(Radiographics. 2000;20:1043-1057.)
© RSNA, 2000
Pneumomediastinum Revisited1
Christopher M. Zylak, MD,
James R. Standen, MD,
George R. Barnes, MD and
Carl J. Zylak, MD
1 From the Department of Radiology, University of Arizona, 1718 E Speedway, PMB 108, Tucson, AZ 85719. Presented as a scientific exhibit at the 1998 RSNA scientific assembly. Received April 1, 1999; revision requested May 17 and received July 15; accepted July 15. Address correspondence to C.M.Z. (e-mail: cmzjz@hotmail.com).
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Abstract
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Pneumomediastinum may result from a variety of causes that may be either intrathoracic (eg, narrowed or plugged airway, straining against a closed glottis, blunt chest trauma, alveolar rupture) or extrathoracic (eg, sinus fracture, iatrogenic manipulation in dental extraction, perforation of a hollow viscus). The radiographic signs of pneumomediastinum depend on the depiction of normal anatomic structures that are outlined by the air as it leaves the mediastinum. These signs include the thymic sail sign, "ring around the artery" sign, tubular artery sign, double bronchial wall sign, continuous diaphragm sign, and extrapleural sign. In distal esophageal rupture, air may migrate from the mediastinum into the pulmonary ligament. Pneumomediastinum may be difficult to differentiate from medial pneumothorax and pneumopericardium. Occasionally, normal anatomic structures (eg, major fissure, anterior junction line) may simulate air within the mediastinum. Iatrogenic entities that may simulate pneumomediastinum include helium in the balloon of an intraaortic assist device. In addition, pneumomediastinum may be simulated by the Mach band effect, which manifests as a region of lucency adjacent to structures with convex borders. The absence of an opaque line, which is typically seen in pneumomediastinum, can aid in differentiation. Computed tomographic (CT) digital radiography and conventional CT can also be helpful in establishing or confirming the diagnosis.
Index Terms: Mediastinum, anatomy, 67.92 • Pneumomediastinum, 67.735 • Pneumopericardium, 55.823 • Pneumothorax, 60.731
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Introduction
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Pneumomediastinum presents a diagnostic challenge because it may have a variety of intrathoracic and extrathoracic causes and may demonstrate radiologic findings that are difficult to differentiate from other disease entities. However, this challenge may be successfully met if one has a good understanding of normal mediastinal anatomy and of the pathophysiology of pneumomediastinum.
In this article, we review the anatomy of the mediastinum in relation to the pleural and extra-pleural spaces. In addition, we discuss and illustrate the various intra- and extrathoracic sources of pneumomediastinum, the radiographic and computed tomographic (CT) manifestations of this disease entity, and related diagnostic challenges and imaging pitfalls.
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Mediastinal Anatomy
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The mediastinum has been defined as the mass of tissues and organs separating the two pleural sacs, located between the sternum in front and the vertebral column behind and extending from the thoracic inlet above to the diaphragm below (1). The mediastinum has traditionally been divided into superior and inferior portions, with the inferior portion being further subdivided into anterior, middle, and posterior segments. In the Zylak method, the mediastinum is divided into three longitudinal compartments extending uninterruptedly from the level of the thoracic inlet to the level of the diaphragm (Fig 1) (2). The anterior mediastinal compartment (prevascular space) includes the thoracic contents anterior to the pericardium. The middle mediastinal compartment (vascular space) includes the pericardium and its contents along with the great vessels. The posterior mediastinal compartment (postvascular space) contains the trachea, esophagus, descending aorta, and azygous vein.
