Pulmonary air leak occurs more frequently in the newborn period than at any other time of life. It occurs when air escapes from the lung into extra-alveolar spaces where it is not normally present. The resulting disorders depend upon the location of the air. The most common conditions are pneumothorax, pneumomediastinum, pulmonary interstitial emphysema, and pneumopericardium. Rarer forms are pneumoperitoneum and subcutaneous emphysema.
Air leak begins with the rupture of an overdistended alveolus . Overdistention may be due to generalized air trapping or uneven distribution of gas. The air dissects along the perivascular connective tissue sheath toward the hilum, resulting in a pneumomediastinum, or into the pleural space, producing a pneumothorax . Less commonly, air may dissect into the pericardial space, subcutaneous tissue, or peritoneal space, causing pneumopericardium, subcutaneous emphysema, and pneumoperitoneum, respectively.
The perivascular connective tissue is more abundant and less dissectible in preterm than in older infants. This predisposes to air trapping in the perivascular space, resulting in pulmonary interstitial emphysema (PIE) .
Data on the incidence of air leak primarily reflect the incidence of pneumothorax, because that disorder is most common. The incidence depends upon factors including birth weight, the presence of lung disease, and the method of detection. In a report from 1930 in which chest radiographs were performed on 702 consecutive newborns, the incidence of spontaneous pneumothorax was 1 to 2 percent of live births, mostly in infants with otherwise normal lungs .
The incidence is higher in preterm infants, who often have pulmonary disease. In a report from the Vermont Oxford database, pneumothorax was reported in 6.3 percent of 26,007 infants with birth weight 500 to 1500 grams in 1999 . Since that published report, the incidence of pneumothorax has decreased to 4.1 percent of 55,921 infants with birth weight 500 to 1500 grams registered in the Vermont Oxford Network database in 2013 .