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Ann Thorac Surg 2008;86:962-966. doi:10.1016/j.athoracsur.2008.04.067
© 2008 The Society of Thoracic Surgeons
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Original Articles: General Thoracic

Spontaneous Pneumomediastinum: A Comparative Study and Review of the Literature

Manuel Caceres, MDa,b, Syed Z. Ali, MDa, Rebecca Braud, BSc, Darryl Weiman, MDb, H. Edward Garrett, Jr, MDb,d,*

a Department of Cardiothoracic Surgery, Appalachian Regional Healthcare, Hazard, Kentucky
b Department of Cardiothoracic Surgery, University of Tennessee HSC, Memphis, Tennessee
c Louisiana State University, Baton Rouge, Louisiana
d Department of Cardiothoracic Surgery, Baptist Memorial Hospital, Memphis, Tennessee

Accepted for publication April 21, 2008.

* Address correspondence to Dr Garrett, 6029 Walnut Grove Rd, Suite 401, Memphis, TN 38120 (Email: egarrettmd{at}cvsclinic.com).


   Abstract
Top
 Abstract
Introduction
 Material and Methods
 Results
 Comment
 Acknowledgments
 References
 
Background: Spontaneous pneumomediastinum (SPM) is an unusual occurrence with few cases reported. It is seen after intrathoracic pressure changes leading to alveolar rupture and dissection of air along the tracheobronchial tree. This study was undertaken to provide a thorough clinical and radiologic analysis of this patient population.

Methods: A retrospective comparative analysis was performed on patients with SPM over 12 years. Patient demographics, clinical presentation, and radiographic and diagnostic studies were recorded. A clinical and radiologic comparison was performed with secondary pneumomediastinum.

Results: Seventy-four patients were identified with a diagnosis of pneumomediastinum. A total of 28 patients with SPM were identified. The major initial complaints were chest pain (54%), shortness of breath (39%), and subcutaneous emphysema (32%). The main triggering events were emesis (36%) and asthma flare-ups (21%). No apparent triggering event was noted in 21% of patients. Chest radiograph was diagnostic in 69%; computed tomography was required in 31%. Esophagram, esophagoscopy, and bronchoscopy were performed on an individual basis and were invariably negative. When compared with secondary pneumomediastinum, SPM is more likely to be discovered by chest radiography, has a lower incidence of pneumothorax and pleural effusion, requires a shorter hospital stay, and has no associated mortality.

Conclusions: Spontaneous pneumomediastinum is a benign condition that often presents with chest pain or dyspnea. It can develop without a triggering event and with no findings on chest radiography. Treatment is expectant and recurrence is low. Secondary causes must be ruled out to avoid an unfavorable outcome.


   Introduction
Top
 Abstract
 Introduction
Material and Methods
 Results
 Comment
 Acknowledgments
 References
 
Pneumomediastinum is defined as the presence of free air in the mediastinum. Its presence is commonly viewed as an ominous finding, with potentially catastrophic complications. Pneumomediastinum is further divided into two groups of patients: spontaneous pneumomediastinum (SPM), without any obvious primary source, and secondary pneumomediastinum, with a specific responsible pathologic event, such as trauma, intrathoracic infections, or violation of the aerodigestive track.

Spontaneous pneumomediastinum was originally described by Louis Hamman in 1939 [1]; thus, the crepitus heard with the heartbeat on chest auscultation is known as the Hamman sign. Spontaneous pneumomediastinum is generally described as a benign condition, presenting in young adults exposed to a sudden pressure change within the intrathoracic cavity. The pathogenesis of spontaneous pneumomediastinum was first postulated by Macklin [2]. According to this report, a sudden increase in intrathoracic pressure results in increased intraalveolar pressure. The pressure differential created within the pulmonary parenchyma leads to alveolar rupture with further leakage of air throughout the interstitium and bronchovascular tissue sheath following a centripetal pattern toward the mediastinum. In clinical practice, SPM often develops as a result of various precipitating events triggering a strong Valsalva maneuver, creating the pressure differential needed for its genesis. In many instances, however, it is difficult to determine the precise source of the mediastinal air, and the diagnosis of SPM remains one of exclusion.