In the discussion of mediastinal disease, it is convenient to define the mediastinum as the area between the thoracic inlet and the diaphragm. However, an "invisible belt" at the diaphragm or a "noose" at the neck does not confine the various layers of connective tissue, blood vessels, and anatomic structures that traverse the mediastinum. The mediastinum communicates with several anatomic structures including the submandibular space (3), the retropharyngeal space, and the vascular sheaths in the neck (4). There is a tissue plane that extends anteriorly from the mediastinum to the retroperitoneal space through the sternocostal attachment of the diaphragm (5). This space is continuous with the flanks and extends to the pelvis (Fig 2) (6). The mediastinum also communicates directly with the retroperitoneum by way of the periaortic and peri-esophageal fascial planes (7,8). Thus, the mediastinum extends beyond the confines established by previous definitions.
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Figure 2a. Spontaneous pneumomediastinum and pneumothorax in a 79-year-old woman with carcinoma of the right upper lobe. (a) Anteroposterior radiograph clearly depicts air extending into the retroperitoneal compartment and outlining the kidney and the retroperitoneal segment of the descending colon. (b) Axial CT scan demonstrates air in the left-sided chest wall and in the mediastinum outlining the vessels. The esophagus is also seen. (c) Axial CT scan demonstrates a left-sided pneumothorax. (d) Axial CT scan shows extension of the pneumothorax into the retroperitoneum and the soft tissues of the thigh.
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Figure 2b. Spontaneous pneumomediastinum and pneumothorax in a 79-year-old woman with carcinoma of the right upper lobe. (a) Anteroposterior radiograph clearly depicts air extending into the retroperitoneal compartment and outlining the kidney and the retroperitoneal segment of the descending colon. (b) Axial CT scan demonstrates air in the left-sided chest wall and in the mediastinum outlining the vessels. The esophagus is also seen. (c) Axial CT scan demonstrates a left-sided pneumothorax. (d) Axial CT scan shows extension of the pneumothorax into the retroperitoneum and the soft tissues of the thigh.
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Figure 2c. Spontaneous pneumomediastinum and pneumothorax in a 79-year-old woman with carcinoma of the right upper lobe. (a) Anteroposterior radiograph clearly depicts air extending into the retroperitoneal compartment and outlining the kidney and the retroperitoneal segment of the descending colon. (b) Axial CT scan demonstrates air in the left-sided chest wall and in the mediastinum outlining the vessels. The esophagus is also seen. (c) Axial CT scan demonstrates a left-sided pneumothorax. (d) Axial CT scan shows extension of the pneumothorax into the retroperitoneum and the soft tissues of the thigh.
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Figure 2d. Spontaneous pneumomediastinum and pneumothorax in a 79-year-old woman with carcinoma of the right upper lobe. (a) Anteroposterior radiograph clearly depicts air extending into the retroperitoneal compartment and outlining the kidney and the retroperitoneal segment of the descending colon. (b) Axial CT scan demonstrates air in the left-sided chest wall and in the mediastinum outlining the vessels. The esophagus is also seen. (c) Axial CT scan demonstrates a left-sided pneumothorax. (d) Axial CT scan shows extension of the pneumothorax into the retroperitoneum and the soft tissues of the thigh.
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Sources of Pneumomediastinum
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There are numerous sources of pneumomediastinum (Table 1). Intrathoracic sources may be encountered in a variety of clinical settings. In asthma, pneumomediastinum is secondary to air trapping due to airway narrowing or mucous plugging. In vomiting, parturition, and weight-lifting, straining against a closed glottis may cause pneumomediastinum. Blunt trauma to the chest, usually from a motor vehicle accident, may also cause pneumomediastinum secondary to alveolar rupture or fracture of the trachea or bronchus (Fig 3).
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Figure 3a. Traumatic pneumomediastinum in a 15-year-old boy who sustained blunt trauma to the thorax following sudden deceleration while skiing. (a) Initial radiograph demonstrates a linear band of mediastinal air paralleling the descending aorta. Extensive consolidation is present in both lungs and represents contusion. Note the medial pneumothorax and the air collection paralleling the left hemidiaphragm, which could represent pneumothorax or extrapleural air. (b) On an anteroposterior radiograph obtained 21 months later, the left hilum remains lower than the right. There is disparity between the two lungs in terms of the number of vessels per unit area. In addition, careful inspection of the major airway reveals a more vertical orientation of the left main bronchus as well as occlusion of the left lower lobe bronchus. (c) Bronchogram demonstrates similar findings, thereby helping confirm the diagnosis (cf a and b). (Courtesy of the Department of Radiology, University of Calgary, Foothills Hospital, Calgary, Alberta, Canada.)
