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 from damage to the esophagus or tracheobronchial tree. Other causes include alveolar rupture and gas-producing infections. Although relatively uncommon, pneumomediastinum can have a high associated morbidity and mortality, especially if sequelae like tension pneumothorax or acute mediastinitis develop.

Classification:

Cases are typically divided into spontaneously occurring events 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. Furthermore, it is useful to note the mechanism and location of introduced air, 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 hallmark symptom for pneumomediastinum. Chest pain 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. On physical examination, subcutaneous emphysema is commonly present in the chest wall or neck. Auscultation of the chest may demonstrate a synchronous "click" with the heartbeat (Hamman's sign). Rarely, patients may develop hemodynamic compromise. Case series report concurrent pneumothorax in 6-32 percent of spontaneous pneumomediastinum.

Review of published case series reveals that the most common signs and symptoms are 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 etiologies can present similarly to pneumomediastinum. Of these other diagnoses, esophageal perforation, which is the most commonly mistaken for pneumomediastinum, needs to be ruled out given the high mortality with which it is associated. Medial pneumothorax can be mistaken for pneumomediastinum on plain radiographs, but CT is helpful in distinguishing between these entities. Other critical etiologies to consider include cardiac ischemia, cardiac tamponade, aortic dissection, mediastinitis, and pulmonary embolism.

How and/or why did the patient develop pneumomediastinum?

Transit 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 sheathes that cover the terminal bronchioles, arteries, and veins. The bronchovascular sheath also interconnects with the pericardium so air introduced from alveolar rupture or from the soft tissues of the neck or chest wall can track along these planes and into the mediastinum.

Tracheal or esophageal mucosal disruption usually occurs from trauma, including from 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, airway obstruction, 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, cocaine). Underlying pulmonary disease, such as interstitial lung disease and 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 ruptures is positive-pressure mechanical ventilation, especially if high airway pressures are present, resulting in barotrauma. Barotrauma is more likely to occur when there is poor respiratory system compliance like Acute Respiratory Distress Syndrome (ARDS), use of high tidal volume or end expiratory pressure, or obstructive lung disease that causes auto-PEEP. Although no specific data is available regarding the rate of pneumomediastinum with mechanical ventilation, there is a suggestion of decreased pneumothorax with the low tidal volume and high PEEP strategies now commonly used in ARDS. Under these circumstances, pneumothorax and pneumomediastinum share similar mechanistic causes.

External introduction of pneumomediastinum results from introduction of outside air, such as that which can be introduced from trauma, surgery (usually mediastinoscopy, tracheostomy, or sternotomy), or pneumoperitoneum. Air can also track from the neck, such as from dental procedures.

Rarely, gas-forming organisms from infections can produce air within the mediastinum. Acute mediastinitis more usually develops concurrently 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?

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 percent of cases. Underlying pulmonary diseases associated with the development of spontaneous pneumomediastinum include asthma, emphysema, interstitial lung disease, and bronchiectasis.

Less commonly associated lung diseases include cystic or cavitary lesions, bronchilitis obliterans, and intrathoracic malignancies. Patients on mechanical ventilation are at higher risk for barotrauma and ensuing pneumomediastinum if they have high airway pressures, such as from poor lung compliance (e.g., ARDS), obstructive ventilation physiology, or ventilator dysynchrony. Pneumomediastinum is much more commonly a 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. Males aged 18-25 accounted for 73.1 percent 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?

Aside from 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.

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 show that the majority of cases are seen 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).

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 (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 1.

PA Chest X-Ray of Pneumomediastinum

Figure 2.

Lateral Chest X-Ray of Pneumomediastinum

Lateral CXR can also show pneumoprecardium, which represents substernal air anterior to the heart (Figure 2), and continuous left hemidiaphragm sign, in which air 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.

Distinguishing pneumomediastinum from pneumothorax and pneumopericardium on CXR can present some difficulties. 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, and CT is often required to differentiate between them.

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, and 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.

Computed tomography (CT) scans are generally recommended, as they are sensitive, can delineate the extent of air, and (in some circumstances) can provide clues to the site of damage that is introducing air into the mediastinum. In addition, radiography of the esophagus with contrast should be considered to rule out esophageal perforation, as it can have a similar clinical presentation and require more aggressive treatment than other causes of pneumomediastinum can. In small case series, routine abdominal CT scans have not been shown to demonstrate routinely the significant associated causes.

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 air extravasation into the mediastinum.

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

The patient's history and CT radiography are typically sufficient to establish the diagnosis. Routine procedures like 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 suggested.

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 air extravasation into the mediastinum.

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, so 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 ultimately resolution of the pneumediastinum.

Supplemental oxygen to aid reabsorption of extra-alveolar gas is often used, but data is largely anecdotal. Tube thoracostomy is rarely required except in cases of tension physiology. Similarly, cases have been reported in the use of 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 and occlusion of the incision and/or catheters.

