Pulmonary Medicine

Non-neoplastic Disorders of the Mediastinum: Pneumomediastinum

What every physician needs to know:

Pneumomediastinum is defined as air in the mediastinal space, and may be associated with pathologic extra-alveolar gas in other sites, including the thoracic cavity, pericardium, subcutaneous soft tissue, peritoneum, and retroperitoneum. Pneumomediastinum usually results from traumatic injury that disrupts mucosal barriers such as those in the esophagus or tracheobronchial tree. Other causes include alveolar rupture (such as may occur in coughing fits) and gas-producing infections. Although relatively uncommon, pneumomediastinum can be associated with high morbidity and mortality, especially if sequelae like tension pneumothorax or acute mediastinitis develop.

Classification:

Cases are typically divided into spontaneously occurring event versus secondary to other causes. They may be further delineated based on where air is introduced into the mediastinum: head/neck/upper respiratory tract, lower respiratory tract, lung, gastrointestinal tract, or external sources. It is also necessary to classify the process by mechanism of air introduction, such as tracheal or esophageal mucosal disruption, alveolar rupture, external introduction, infectious, or idiopathic.

Are you sure your patient has pneumomediastinum? What should you expect to find?

Substernal pain is the most common symptom of pneumomediastinum, although this symptom has many other potential etiologies. Chest pain due to pneumomediastinum is often pleuritic, and it may radiate to the neck or back. Other common symptoms include dyspnea, cough, dysphagia, odynophagia, lightheadedness, and dysphonia. A distinctive, higher-pitch change in the voice can occur if subcutaneous emphysema involves the soft tissues of the neck due to compression in the retropharyngeal and lateral pharyngeal spaces near the glottis. On physical examination, crepitus (subcutaneous emphysema) is commonly present in the chest wall or neck. Although there may be swelling from air in the chest, the pressure differential between the large airways and the tissues of the neck are such that the trachea usually remains midline and patent. Stridor or airway compromise is rare. Auscultation of the chest may demonstrate a synchronous "click" with the heartbeat (Hamman's sign). Rarely, patients may develop hemodynamic compromise. Case series have reported concurrent pneumothorax in 6-32% of cases of spontaneous pneumomediastinum.

Review of published case series revealed that the most common signs and symptoms were chest pain (61%), cough (41%), dyspnea, subcutaneous emphysema (40%), persistent cough (20%), neck pain (17%), dysphagia (14%), and Hamman's sign (14%).

Beware: there are other diseases that can mimic pneumomediastinum.

A number of other musculoskeletal, pulmonary/pleural, cardiac, and esophageal entities can present similarly to pneumomediastinum. Of these other diagnoses, esophageal perforation, which is the most common cause of secondary pneumomediastinum, needs to be ruled out given the high associated mortality of this etiology of pneumomedisatinum. Medial pneumothorax can be mistaken for pneumomediastinum on plain x-ray, but chest computed tomography (CT) could distinguish pneumomediastinum from medial pneumothorax. Other critical disease processes to consider include cardiac ischemia, cardiac tamponade, aortic dissection, mediastinitis and pulmonary embolism.

How and/or why did the patient develop pneumomediastinum?

Migration of air from alveolar damage into the mediastinum along the bronchovascular sheath was first demonstrated in 1939 by Macklin. The deep layer of the cervical fascia in the neck encases the trachea and esophagus. This tissue plane extends to the hila of the lungs and connects with the bronchovascular sheath that covers the terminal bronchioles, arteries, and veins. The bronchovascular sheath also interconnects with the pericardium and thus air introduced from alveolar rupture or from the soft tissues of the neck or chest wall can track anywhere along these planes and into the mediastinum.

Tracheal or esophageal mucosal disruption usually occurs from trauma, including procedural manipulation like endoscopy, endotracheal intubation, transesophageal echocardiography, and other manipulations of the tracheobronchial tree or esophagus. Less common causes of mucosal disruption include tumor invasion and emesis (Boerhaave's syndrome).