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Figure 3b. Traumatic pneumomediastinum in a 15-year-old boy who sustained blunt trauma to the thorax following sudden deceleration while skiing. (a) Initial radiograph demonstrates a linear band of mediastinal air paralleling the descending aorta. Extensive consolidation is present in both lungs and represents contusion. Note the medial pneumothorax and the air collection paralleling the left hemidiaphragm, which could represent pneumothorax or extrapleural air. (b) On an anteroposterior radiograph obtained 21 months later, the left hilum remains lower than the right. There is disparity between the two lungs in terms of the number of vessels per unit area. In addition, careful inspection of the major airway reveals a more vertical orientation of the left main bronchus as well as occlusion of the left lower lobe bronchus. (c) Bronchogram demonstrates similar findings, thereby helping confirm the diagnosis (cf a and b). (Courtesy of the Department of Radiology, University of Calgary, Foothills Hospital, Calgary, Alberta, Canada.)
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Figure 3c. Traumatic pneumomediastinum in a 15-year-old boy who sustained blunt trauma to the thorax following sudden deceleration while skiing. (a) Initial radiograph demonstrates a linear band of mediastinal air paralleling the descending aorta. Extensive consolidation is present in both lungs and represents contusion. Note the medial pneumothorax and the air collection paralleling the left hemidiaphragm, which could represent pneumothorax or extrapleural air. (b) On an anteroposterior radiograph obtained 21 months later, the left hilum remains lower than the right. There is disparity between the two lungs in terms of the number of vessels per unit area. In addition, careful inspection of the major airway reveals a more vertical orientation of the left main bronchus as well as occlusion of the left lower lobe bronchus. (c) Bronchogram demonstrates similar findings, thereby helping confirm the diagnosis (cf a and b). (Courtesy of the Department of Radiology, University of Calgary, Foothills Hospital, Calgary, Alberta, Canada.)
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Extrathoracic sources of pneumomediastinum result from the dissection of air along the pathways described previously. Pneumomediastinum may be due to sinus fracture (Fig 4) or to iatrogenic manipulation following dental extraction (3). In addition, air may dissect into the mediastinum from the retroperitoneal space following perforation of a hollow viscus (Figs 5, 6).
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Figure 4a. Traumatic pneumomediastinum in a patient who was struck in the face with a baseball bat. (a) Preoperative anteroposterior radiograph surprisingly demonstrates pneumomediastinum outlining the left subclavian artery and possibly the left carotid artery (tubular artery sign). (b) Lateral radiograph demonstrates the "ring around the artery" sign. (c) Axial CT scan demonstrates evidence of extensive trauma to the sinus regions, with fracture of both the anterior and posterior walls of the left maxillary sinus. This injury was believed to be the source of the pneumomediastinum.
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Figure 4b. Traumatic pneumomediastinum in a patient who was struck in the face with a baseball bat. (a) Preoperative anteroposterior radiograph surprisingly demonstrates pneumomediastinum outlining the left subclavian artery and possibly the left carotid artery (tubular artery sign). (b) Lateral radiograph demonstrates the "ring around the artery" sign. (c) Axial CT scan demonstrates evidence of extensive trauma to the sinus regions, with fracture of both the anterior and posterior walls of the left maxillary sinus. This injury was believed to be the source of the pneumomediastinum.