Secondary pneumomediastinum comprises the bulk of experience with pneumomediastinum. In the majority of cases, it is initiated by blunt or penetrating trauma, recent interventions in the esophageal or tracheobronchial tree, rupture of a hollow viscus, tissue dissection originating from a spontaneous pneumothorax, or pulmonary or mediastinal infection by gas-forming organisms [3]. Spontaneous pneumomediastinum is often difficult to differentiate from more subtle causes of secondary pneumomediastinum, such as contained esophageal perforation, minor tears in the central tracheobronchial tree, and smoldering pulmonary and mediastinal infections. It is essential to confirm the absence of a secondary pathologic event responsible for the presence of mediastinal air.

The purpose of this study is to report our experience in the management of 28 patients with SPM. Their clinical presentation, diagnostic evaluation, radiologic findings, and outcome are analyzed. A comparison with secondary pneumomediastinum with regard to radiologic findings and major outcome variables is conducted. An extensive review of the literature is provided.


   Material and Methods
Top
 Abstract
 Introduction
 Material and Methods
Results
 Comment
 Acknowledgments
 References
 
A retrospective comparative study of the cases of spontaneous pneumomediastinum diagnosed over a 12-year period is presented. Patients with SPM and a cohort of patients presenting with secondary pneumomediastinum during the same time frame were compared. Approval to conduct this retrospective study was granted by the Baptist Memorial Hospital and University of Tennessee (UT) Institutional Review Boards, and individual patient consent was waived. Data collection was performed following strict guidelines to protect patient information.

Between January 1995 and June 2006, all patients with an ICD code of mediastinal emphysema- pneumomediastinum were identified at Baptist Memorial Hospital and the Regional Medical Center, both teaching institutions of the UT Health Science Center. All charts were reviewed for demographic data, clinical presentation, radiologic studies, diagnostic interventions, and outcome. Spontaneous pneumomediastinum was defined as the radiologic confirmation of air within the tissue planes of the mediastinum without any obvious underlying etiology. If a pneumothorax was present, the patient was included only in the absence of pulmonary pathology explaining the initial event (eg, blebs, bullae, pulmonary emphysema). A triggering factor was defined as the most likely immediate event precipitating the SPM.

Spontaneous pneumomediastinum was excluded and the pneumomediastinum considered secondary in all cases of trauma-related admissions, recent aerodigestive tract interventions, recent thoracic, cardiac, abdominal, or cervical surgical interventions, cervical soft tissue or intrathoracic infection, esophageal perforation, human immunodeficiency virus disease with suspected Pneumocystis carinii pneumonia or recent endotracheal intubation. Pneumomediastinum was not routinely identified and coded in patients with penetrating trauma; therefore, these patients were excluded from the group of patients evaluated with secondary pneumomediastinum.

The variables assessed in both cohorts of pneumomediastinum patients were age, sex, chest pain, respiratory distress, presence of pneumothorax or pleural effusion during hospitalization, sensitivity of chest roentgenogram to diagnose pneumomediastinum, presence of subcutaneous emphysema on physical examination, presence of subcutaneous emphysema on computed tomography (CT) scan of the chest, hospital stay, and mortality.

Statistical Analysis
Categorical variables are expressed as percentages and continuous variables are expressed as mean ± SD. Univariate analysis using a {chi}2 test or Fisher's exact test when indicated was performed for categorical variables, whereas Student's t test was used when comparing continuous variables. A p value of less than 0.05 was considered statistically significant.


   Results
Top
 Abstract
 Introduction
 Material and Methods
 Results
Comment
 Acknowledgments
 References
 
A total of 74 patients were identified with mediastinal emphysema or pneumomediastinum. According to the inclusion and exclusion criteria previously defined, SPM was identified in 28 patients. The average age was 27 ± 17 years with a range from 3 to 71. Fifty-seven percent (16 of 28) were male.

The most frequently reported symptom was chest pain in 54% (15 of 28), followed by dyspnea in 39% (11 of 28) and cough in 32% (9 of 28). Relevant findings on physical examination included subcutaneous emphysema in 32% (9 of 28) and neck swelling in 14% (4 of 28). Pneumothorax was present in 7% of patients (2 of 28) upon admission. The pneumothorax in each patient was small and evident only on the CT scan of the chest (Table 1). No pathologic lung abnormality (eg, bleb, cavity, bullae) was identified as the etiology of the pneumothorax. Laboratory analysis included complete cell count, electrolytes, and arterial blood gases. The white blood cell count was elevated in 39% of patients (11 of 28). The remainder of the laboratory work performed was otherwise unremarkable.


