Chapter 17 Diffuse Mediastinal Abnormalities
Unlike focal mediastinal masses, which usually can be localized within a single mediastinal compartment, diffuse mediastinal abnormalities almost always involve more than one compartment of the mediastinum and therefore preclude classification by the traditional compartmentalization method. The common feature of these entities is that they all may present with diffuse mediastinal widening on chest radiographs.
RADIOLOGIC APPROACH TO DIFFUSE MEDIASTINAL WIDENING
Recognition and evaluation of diffuse mediastinal widening on plain radiographs can be challenging, even for experienced radiologists. The first challenge is recognizing the abnormality. Accurate recognition is particularly difficult on anteroposterior portable supine radiographs because they result in magnification of normal mediastinal structures. The mediastinum often must be assessed on portable radiographs, especially for trauma patients, even though posteroanterior and lateral chest radiographs are superior. Assessment of the mediastinum is also difficult in older patients with atherosclerotic vascular disease, because the mediastinum may appear wide because of tortuosity of the aorta and great vessels. Comparison with prior radiographs is particularly helpful in evaluating this population.
After recognizing diffuse mediastinal widening, its cause must be determined, which can be a difficult task based on chest radiographic findings alone. Assessment of the mediastinal contours and normal mediastinal landmarks is essential. Subtle alterations are often best appreciated as a change in appearance from previous radiographs. The identification of diffuse mediastinal widening on plain radiographs, particularly when accompanied by abnormalities of the normal mediastinal landmarks, usually requires further evaluation with computed tomography (CT) or magnetic resonance imaging (MRI).
DIFFUSE MEDIASTINAL ABNORMALITIES
Mediastinal lipomatosis is the diffuse accumulation of excess unencapsulated fat within the mediastinum. This benign condition is usually seen in adult patients and may be associated with Cushing’s syndrome, exogenous steroid use, and obesity.
Fat accumulation is usually most prominent in the anterior and superior portions of the mediastinum, where it surrounds the great vessels and results in lateral displacement of the pleural reflections. It may be detected in other parts of the mediastinum, including the cardiophrenic angles, paravertebral regions, retrocrural, and subcostal regions.
The appearance on chest radiographs and CT depends on the distribution of excess fat deposition. Accumulation of fat in the anterior and superior portions of the mediastinum results in smooth widening of the anterior and superior mediastinal contours as seen on chest radiographs (Fig. 17-1). An important feature is the lack of mass effect on the trachea and esophagus, structures that are often displaced or compressed by other mediastinal abnormalities. Excess fat deposits within the cardiophrenic angles result in cardiophrenic angle “masses,” and excess fat within the paravertebral regions may result in bilateral lateral displacement of the paraspinal lines.
Figure 17-1 Mediastinal lipomatosis. The frontal chest radiograph reveals smooth widening of the superior mediastinum (arrows). Notice the absence of a mass effect on the trachea, which has a midline position.
A definitive diagnosis of mediastinal lipomatosis may be made on CT (Fig. 17-2). Fat is recognized on CT by its low CT numbers, which typically vary from −70 to −130 Hounsfield units (HU). Although CT is considered the imaging modality of choice, the diagnosis also can be made by MRI. On MRI, fat demonstrates bright signal intensity on T1-weighted images. Using a fat-suppression sequence results in suppression of the normally bright T1 signal from fat tissue and helps to differentiate it from other tissues with bright T1 signal. An important feature of mediastinal lipomatosis on CT or MRI is a homogeneous appearance of the mediastinal fat. An inhomogeneous appearance, such as the presence of high-attenuation foci within the fat, should raise the suspicion of a superimposed process, such as mediastinal hemorrhage or neoplastic infiltration.
Figure 17-2 Mediastinal lipomatosis. A, Axial, noncontrast CT of the chest at the level of the brachiocephalic vessels shows a large amount of fat within the mediastinum that surrounds the vessels and results in lateral displacement of the pleural reflections (arrows). Notice the normal midline position of the trachea (T) and esophagus (E). B, Axial CT of the chest at the level of the diaphragm reveals excess fat within the cardiophrenic angles (paired arrows).