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Figure 4c. Traumatic pneumomediastinum in a patient who was struck in the face with a baseball bat. (a) Preoperative anteroposterior radiograph surprisingly demonstrates pneumomediastinum outlining the left subclavian artery and possibly the left carotid artery (tubular artery sign). (b) Lateral radiograph demonstrates the "ring around the artery" sign. (c) Axial CT scan demonstrates evidence of extensive trauma to the sinus regions, with fracture of both the anterior and posterior walls of the left maxillary sinus. This injury was believed to be the source of the pneumomediastinum.
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Figure 5. Traumatic pneumomediastinum in a patient who had sustained a gunshot injury. Anteroposterior radiograph obtained 9 days after exploration and repair of the duodenum, right kidney, and liver demonstrates elevation of the pleura away from the left pulmonary artery and ascending aorta secondary to air in the mediastinum.
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Figure 6a. Pneumomediastinum in a 44-year-old man who had donated the left kidney. Anteroposterior (a) and lateral (b) radiographs obtained 1 day after organ donation show streaks of air within the mediastinal fat, producing a rather bizarre appearance.
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Figure 6b. Pneumomediastinum in a 44-year-old man who had donated the left kidney. Anteroposterior (a) and lateral (b) radiographs obtained 1 day after organ donation show streaks of air within the mediastinal fat, producing a rather bizarre appearance.
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The terminal air spaces of the lung are the most common source of pneumomediastinum. Macklin and Macklin (9) first demonstrated that pneumomediastinum can result when air from ruptured alveoli migrates to the mediastinum, a finding that has been confirmed by other investigators (10). In the presence of a pressure gradient between an alveolus and the interstitium, air ruptures from the alveolus into the perivascular and peribronchial fascial sheath (Figs 7, 8). At the lung roots, the fascial sheath must also rupture before pneumomediastinum develops. In their study, Macklin and Macklin (9) inflated the lower lobe of the right lung in a cat that was under anesthesia. After alveolar rupture had been produced, hot gelatin containing minute grains of carmine was introduced through the same catheter that had admitted air into the lung. Subsequent histologic analysis revealed the defined pathway. Continued insufflation caused an overflow of air into the retroperitoneum, anterior mediastinum, and subcutaneous tissues of the neck and axilla (9).
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Figure 7a. Pneumomediastinum due to alveolar rupture. (a) Photograph of a resected lung specimen from a cat that had undergone intratracheal insufflation with air to pressures of 100 cm of water demonstrates interstitial emphysema, which manifests as air in the sheaths surrounding the pulmonary artery (a) and vein (v). b = bronchus. (Reprinted, with permission, from reference 6.) (b) Photograph of a resected lung specimen from a dog that had undergone insufflation at similar pressures again demonstrates air surrounding only the vessels. (c) Photomicrograph of tissue obtained from the dog demonstrates air in a perivenous space. (Fig 7b, 7c courtesy of Lee Rogers, MD.)
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Figure 7b. Pneumomediastinum due to alveolar rupture. (a) Photograph of a resected lung specimen from a cat that had undergone intratracheal insufflation with air to pressures of 100 cm of water demonstrates interstitial emphysema, which manifests as air in the sheaths surrounding the pulmonary artery (a) and vein (v). b = bronchus. (Reprinted, with permission, from reference 6.) (b) Photograph of a resected lung specimen from a dog that had undergone insufflation at similar pressures again demonstrates air surrounding only the vessels. (c) Photomicrograph of tissue obtained from the dog demonstrates air in a perivenous space. (Fig 7b, 7c courtesy of Lee Rogers, MD.)
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Figure 7c. Pneumomediastinum due to alveolar rupture. (a) Photograph of a resected lung specimen from a cat that had undergone intratracheal insufflation with air to pressures of 100 cm of water demonstrates interstitial emphysema, which manifests as air in the sheaths surrounding the pulmonary artery (a) and vein (v). b = bronchus. (Reprinted, with permission, from reference 6.) (b) Photograph of a resected lung specimen from a dog that had undergone insufflation at similar pressures again demonstrates air surrounding only the vessels. (c) Photomicrograph of tissue obtained from the dog demonstrates air in a perivenous space. (Fig 7b, 7c courtesy of Lee Rogers, MD.)