Invasive procedures like 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, and can be followed by surgical repair if the site of trauma does not spontaneously resolve with supportive management alone. Unlike other causes of pneumomediastinum, if there is concern about esophageal rupture, prompt surgical intervention should be sought, given the 30-50 percent 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; 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 for this development. 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 suggest less optimal outcomes with pericardiocentesis or percutaneous drain placement.

Empiric antibiotics are frequently used in published cases, but the need for them is largely unsupported. The exception is when a patient has likely contamination of the mediastinal space with infected material, such as in cases of esophageal rupture, trauma, or recent surgery. Antibiotics are generally recommended in these circumstances, given 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 suggests improvement in symptoms after a mean of two days and complete radiographic resolution in approximately one week. Length of hospitalization for spontaneous cases averages 4.1 ± 2.3 days. The majority of outcome-related data in the literature is comprised of retrospective case series. Recurrence of spontaneous pneumomediastinum is less than 1%.

Mortality in secondary pneumomediastinum is similarly low, although the need for chest tube thoractomy is greater than in spontaneous cases. However, mortality is significantly worse with the development of tension physiology, with up to 50 percent mortality in cases of tension pneumopericardium. However, for patients on mechanical ventilation, development of pneumomediastinum or pneumothorax has been reported to have 55 percent and 65 percent mortality, respectively.

What other considerations exist for patients with pneumomediastinum?

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

What’s the evidence?

Bejvan, S, Godwin, J. "Pneumomediastinum: old signs and new signs". AJR. vol. 166. 1996. pp. 1041-1048.

(A discussion of radiographic findings associated with pneumomediastinum.)

Caceres, M, Braud, R, Maekawa, R. "Secondary pneumomediastinum: a retrospective comparative analysis". Lung. vol. 187. 2009. pp. 341-346.

(Analysis of forty-five cases of secondary pneumomediastinum and a review of the literature.)

Dajer-Fadel, WL, Aguero-Sanchez, R, Ibarra-Perez, C. "Systematic review of spontaneous pneumomediastinum: a survey of 22 years' data". Asian Cardiovasc Thorac Ann. 2013; Nov 5.

(Retrospective literature review of 600 reported cases of spontaneous pneumomediastinum.)

Iyer, V, Joshi, A, Ryu, J. "Spontaneous pneumomediastinum: analysis of 62 consecutive adult patients". Mayo Clin Proc. vol. 84. 2009. pp. 417-421.

(Retrospective review of findings from sixty-two patients with spontaneous pneumomediastinum.)

Koullias, G, Korkolis, D, Wang, X. "Current assessment and management of spontaneous pneumomediastinum: experience in 24 adult patients". Eur J Cardiothorac Surg. vol. 25. 2004. pp. 852-855.

(Retrospective review of findings from twenty-four patients with spontaneous pneumoediastinum.)

Leray, V, Bourdin, G, Flandreau, G. "A case of pneumomediastinum in a patient with acute respiratory distress syndrome on pressure support ventilation". Respir Care. vol. 55. 2010. pp. 770-773.

(Case report and discussion of physiology associated with pneumomediastinum and mechnical ventilation.)

Macia, I, Moya, J, Ramos, R. "Spontaneous pneumomediastinum: 41 cases". Eur J Cardiothorac Surg. vol. 31. 2007. pp. 1110-1114.

(Retrospective review of findings from forty-one patients with spontaneous pneumomediastinum.)

Macklin, CC. "Transport of air along sheaths of pulmonic blood vessels from alveoli to mediastinum: clinical implications". Arc Intern Med. vol. 64. 1939. pp. 913-926.

(Original description of pathophysiology of alveolar rupture resulting in pneumomediastinum.)

Newcomb, A, Clark, C. "Spontaneous pneumomediastinum: a benign curiosity or a significant problem". Chest. vol. 128. 2005. pp. 3298-3302.

(Retrospective review of findings from eighteen patients with spontaneous pneumomediastinum.)

Petteruti, F, Stassano, P, De Luca, G. "Tension pneumopericardium and pneumothorax during spontaneous ventilation". J Thorac Cardiovasc Surg. vol. 133. 2007. pp. 829-830.

(Description of the development of tension physiology with negative-pressure ventilation and complications of tension pneumomediastinum.)

Takada, K, Matsumoto, S, Hiramatsu, T. "Management of spontaneous pneumomediastinum based on clinical experience of 25 cases". Respir Med. vol. 102. 2008. pp. 1329-1334.

(Retrospective description of management of twenty-five patients with spontaneous pneumomediastinum.)

Takada, K, Matsumoto, S, Hiramatsu, T. "Spontaneous pneumomediastinum: an algorithm for diagnosis and management". Ther Adv Respir Dis. vol. 3. 2009. pp. 301-307.

(Summary and review of published cases of spontaneous pneumomediastinum.)

Zylak, C, Standen, J, Barnes. "Pneumomediastinum revisited". Radiographics. vol. 20. 2000. pp. 1043-1057.

(Review of clinical findings, with a focus on radiographic findings associated with pneumomediastinum.)
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