Pneumomediastinum is the presence of air in the mediastinal soft tissues. Primary pneumomediastinum is caused by increased pressure within the airways and alveoli that results in airway rupture and dissection of interstitial air along peribronchial and perivascular planes into the mediastinum. The underlying mechanisms for development of pneumomediastinum include elevation of airway pressures and/or dramatic decreases in intravascular pressure, causing a relative pressure gradient in the perivascular spaces.¹⁵ Once a breach of airway integrity occurs, air can dissect relatively freely along bronchovascular structures and extend centrally into the mediastinum. From the mediastinum, air under pressure can dissect along tissue planes into the neck, retroperitoneum, subcutaneous tissue, and epidural space. Although such dissection of air is usually limited to the soft tissues, rupture of the visceral or parietal pleura may result in secondary pneumothorax. Even less commonly, air may dissect through the peritoneum, causing pneumoperitoneum. Air dissection along the pulmonary veins causing pneumopericardium has been induced experimentally¹⁵ and reported clinically.²²

Spontaneous pneumomediastinum has been reported most frequently in young patients, with an incidence of 1:25,000 in those between 5 and 34 years of age.¹⁰ A true incidence is difficult to assess, as many episodes probably go undiagnosed because symptoms are too minimal for patients to seek medical care. Additionally, pneumomediastinum may be too small to be detected by chest radiograph, causing symptoms to be ascribed to musculoskeletal pain or other causes.¹¹ Approximately 78% to 85% of cases occur in males.^1,5,14,19,20 Although an association is described with tall, thin body habitus,^1,9,10 in one series, 10 of 18 patients were obese.¹⁹

Primary pneumomediastinum pathology or physiology is usually associated with conditions that result in a marked increase of intrapulmonary pressures (Table 172-1). COPD, bronchitis, or asthma exacerbations account for 40% to 50% of patients with primary pneumomediastinum. Severe anorexia nervosa has been reported, with a disproportionate incidence suggesting that malnutrition in this group may play a role.²⁶ Illicit drug use associated with primary pneumomediastinum has typically involved inhalational drugs used in conjunction with prolonged breath holding.¹⁸ However, primary pneumomediastinum has also been seen in noninhalational use of speed (methyl-dimethoxyamphetamine) or ecstasy (methylene-dioxy-metamphetamine), wherein high levels of physical activity may have caused alveolar rupture. Strenuous physical exercise with frequent yelling, such as that experienced by military recruits in training or during sports activities, is sufficient to cause pneumomediastinum.^7,16,17

Most patients are symptomatic at the time of presentation, as asymptomatic patients would rarely undergo the sort of testing that would detect primary pneumomediastinum. The signs and symptoms in adults and adolescents from several series are summarized in Table 172-2. Pain is typically pleuritic, associated with shortness of breath, and described as retrosternal and radiating to the neck or back. Patients presenting with chest pain and anxiety of unclear origin with normal chest radiographs should be considered for computed tomography (CT) of the chest, as this may diagnose pneumomediastinum and minimize further unnecessary diagnostic evaluation (Fig. 172-1).

Physical findings range from normal exam to profound subcutaneous emphysema, with anxiety, tachycardia, and tachypnea. In over half of the cases, auscultation will reveal a precordial crunching sound synchronous with the heartbeat (Hamman’s sign). This may be accentuated with expiration and when the patient is lying in the left lateral decubitus position.

Although they may be anxious, patients with primary pneumomediastinum generally appear well. Vital signs are usually normal, although anxiety and pain can cause mild tachycardia and tachypnea. Low-grade fever (38°C–38.5°C) has been associated with as many as 22% of patients.¹⁹ Dissection of air
into the soft palate may give the patient’s voice a nasal quality (rhinolalia⁹). However, the dissection of air does not progress submucosally or in the tissues of the larynx and will therefore not cause airway obstruction.