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Figure 8a. Pneumomediastinum in a 41-year-old woman with pneumonia who required ventilation with positive end-expiratory pressure. Axial CT scans show pneumomediastinum (a) and air in the perivascular and peribronchial sheaths (b) (arrows). (Courtesy of J.P. Locksmith, MD, Medical College of Georgia, Augusta.)
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Figure 8b. Pneumomediastinum in a 41-year-old woman with pneumonia who required ventilation with positive end-expiratory pressure. Axial CT scans show pneumomediastinum (a) and air in the perivascular and peribronchial sheaths (b) (arrows). (Courtesy of J.P. Locksmith, MD, Medical College of Georgia, Augusta.)
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Radiographic Signs of Pneumomediastinum
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The radiographic signs of pneumomediastinum depend on the depiction of normal anatomic structures that are outlined by the air as it leaves the mediastinum (Table 2). If there is sufficient air, the thymus can become elevated to produce the thymic sail sign (Fig 9) (11). Air anterior to the pericardium (pneumoprecardium) is a frequent manifestation and requires a lateral view of the chest for diagnosis (Fig 10). Air surrounding the pulmonary artery or either of its main branches can result in the ring around the artery sign (12), particularly when the air surrounds the intramediastinal segment of the right pulmonary artery (Fig 10). When there is air adjacent to the major branches of the aorta, both sides of the vessel are depicted: Mediastinal air outlines the medial side, and the aerated lung marginates the lateral side (tubular artery sign) (Fig 11).
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Figure 9a. Thymic sail sign in a 4-year-old patient who had inhaled gasoline fumes. (a) Initial anteroposterior radiograph demonstrates findings consistent with pulmonary edema. (b) Anteroposterior radiograph obtained 50 minutes later shows air in the subcutaneous tissues of the neck, a finding that is consistent with pneumomediastinum. The thymus is outlined by air (thymic sail sign). However, there is also a well-defined opaque line paralleling the left border of the heart and extending to the level of the pulmonary artery that represents the pericardium. (c) Anteroposterior radiograph clearly depicts the tube that was inserted into the pericardium and helped successfully relieve the tamponade. The patient made an uneventful recovery.
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Figure 9b. Thymic sail sign in a 4-year-old patient who had inhaled gasoline fumes. (a) Initial anteroposterior radiograph demonstrates findings consistent with pulmonary edema. (b) Anteroposterior radiograph obtained 50 minutes later shows air in the subcutaneous tissues of the neck, a finding that is consistent with pneumomediastinum. The thymus is outlined by air (thymic sail sign). However, there is also a well-defined opaque line paralleling the left border of the heart and extending to the level of the pulmonary artery that represents the pericardium. (c) Anteroposterior radiograph clearly depicts the tube that was inserted into the pericardium and helped successfully relieve the tamponade. The patient made an uneventful recovery.
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Figure 9c. Thymic sail sign in a 4-year-old patient who had inhaled gasoline fumes. (a) Initial anteroposterior radiograph demonstrates findings consistent with pulmonary edema. (b) Anteroposterior radiograph obtained 50 minutes later shows air in the subcutaneous tissues of the neck, a finding that is consistent with pneumomediastinum. The thymus is outlined by air (thymic sail sign). However, there is also a well-defined opaque line paralleling the left border of the heart and extending to the level of the pulmonary artery that represents the pericardium. (c) Anteroposterior radiograph clearly depicts the tube that was inserted into the pericardium and helped successfully relieve the tamponade. The patient made an uneventful recovery.