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  		Table 1 Spontaneous Pneumomediastinum, Clinical Findings on Presentation
 
Radiologic images upon admission included chest roentgengram in 93% of patients (26 of 28) and chest CT scan in 71% (20 of 28). The chest roentgengram revealed mediastinal air in 69% (18 of 26) and subcutaneous air in 35% (9 of 26). The CT scan of the chest revealed mediastinal air in 100% (20 of 20) and subcutaneous air in 40% (8 of 20) of the patients assessed. Additional evaluation was performed on an individual basis, including contrast esophagram in 57% (16 of 28), esophagoscopy in 11% (3 of 28), and fiberoptic bronchoscopy in 4% (1 of 28). These interventions were invariably negative.

Medical history predisposing to the development of SPM included smoking in 29% patients (8 of 28), asthma in 14% (4 of 28), idiopathic pulmonary fibrosis in 7% (2 of 28), and chronic obstructive pulmonary disease in 4% (1 of 28). Inhalational drugs, although an established precipitating event for SPM, were not found in any of our patients (Table 2).


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  		Table 2 Predisposing Conditions in the Cohort of Spontaneous Pneumomediastinum Patients
 
Among the suspected triggering factors in the origin of SPM, emesis was noted to be the predominant initiating event in 36% of patients (10 of 28). Asthma exacerbation was seen in 21% (6 of 28) and cough in 7% (2 of 28). Additional triggering events noted were physical activity, defecation, and choking episodes. There was no apparent triggering factor for mediastinal air in 21% of patients (6 of 28; Table 3).


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  		Table 3 Triggering Event in the Origin of Spontaneous Pneumomediastinum
 
In the majority of cases, patients were admitted to the hospital, placed on oxygen, and treated expectantly. In many instances, oral intake was briefly withheld under the suspicion of esophageal perforation. Average hospital stay was 3 days (range, 0 to 11). There were no in-hospital deaths. Follow-up (1 to 10 years) was obtained in 79% of patients (22 of 28), and no recurrences were reported.

Among the patients with pneumomediastinum, 46 cases did not fulfill the criteria previously defined for SPM; this group of patients was considered to have secondary pneumomediastinum. This second cohort included patients in whom pneumomediastinum developed as a result of blunt thoracic trauma in 39% (18 of 46), barotrauma in 33% (15 of 46), P carinii pneumonia in human immunodeficiency virus patients in 6% (3 of 46), pneumothorax with pulmonary cavitary lesions in 6% (3 of 46), esophageal perforation in 4% (2 of 46), meconium aspiration in the newborn in 4% (2 patients), surgical intervention (tracheostomy in 2% [1 patient] and thyroidectomy in 2% [1 patient]), and amiodarone-induced lung injury in 2% (1 patient).

This group of 46 patients with secondary pneumomediastinum was compared with the original cohort of patients with SPM. Patients with secondary pneumomediastinum were of older age (39 versus 27 years, p < 0.05), were less likely to be diagnosed with chest roentgengram (47% versus 69%, p < 0.05), more likely to have an associated pneumothorax (56% versus 14%, p < 0.001), more likely to have a chest tube placed (46% versus 7%, p < 0.001), more likely to have an associated pleural effusion (12% versus 0%, p < 0.001), had a longer hospital stay (19 versus 3 days, p < 0.001), and were more likely to die (39% versus 0%, p < 0.001). In addition, patients with secondary pneumomediastinum were found to have a higher presence of clinical and radiologic subcutaneous emphysema compared with SPM patients (45% versus 32%, p < 0.05, and 64% versus 40%, p < 0.05, respectively; Table 4).


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  		Table 4 Spontaneous Pneumomediastinum Compared With Secondary Pneumomediastinum
 

   Comment
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
Acknowledgments
 References
 
Pneumomediastinum is viewed by the medical community as an ominous sign with potentially devastating complications. The differential diagnosis for chest pain, dyspnea, or subcutaneous emphysema focuses on cardiac or pulmonary sources, including acute coronary events, pericarditis, pulmonary embolism, pneumonia, and pneumothorax. Esophageal perforation, spasm, and reflux disease are also included in the differential diagnosis. Once these etiologic factors are excluded, a high degree of suspicion is required to consider SPM as the underlying condition.

Spontaneous pneumomediastinum is an unusual occurrence, with few reports in the literature [4–15]. According to our data, chest pain, shortness of breath, and subcutaneous emphysema are the most common presenting complaints. These findings are consistent with prior reports in the literature [4, 6–11].

