Respiratory distress, often minimal. 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. 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. The thymus is often well outlined by the mediastinal air. The continuous diaphragm sign may also be seen in pneumopericardium. Pneumopericardium - air does not dissect above the level of the great vessels, unlike in pneumomediastinum where air may dissect up into the neck.