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Figure 10a. Pneumomediastinum in an 18-year-old asthmatic patient who presented with shortness of breath and sternal discomfort. (a) Anteroposterior radiograph demonstrates the continuous diaphragm sign. (b) Lateral radiograph shows air surrounding the right pulmonary artery (ring around the artery sign). Air is present anterior to the pericardium (pneumoprecardium).
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Figure 10b. Pneumomediastinum in an 18-year-old asthmatic patient who presented with shortness of breath and sternal discomfort. (a) Anteroposterior radiograph demonstrates the continuous diaphragm sign. (b) Lateral radiograph shows air surrounding the right pulmonary artery (ring around the artery sign). Air is present anterior to the pericardium (pneumoprecardium).
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Figure 11a. Tubular artery sign in a 28-year-old man who sustained blunt trauma. (a) Anteroposterior radiograph demonstrates an air-fluid level projecting at the left costophrenic angle surrounded by increased airspace opacity. These findings represent a laceration with surrounding contusion. The tubular structure seen arising from the top of the aorta represents displacement of the left subclavian artery by air in the mediastinum. The mediastinal air outlines the medial side of the artery, and the aerated lung marginates its lateral border. The left common carotid artery is also faintly visible. (b) Follow-up radiograph obtained 27 days later demonstrates resolution of the pneumomediastinum and lung contusion. The resolving pneumatocele is now readily apparent.
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Figure 11b. Tubular artery sign in a 28-year-old man who sustained blunt trauma. (a) Anteroposterior radiograph demonstrates an air-fluid level projecting at the left costophrenic angle surrounded by increased airspace opacity. These findings represent a laceration with surrounding contusion. The tubular structure seen arising from the top of the aorta represents displacement of the left subclavian artery by air in the mediastinum. The mediastinal air outlines the medial side of the artery, and the aerated lung marginates its lateral border. The left common carotid artery is also faintly visible. (b) Follow-up radiograph obtained 27 days later demonstrates resolution of the pneumomediastinum and lung contusion. The resolving pneumatocele is now readily apparent.
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Occasionally, air can reside next to a major bronchus, allowing clear depiction of the bronchial wall and producing the double bronchial wall sign (Fig 12). The continuous diaphragm sign (Fig 10) (13) is produced by air trapped posterior to the pericardium, which gives the appearance of a continuous collection of air at anteroposterior radiography. Air from the mediastinum can extend laterally between the parietal pleura and the diaphragm to produce the extrapleural sign (Fig 14) (14). Occasionally, air will migrate from the mediastinum into the pulmonary ligament secondary to rupture of the distal esophagus (Fig 13) (15).
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Figure 12a. Double bronchial wall sign. (a) Radiograph obtained in a 35-year-old asthmatic patient demonstrates air in the mediastinum and left main bronchus, which allows visualization of both sides of the bronchial wall. (b) Axial CT scan obtained in a different patient helps explain this sign. Air in the mediastinum outlines the posterior aspect of the inferior wall of the left mainstem bronchus, whereas air in the bronchus outlines its anterior surface.
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Figure 12b. Double bronchial wall sign. (a) Radiograph obtained in a 35-year-old asthmatic patient demonstrates air in the mediastinum and left main bronchus, which allows visualization of both sides of the bronchial wall. (b) Axial CT scan obtained in a different patient helps explain this sign. Air in the mediastinum outlines the posterior aspect of the inferior wall of the left mainstem bronchus, whereas air in the bronchus outlines its anterior surface.
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Figure 14a. Pseudomediastinum in a 78-year-old woman who underwent endoscopic dilation of an esophageal stricture. The patient developed chest pain following the procedure. (a) CT digital radiograph demonstrates extravasation of barium into the mediastinum. (b) Axial CT scan shows mediastinal air outlining the descending aorta. Note the paraesophageal collection of contrast material in the pulmonary ligament (arrow). (c) CT scan demonstrates a collection of contrast material extending over the hemidiaphragm (arrow). The extrapleural location was confirmed at surgery.