In our study, SPM presented with similar frequency among men and women. This finding has also been reported by Mondello and coworkers [5] in their review of 18 cases and by Weissberg and Weissberg [7] in their review of 22 cases. In other studies, however, this condition has presented mainly in men [4, 6, 10]. Our review suggests that this condition presents predominantly in healthy young adults, consistent with other published reports [4–6].

According to previously published series, the presence of a predisposing condition ranges widely, from 20% to 80% [4, 6, 16, 17]. In our study, only 39% of cases presented with any of these risk factors. This may be explained by the variability among evaluating physicians in seeking and documenting predisposing conditions.

Most series describe the presence of a triggering event before the development of SPM. Numerous immediate preceding events have been noted, including forceful emesis [4], intense coughing [7], inhalational drugs [8], physical activity [18], intense screaming [11], spirometry [19], childbirth [20], bronchospasm [11], and playing of wind instruments [1]. There are variable data in the literature regarding triggering events responsible for the origin of the SPM; however, a compilation of reported triggering events lists the most frequent ones as emesis [4], intense physical activity [18], coughing [3, 5], and intense screaming [7].

There is agreement among published reports that in a significant number of patients, there is no evidence of a triggering event responsible for SPM. Among the reports listed in the bibliography, the presence of a precipitating event ranges from 39% to 100%. After combining all the series reported and adjusting for the sample size in each study, 32% of the patients had SPM without having a precipitating event identified. In our study, there was no evidence of a precipitating event in 21% of the patients.

The diagnosis of SPM is revealed by radiographic examination. A review of prior studies of SPM focuses primarily on the clinical presentation without discussion of the radiologic findings and no comparison with the overall population of patients with pneumomediastinum. The largest study was reported by Macia and associates [4] and included 41 patients over a period of 16 years, followed by a study by Campilo-Soto and colleagues [6] who reported 36 patients. In both publications, the diagnosis was made by chest radiograph alone, possibly overlooking patients identifiable only by CT. In our series, only 69% of the cases were discovered by chest radiograph, with the remaining 31% found on chest CT scan. There is only one report in the literature describing the correlation between chest radiograph and CT scan for the diagnosis of SPM. Kaneki and coworkers [10] described 33 patients with SPM, but 30% of the cases of SPM were missed with a plain chest x-ray film and were diagnosed only by CT scan of the chest (Fig 1). Computed tomography has become the gold standard for diagnosing pneumomediastinum.


Figure 1
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  		Fig 1. Spontaneous pneumomediastinum with extensive mediastinal and subcutaneous emphysema, discovered by computed tomography scan of the chest.

 
The lack of sensitivity of the plain chest x-ray film to detect secondary pneumomediastinum is explained by the severity of the pathology. In general, SPM is a benign condition with preserved lung fields and mediastinal structures. In cases of secondary pneumomediastinum, as seen in blunt thoracic trauma and intrathoracic septic processes, there is additional pathology involved that may obscure otherwise distinct radiographic findings of mediastinal air. In our series, in patients with secondary pneumomediastinum, plain chest x-ray film was found to have a higher incidence of associated pneumothorax and pleural effusion and a lower sensitivity for the diagnosis. In addition, subcutaneous air was more frequently seen by CT scan in secondary pneumomediastinum as compared with SPM (64% versus 40%, respectively; p < 0.05). This comparative analysis has not been previously reported.

Recurrence of spontaneous pneumomediastinum has been rarely reported. The first case was described by Yellin and colleagues [17] in 1983. Recurrence has been reported in 5 additional patients [4, 9, 11, 14, 18]. No recurrence was identified in our patients with 79% follow-up during a 1- to 10-year period.

In conclusion, SPM is a benign condition presenting primarily in young adults, with an uneventful recovery and unlikely recurrence. The clinical presentation in the majority of cases involves chest pain, shortness of breath, or subcutaneous emphysema. A triggering event with a sudden increase in intrathoracic pressure is frequently seen, and emesis is the most frequent inciting event. Diagnosis requires a high level of suspicion as more than 30% of patients present without any precipitating factor, and more than 30% of cases can be missed by a plain chest radiograph. Other sources of mediastinal air need to be excluded, because secondary pneumomediastinum may have an unfavorable outcome if not diagnosed promptly.


   Acknowledgments
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
 Acknowledgments
References
 
The authors thank Pansy Adams, library technician, KCTCS Library, for her helpful assistance in preparation of this manuscript.


   References
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
 Acknowledgments
 References
 

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