’s disease and cystic medial necrosis in Marfan’s syndrome. Inflammatory aneurysms have a predilection for the ascending aorta while atherosclerosis most commonly involves the descending aorta.

Fig. 11.7a, b Elongated, ectatic thoracic aorta.

Fig. 11.8a–c Traumatic aortic injury with rupture giving hemothorax and mediastinal hemorrhage.