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Figure 14b. Pseudomediastinum in a 78-year-old woman who underwent endoscopic dilation of an esophageal stricture. The patient developed chest pain following the procedure. (a) CT digital radiograph demonstrates extravasation of barium into the mediastinum. (b) Axial CT scan shows mediastinal air outlining the descending aorta. Note the paraesophageal collection of contrast material in the pulmonary ligament (arrow). (c) CT scan demonstrates a collection of contrast material extending over the hemidiaphragm (arrow). The extrapleural location was confirmed at surgery.
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Figure 14c. Pseudomediastinum in a 78-year-old woman who underwent endoscopic dilation of an esophageal stricture. The patient developed chest pain following the procedure. (a) CT digital radiograph demonstrates extravasation of barium into the mediastinum. (b) Axial CT scan shows mediastinal air outlining the descending aorta. Note the paraesophageal collection of contrast material in the pulmonary ligament (arrow). (c) CT scan demonstrates a collection of contrast material extending over the hemidiaphragm (arrow). The extrapleural location was confirmed at surgery.
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Figure 13. Extrapleural sign in a 26-year-old woman with esophageal rupture. Anteroposterior radiograph demonstrates a linear area of lucency paralleling the descending aorta (arrow) representing air within the mediastinum. A collection of air is seen just lateral to the lower descending aorta (black arrowheads). This air presumably resides within the pulmonary ligament. Note also the small collection of air in the left pleural space (white arrowheads).
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Diagnostic Challenges and Pitfalls
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Diagnostic challenges include differentiating pneumomediastinum from medial pneumothorax and pneumopericardium. If there is no other evidence of pneumothorax, medial collections of air within the pleural space are very difficult to differentiate from pneumomediastinum (Fig 15).
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Figure 15a. Medial pneumothorax. (a) Anteroposterior radiograph demonstrates a left-sided medial pneumothorax with placement of a catheter aspiration tube for simple pneumothorax. Prior radiographs (not shown) demonstrated total collapse of the left lung. (b) On an anteroposterior radiograph obtained 1 day later, the residual medial pneumothorax (arrow) is very difficult to distinguish from pneumomediastinum.
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Figure 15b. Medial pneumothorax. (a) Anteroposterior radiograph demonstrates a left-sided medial pneumothorax with placement of a catheter aspiration tube for simple pneumothorax. Prior radiographs (not shown) demonstrated total collapse of the left lung. (b) On an anteroposterior radiograph obtained 1 day later, the residual medial pneumothorax (arrow) is very difficult to distinguish from pneumomediastinum.
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Air within the pericardium is suspected when the pericardial sac itself is visualized (Fig 9). The line formed by the pneumopericardium will be confined to the length of the pericardial sac. Rarely, patients with congenital partial absence of the pericardium will present with a spontaneous pneumothorax. In such cases, air outlines the pericardium and its contents, particularly the roots of the great vessels (Fig 16). The embryologic development of the pleuropericardial membrane is such that a defect in the pericardium is associated with a
communicating defect in the pleura. Subpulmonary pneumothorax (Fig 17) and pneumoperitoneum (Fig 18) can sometimes be difficult to differentiate from an extrapleural collection of air. Decubitus views can often help make this distinction.
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Figure 16a. Spontaneous pneumothorax in a 22-year-old man. (a) On an anteroposterior radiograph, the configuration of the heart is unusual in that the left ventricle extends quite far laterally. A linear band of air is seen paralleling the right border of the heart and terminating abruptly near the hilum (arrow). Air is seen outlining the main and right pulmonary arteries. These findings suggest a partial congenital pericardial defect that allows communication between the pleural and pericardial sacs. (b) Lateral radiograph demonstrates the ring around the artery sign. (Courtesy of the Department of Radiology, University of Calgary, Foothills Hospital, Calgary, Alberta, Canada.)