Diffuse mediastinitis may be acute or chronic. Both forms are most often caused by infections. Acute mediastinitis is often the result of a bacterial infection, and chronic mediastinitis is more often related to a granulomatous infection, such as histoplasmosis. Patients with acute mediastinitis usually present with an acute onset of symptoms, including fever and leukocytosis, whereas patients with chronic mediastinitis are often asymptomatic. If symptoms occur, they usually result from compression of mediastinal structures.
Acute mediastinitis may occur after esophageal perforation, from extension of an infectious process from thoracic and extrathoracic structures (especially from the neck), and as an infrequent complication of cardiac surgery (Box 17-1). Most cases are caused by esophageal perforation.
Box 17-1 Causes of Acute Mediastinitis
Iatrogenic (after esophagoscopy or esophageal dilation)
Impacted foreign body (chicken bone, sharp objects)
Obstructing esophageal neoplasm
Trauma (penetrating trauma more than blunt trauma)
EXTENSION OF INFECTION FROM ADJACENT SPACES
Pharynx (retropharyngeal or nasopharyngeal abscess)
Retroperitoneum (pancreatic pseudocyst)
EXTENSION OF INFECTION FROM ADJACENT THORACIC STRUCTURES
Patients with esophageal perforation frequently present with fever, leukocytosis, dysphagia, and retrosternal chest pain, which often radiates into the neck. On physical examination, they may demonstrate subcutaneous emphysema and Hamman’s sign, a crunching or rasping sound that is synchronous with the heartbeat and heard on auscultation over the cardiac apex and that is associated with pneumomediastinum.
Chest radiographic findings include diffuse widening of the mediastinum and pneumomediastinum (Fig. 17-3). Associated pleural abnormalities are usually left sided and include pneumothorax and empyema. When the diagnosis is delayed, complications may include mediastinal abscess formation and rupture of the abscess into the adjacent bronchus (i.e., esophagobronchial fistula) and pleura (i.e., esophagopleural fistula, often with subsequent development of empyema). The diagnosis of esophageal perforation can be confirmed by fluoroscopic examination after administration of water-soluble contrast, which demonstrates extravasation of contrast at the site of perforation (Fig. 17-4). In complicated cases that have progressed to mediastinal abscess formation, CT may be helpful in identifying the precise location and extent of fluid collections (Fig. 17-5).
Figure 17-3 Acute mediastinitis caused by esophageal perforation (i.e., Boerhaave’s syndrome). The frontal radiograph of the chest reveals an abnormal linear lucency (arrows) adjacent to the descending aortic interface, consistent with pneumomediastinum.
Figure 17-4 Acute mediastinitis caused by esophageal perforation (i.e., Boerhaave’s syndrome). Coned-down image of the distal esophagus after administration of water-soluble contrast shows extravasation of contrast (arrows) from the distal esophagus, consistent with an esophageal perforation.
Figure 17-5 Mediastinal abscess after esophageal perforation from penetrating trauma. A, Axial contrast-enhanced CT of the chest at the level of the carina (C) reveals a large, subcarinal fluid collection (arrows) containing foci of gas, consistent with a mediastinal abscess. AA ascending aorta; DA, descending aorta; S, superior vena cava. B, Coned-down image of the upper thoracic esophagus after administration of water-soluble contrast media shows extravasation of contrast (arrows), consistent with an esophageal perforation. AA, aortic arch; E, esophagus.
Prompt diagnosis and treatment of esophageal perforation are critical. Very high morbidity and mortality rates are associated with delay in diagnosis beyond 24 hours.
Other Causes of Acute Mediastinitis
Other causes of acute mediastinitis are less common. They include extension of infection from adjacent thoracic structures, including the lungs, pleura, pericardium and mediastinal lymph nodes; extension of infection from adjacent anatomic regions, especially from the neck; traumatic tracheobronchial rupture; and cardiac surgery (an infrequent postoperative complication).
Chest radiographs may demonstrate diffuse mediastinal widening and findings associated with a mediastinal abscess, including gas bubbles or an air-fluid level. Pneumomediastinum and pneumothorax, findings frequently associated with esophageal perforation, are not usually seen with other causes of acute mediastinitis. An important exception is traumatic tracheobronchial rupture, which frequently manifests with pneumomediastinum and pneumothorax.