While alveolar rupture typically results in pneumothorax, air can track along the fascia lining the tracheobronchial tree, resulting in pneumomediastinum. Spontaneous pneumomediastinum is typically associated with an increase in intrathoracic pressure against a closed glottis (Valsalva maneuver), airway obstruction (such as occurs in asthma), or extreme changes in lung volume. Spontaneous pneumomediastinum has been described in patients after severe coughing or emesis, asthma exacerbation, parturition, Valsalva maneuvers, exercise, scuba diving and abuse of inhaled drugs (e.g., marijuana and cocaine). Underlying pulmonary disease, such as interstitial lung disease and chronic obstructive lung disease (COPD), and certain intrathoracic infections (e.g., Pneumocystis jiroveci) are associated with development of spontaneous pneumomediastinum. Blunt trauma to the chest wall has also been described as an instigating cause.

An additional secondary cause of alveolar rupture is positive-pressure mechanical ventilation, especially if high peak airway pressures are present, resulting in barotrauma. Barotrauma is more likely to occur when there is poor lung compliance as occurs in acute respiratory distress syndrome (ARDS); high tidal volume and/or positive end expiratory pressure (PEEP); or development of auto-PEEP from chronic obstructive pulmonary disease. Although no specific data is available regarding the rate of pneumomediastinum with mechanical ventilation, there is a suggestion of decreased pneumothorax rate with the low tidal volume strategies now commonly used in ARDS. Under these circumstances, pneumothorax and pneumomediastinum share similar mechanistic causes.

External introduction of air can cause pneumomediastinum, such as air introduced from trauma, surgery (usually mediastinoscopy, tracheostomy, or sternotomy), or pneumoperitoneum. Air can also track from the oropharynx or neck, such as in dental procedures or retropharyngeal abscess. Certain pieces of equipment such as high-powered sandblasters and/or power-washers can introduce air in the setting of an accident involving exposure of the aerodigestive tract to these high-pressured devices.

Rarely, gas-forming organisms from infections can produce air within the mediastinum. Acute mediastinitis may present with pneumomediastinum after introduction of organisms into the mediastinal soft tissue, such as from disruption of the esophageal mucosa.

Tension physiology rarely occurs with spontaneous ventilation, but it can develop in the mediastinum or pericardium if a one-way-valve effect occurs from damaged tissue that selectively allows air passage only during inspiration. Tension pneumomediastinum is far more common with positive-pressure mechanical ventilation.

Which individuals are at greatest risk of developing pneumomediastinum?

Once associated high-morbidity entities such as esophageal perforation and/or acute mediastinitis have been ruled out, history should be directed at identifying precipitating factors that increase the risk for alveolar rupture or traumatic injury to the respiratory or gastrointestinal tract, including emesis, use of inhaled recreational drugs, coughing, and sports/physical exercise. However, predisposing triggers are identified in only about 40% of cases. Underlying pulmonary diseases associated with the development of spontaneous pneumomediastinum include asthma, emphysema, interstitial lung disease, and bronchiectasis.

Less commonly associated lung pathology includes cystic or cavitary lesions, bronchiolitis obliterans, and intrathoracic malignancy. Patients on mechanical ventilation are at higher risk of barotrauma and ensuing pneumomediastinum if high airway pressures results from poor lung compliance (e.g., ARDS), obstructive ventilation physiology, or ventilator dyssynchrony. Pneumomediastinum is a much more common complication of positive-pressure ventilation, although case reports have described association with negative-pressure modes of mechanical ventilation.

True idiopathic spontaneous pneumomediastinum is rare (0.001%-0.01% in adult inpatients) and is more common in young males than in other groups. Men aged 18-25 accounted for 73.1% of cases of spontaneous pneumomediastinum in a review of published data.

Patients with secondary causes of pneumomediastinum usually present with a history that readily suggests an etiology of prior trauma or procedural instrumentation.

What laboratory studies should you order to help make the diagnosis, and how should you interpret the results?