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Figure 16b. Spontaneous pneumothorax in a 22-year-old man. (a) On an anteroposterior radiograph, the configuration of the heart is unusual in that the left ventricle extends quite far laterally. A linear band of air is seen paralleling the right border of the heart and terminating abruptly near the hilum (arrow). Air is seen outlining the main and right pulmonary arteries. These findings suggest a partial congenital pericardial defect that allows communication between the pleural and pericardial sacs. (b) Lateral radiograph demonstrates the ring around the artery sign. (Courtesy of the Department of Radiology, University of Calgary, Foothills Hospital, Calgary, Alberta, Canada.)
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Figure 18a. Pneumoperitoneum. (a) Anteroposterior radiograph demonstrates pneumoperitoneum simulating extrapleural and subpulmonary air collections. (b) CT scan helps confirm the intraperitoneal location of the air. Note also the extensive subcutaneous emphysema and the air extending from the mediastinum into the retroperitoneum.
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Figure 18b. Pneumoperitoneum. (a) Anteroposterior radiograph demonstrates pneumoperitoneum simulating extrapleural and subpulmonary air collections. (b) CT scan helps confirm the intraperitoneal location of the air. Note also the extensive subcutaneous emphysema and the air extending from the mediastinum into the retroperitoneum.
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Occasionally, normal anatomic structures may simulate air within the mediastinum. With lordotic positioning, the superior aspect of a major fissure can manifest as a white line, thereby mimicking pneumomediastinum or medial pneumothorax (Fig 19) (16). On occasion, the anterior junction line can also give this spurious impression, particularly if the patient is imaged in a slight degree of obliquity or lordosis (Fig 20).
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Figure 19. Major fissure simulating disease. Radiograph demonstrates the superior aspect of a major fissure simulating pneumomediastinum or medial pneumothorax (arrows). This finding was confirmed at fluoroscopy.
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Figure 20. Anterior junction line simulating pneumomediastinum. Radiograph obtained with the patient in a somewhat lordotic position demonstrates the normal anterior junction line (arrows).
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In addition, pneumomediastinum may be simulated by the Mach band effect (17). The Mach band effect is associated with convex surfaces, appearing as a region of lucency adjacent to structures with convex borders (Fig 21). The absence of an opaque line, which is typically seen in pneumomediastinum, can aid in differentiation.
Iatrogenic entities that may simulate pneumomediastinum include helium in the balloon of an intraaortic assist device (Fig 22). This device is introduced into the descending aorta via the femoral artery with its tip just distal to the left subclavian artery. The device is intended to aid in coronary artery perfusion.
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Figure 22a. Iatrogenic simulation of pneumomediastinum due to helium in an intraaortic balloon assist device. (a) Anteroposterior radiograph obtained in a cardiac intensive care unit patient demonstrates global cardiomegaly. The metallic tip of an intraaortic balloon assist device is seen projecting just inferior to the aortic knob. A linear area of lucency representing helium in the intraaortic assist device is seen paralleling the spine. (b) Radiograph shows the assist device. The device has been inflated and placed in a container of water, which serves as the surrounding medium.
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Figure 22b. Iatrogenic simulation of pneumomediastinum due to helium in an intraaortic balloon assist device. (a) Anteroposterior radiograph obtained in a cardiac intensive care unit patient demonstrates global cardiomegaly. The metallic tip of an intraaortic balloon assist device is seen projecting just inferior to the aortic knob. A linear area of lucency representing helium in the intraaortic assist device is seen paralleling the spine. (b) Radiograph shows the assist device. The device has been inflated and placed in a container of water, which serves as the surrounding medium.
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Conclusions
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Pneumomediastinum presents a diagnostic challenge that may be successfully met if one understands the pathophysiology of this disease entity. Careful examination of chest radiographic findings is crucial in the diagnosis of pneumomediastinum. Correct assessment of the radiologic signs of pneumomediastinum can help the clinician plan appropriate treatment in affected patients.
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Abstract
Introduction
