Pneumomediastinum is defined as air present in the mediastinum and less frequently referred to as mediastinal emphysema. The mediastinum is the space in the central thorax bounded laterally by the parietal pleura of the lungs, superiorly by the thoracic outlet, inferiorly by the diaphragm, anteriorly by the sternum, and posteriorly by the thoracic vertebral column. Vital structures, such as the heart, tracheobronchial tree, and the esophagus, reside within the mediastinum. Pneumomediastinum develops when air extravasates from within the airways, lungs, or esophagus and migrates into the mediastinal space. Extravasated air may then dissect contiguously into the neighboring cervical subcutaneous tissues, epidural space, pericardium, and/or the peritoneal cavity. As a clinical entity, pneumomediastinum is rare and typically self-limited. Once diagnosed and serious concomitant pathology excluded, management is largely focused on symptomatic treatment.
Spontaneous or primary pneumomediastinum occurs in otherwise healthy individuals with no identifiable cause. Secondary pneumomediastinum includes all cases in which an underlying cause such as trauma can be elicited. The most common cause of pneumomediastinum is alveolar rupture.
Risk Factors for Spontaneous Pneumomediastinum
Causes of Secondary Pneumomediastinum
1. Intrinsic Lung and Airway Causes:
2. Iatrogenic Causes:
3. Traumatic Causes:
Pneumomediastinum is rare, with a reported prevalence as low as 0.002%. Spontaneous pneumomediastinum most commonly presents in younger males of tall stature with low BMI (76% of cases). One explanation for this predilection for younger adult patients is that mediastinal tissues in this population are less fibrotic compared to older patients and thus allow for easier air extravasation and subsequent spread. However, it is not well described in the pediatric literature except in neonates.
Patients with sepsis-induced acute respiratory distress syndrome air leaks of any type excluding pneumothorax occurred in 3.7% of patients. In adult patients with blunt chest trauma, up to 10% had evidence of pneumomediastinum.
Spontaneous pneumomediastinum typically develops in a three-step process called the Macklin phenomenon:
Intra-alveolar pressure may increase from excessive vomiting or coughing, strenuous physical exertion, or Valsalva maneuvers (e.g., childbirth). Airway disease, such as asthma and COPD, causes bronchial constriction, which predisposes patients to airway injury due to baseline increased alveolar pressures. Forceful inhalation during illicit drug use may cause changes in intra-alveolar pressure or direct barotrauma, which similarly increases the risk for pneumomediastinum.
Secondary pneumomediastinum is typically caused by trauma to the trachea or esophagus. Penetrating trauma (i.e., stab wounds) or blunt trauma (i.e., motor vehicle accident) may cause tracheal or esophageal injury. Though not well described, blasts may also cause secondary pneumomediastinum. Iatrogenic trauma (e.g., endoscopy or endotracheal intubation) may result in similar injuries.
In gathering the history, it is important to identify any existing risk factors for pneumomediastinum as well as preceding events. Key elements of the history include:
The most common symptom of pneumomediastinum is retrosternal chest pain (60 to 100% of patients), which may radiate to the neck or back. Pain is typically acute in onset and may be preceded by excessive coughing or vomiting. Other symptoms include:
The most common sign on physical exam is subcutaneous emphysema (70% of patients). An uncommon but specific sign that may be present is Hamman’s sign, a mediastinal crunch or click synchronous with the heart sounds auscultated over the cardiac apex. Other signs include:
Rarely, patients may present with malignant pneumomediastinum, which is severe pneumomediastinum that obstructs the great vessels of the heart resulting in cardiac tamponade physiology.
Pneumomediastinum is usually diagnosed via anterior chest X-ray. The most common finding is air outlining mediastinal structures (90% of cases). Lateral chest x-rays are rarely needed. Other signs on chest x-ray include:
If chest x-ray findings are equivocal, a computed tomography (CT) scan of the chest can definitively rule in or rule out pneumomediastinum. CT scans can detect very small amounts of air in the mediastinum or subcutaneous tissues and can differentiate between pneumomediastinum and pneumopericardium, which can be difficult to do on a chest X-ray.
In emergent situations, ultrasound may be helpful in identifying pneumomediastinum. The “air gap sign” may be present, which appears as broadband of sonographic echoes that obscure the cardiac structures beneath. Other findings may include air adjacent to the heart or diaphragm and subcutaneous hyperechoic white foci suggestive of air bubbles in the soft tissue. 
Additional studies such as bronchoscopy, esophagoscopy, or esophagography are typically not required in most cases of pneumomediastinum. Obtaining these additional tests is warranted, given the clinical context. In blunt trauma, the absence of aerodigestive injury on multidetector CT in the setting of pneumomediastinum is adequate to rule out additional traumatic sequelae related to the aerodigestive system. However, posterior mediastinal air, the involvement of all mediastinal compartments, and concomitant hemothorax are associated with increased mortality in blunt trauma patients.
Most patients with pneumomediastinum diagnosed on imaging studies ultimately prove not to have serious mediastinal organ injury and only require symptomatic management, as spontaneous pneumomediastinum is usually self-limited and rarely recurs. These patients are typically well-appearing and hemodynamically stable. Treatment in these patients includes bed rest, oxygen administration as needed, antitussives, and analgesia. Prophylactic antibiotics are not typically needed in spontaneous pneumomediastinum but may be indicated in patients with aerodigestive injury secondary to trauma or instrumentation. Most spontaneous pneumomediastinum patients can be discharged with close outpatient follow-up or admitted for brief inpatient observation. Similarly, blunt trauma patients with isolated pneumomediastinum may also be able to be managed conservatively. In patients who are in distress, febrile, or with concern for secondary pneumomediastinum, hospital admission is indicated for observation, additional diagnostic testing, and/or surgery. Those with emesis, dysphagia, associated traumatic injuries, hemodynamic instability, pleural effusion, signs of infection, or pneumoperitoneum should undergo a contrast swallow study. Patients with extensive subcutaneous emphysema may necessitate surgical decompression, and those with pneumothorax may require chest tube placement. Finally, in patients presenting with malignant pneumomediastinum, video-assisted thoracoscopic surgery, or thoracotomy may be necessary.
Most cases of pneumomediastinum are self-resolving and require only symptomatic treatment and/or observation.
Complications include pneumothorax, extensive subcutaneous emphysema, malignant pneumomediastinum, and pneumopericardium. Other complications may be specific to the cause (i.e., mediastinitis in patients with a perforated esophagus).
Most cases of pneumomediastinum are spontaneous. Therefore, abstinence from smoking and/or recreational drug use are a few measures that can appreciably lower the risk of developing this condition. There is a paucity of literature regarding when those with pneumomediastinum may fly or return to contact sports. However, it is generally advisable that patients refrain from flying or contact sports until asymptomatic.
Most patients with pneumomediastinum have a good prognosis. However, in the rare subset of patients with malignant pneumomediastinum, hemodynamic instability, or secondary pneumomediastinum, the expertise and cooperative efforts of intensivists, surgeons, respiratory therapists, and nurses may be required.
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