Except for imaging studies, laboratory studies are non-specific. Some patients may present with mild leukocytosis. Non-specific ST-T wave changes or ST elevation may be seen on ECG, similar to what one may see with pericarditis.

What imaging studies will be helpful in making or excluding the diagnosis of pneumomediastinum?

Plain chest x-ray (CXR) is the most common initial study, and retrospective case series have shown that the majority of cases were identified on initial CXR. Sensitivity of CXR is more than 80% and CT is 100% sensitive at detecting air in the mediastinum. Radiolucency associated with pneumomediastinum is sometimes more obvious on lateral films than on other views. Some of the more notable CXR findings include elevated mediastinal pleura, where radiolucent streaks of air lift the mediastinal pleura and can extend into the neck or chest wall. Such elevated pleura are often most easily visualized along the left heart border (Figure 1).

Figure 1:

PA Chest X-Ray of Pneumomediastinum

Another notable CXR finding involves the outline of the aorta. Air lucency created by both the mediastinal parietal pleura and visceral pleura of the lung can outline the ascending aorta, aortic arch, descending aorta, and other vascular branches (Figure 1 and Figure 2). A continuous diaphragm sign may also appear on frontal CXR view, as air separates the heart from the superior surface of the diaphragm on the left (Figure 1). Naclerio's V sign is a radiographic "V" created by air lining the descending aorta intersecting with air along the left hemidiaphram (Figure 1).

Figure 2:

Lateral Chest X-Ray of Pneumomediastinum

Lateral CXR can also show pneumopericardium, which represents substernal air anterior to the heart (Figure 2). A lateral view of the continuous left hemidiaphragm sign allows complete visualization of the normally obscured anterior left hemidiaphragm (Figure 2). There may also be a ring-around-the-artery sign, where air outlines the right pulmonary artery (Figure 2) and a halo sign, where a large pneumopericardium surrounds and outlines the entire heart like a halo.

It may be difficult to distinguishing pneumomediastinum from pneumothorax on CXR. Apical pneumomediastinum can be mistaken for an apical pneumothorax but can be differentiated with a contralateral lateral decubitus film. Apical pneumomediastinal air is trapped within tissue planes and will not shift with a decubitus film, whereas a non-loculated pneumothorax will travel to the non-dependent portion of the thorax. Medial pneumothorax and pneumomediastinum may appear similar on plain radiographs because of obstruction by mediastinal structures. A CT is often required to differentiate.

Pneumopericardium can also be difficult to differentiate from pneumomediastinum. Unless the patient recently had cardiac surgery, pneumopericardium is far less common than pneumomediastinum. Observation of any pericardial thickening or effusion relative to the radiographic air lucency can also be helpful in distinguishing between the two etiologies. In addition, involvement of the proximal ascending aorta implies pneumomediastinum. Caution should be used when trying to distinguish these entities radiographically, as pneumomediastinum, pneumothorax, and pneumopericardium can occur concurrently.

Chest CT is generally recommended as it can delineate the extent of air, and (in some circumstances) provides clues to the site of damage that allowed for air introduction into the mediastinum with high sensitivity. In addition, contrast esophagram with water-soluble agent (caution is advised as some can cause pneumonitis if aspirated) or thin barium should be considered to rule out esophageal perforation. Esophageal perforation can have a similar clinical presentation and require more aggressive treatment than other causes of pneumomediastinum. In a small case series, routine abdominal CT failed to reliably identify the etiology and therefore is not necessary unless an associated abdominal process is suspected such as might be the case in poly-trauma.

What non-invasive pulmonary diagnostic studies will be helpful in making or excluding the diagnosis of pneumomediastinum?

The diagnosis is typically made by radiographic evidence of mediastinal air outside of the aerodigestive tract.

What diagnostic procedures will be helpful in making or excluding the diagnosis of pneumomediastinum?

