Pneumomediastinum (PMS)

Michael P. D'Alessandro, M.D.
Peer Review Status: Internally Peer Reviewed

Etiology/Pathophysiology:
In the newborn, the periphery of the lung is hyperinflated. Due to uneven distention of alveoli and poorly developed collateral air channels, a pressure gradient develops between the alveoli and the interstitial spaces. This causes alveoli to rupture into the interstitium, with air then extending into the interstitium and then dissecting centrally to the hila and mediastinum along the perivascular sheaths/bronchovascular bundles. The pneumomediastinum may extend under the diaphragm by dissecting along the esophageal hiatus and other diaphragmatic foramina. Air can accumulate in the retroperitoneum or dissect peripherally through the mesenteric perivascular spaces to rupture into the peritoneal cavity. Air can dissect upward into the neck as well. Spontaneous pneumomediastinum usually only causes minimal respiratory distress and resolves with conservative therapy. Non spontaneous pneumomediastinum is most often due to air leak complications in patients on mechanical ventilation.

Pathology:
Not applicable.

Imaging Findings:
Air is seen in the mediastinum, usually collecting in the anterior and middle mediastinum. The thymus is often well outlined by the mediastinal air. A "continuous diaphragm sign" may be seen, with air in the retrocardiac space outlining the diaphragm under the heart. The continuous diaphragm sign may also be seen in pneumopericardium.

DDX:

• Pneumopericardium - air does not dissect above the level of the great vessels, unlike in pneumomediastinum where air may dissect up into the neck.

References:
See References Chapter.

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