The main causes of pneumomediastinum are
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Alveolar rupture with dissection of air into the interstitium of the lung with translocation to the mediastinum
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Esophageal perforation
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Esophageal or bowel rupture with dissection of air from the neck or the abdomen into the mediastinum
The primary symptom is substernal chest pain which can, on occasion, be severe.
Physical examination may show subcutaneous emphysema, usually in the suprasternal notch, along with a crunching or clicking noise synchronous with the heartbeat; this noise is best heard over the heart when the patient is in the left lateral decubitus position (Hamman sign).
The diagnosis is confirmed by chest x-ray, which shows air in the mediastinum.
Treatment
Treatment usually is not necessary, although tension pneumomediastinum with compression of mediastinal structures (rare) can be relieved with needle aspiration, leaving the needle open to the atmosphere as is done with tension pneumothorax. Hospital admission is required if pneumomediastinum is secondary to esophageal or bowel rupture but not necessarily if secondary to alveolar rupture.