The patient's history and CT chest are typically sufficient to establish the diagnosis, with a low threshold to do an esophagram if there is any possibility of esophageal perforation. Routine procedures such as bronchoscopy and esophagogastroduodenoscopy (EGD) have not been shown to be beneficial and should be reserved for cases in which trauma to the respiratory or gastrointestinal tract are considerations.

What pathology/cytology/genetic studies will be helpful in making or excluding the diagnosis of pneumomediastinum?

The diagnosis is typically made by radiographic evidence of mediastinal air outside of the aerodigestive tract.

If you decide the patient has pneumomediastinum, how should the patient be managed?

The majority of patients with spontaneous pneumomediastinum do not develop hemodynamic or respiratory compromise, and therefore treatment is largely supportive. Patients should be followed with close hemodynamic monitoring to ensure that tension physiology does not develop. Otherwise, serial imaging is useful in monitoring for stability and ultimate resolution of the pneumediastinum.

Supplemental oxygen to aid reabsorption of extra-alveolar gas is often used, but data is largely anecdotal. Catheter or tube thoracostomy is rarely required except in cases of tension physiology or if there is an associated pneumothorax. Similarly, cases have been reported using skin incisions or small-bore catheter insertion into the subcutaneous tissue for relief of tension subcutaneous emphysema, which can be associated with mediastinal emphysema. However, these maneuvers are limited by issues like infection risk and occlusion of the incision and/or catheters.

Invasive procedures such as bronchoscopy and EGD to evaluate for the source of damage to the respiratory or gastrointestinal tract are primarily situational, based on a history of known traumatic insult. They can be followed by surgical drainage or repair if the site of trauma does not resolve spontaneously with supportive management alone. Unlike other causes of pneumomediastinum, if there is concern about esophageal perforation, prompt surgical intervention should be sought, given that this etiology is associated with a 30-50% mortality from ensuing mediastinitis.

Ventilator strategies to minimize barotrauma should be used for patients with pneumomediastinum associated with mechanical ventilation. Specific ventilator-management strategies to minimize PEEP and decrease airway pressures have been described and are now widely used in diseases like ARDS. In addition, specific concerns include the development of tension pneumomediastinum that is due to positive-pressure ventilation; catheter or tube thoracostomy is usually required in this setting because of the development of tension physiology.

Tension pneumomediastinum may result in tension pneumothorax, so the patient should be closely monitored. Empiric chest tube placement is controversial, although some suggest using this approach when a physician is not always readily available to intervene should tension pneumothorax develop. Tension pneumopericardium is also potentially life-threatening, so emergent surgical evaluation for relief of tension physiology via thoracotomy, thoracoscopy, or subxiphoid incisions should be obtained. Case reports suggested less optimal outcomes with pericardiocentesis or percutaneous drain placement.

Empiric antibiotics were frequently used in published cases, but the need for them is not supported by the literature. Nevertheless, if communication between the mediastinum and aerodigestive tract is suspected (such as the case in esophageal perforation, trauma, or recent surgery), antibiotics are warranted to minimize contamination of the mediastinal space with aeordigestive organisms or infected material. The rationale for is the high level of mortality associated with acute mediastinitis.

What is the prognosis for patients managed in the recommended ways?

The majority of cases of pneumomediastinum are self-limited and resolve without invasive therapeutic measures. Published literature suggested improvement in symptoms after a mean of two days and complete radiographic resolution in approximately one week. Length of hospitalization for spontaneous cases averaged 4.1 ± 2.3 days. The majority of outcome-related data in the literature was comprised of retrospective case series. Recurrence of spontaneous pneumomediastinum was less than 1%.

Mortality in secondary pneumomediastinum is similarly low, although the need for chest tube thorastomy is greater than in spontaneous cases. However, mortality is significantly worse with the development of tension physiology, with up to 5% mortality in cases of tension pneumopericardium. For patients on mechanical ventilation, development of pneumomediastinum or pneumothorax has been reported to have 55% and 65% mortality, respectively.

What other considerations exist for patients with pneumomediastinum?

Primary considerations for the diagnosis and treatment of the disease have been outlined above.

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