A peribronchial abscess forms that communicates with the airway permitting entry of air into this abscess & when exudate clears, there is development of a large thin walled cavity- the pneumatocele. The growth of the pneumatocele occurs because of a ball valve obstruction at the site of perforation of airway into the cavity.

The pneumatocele is formed by distension of small bronchioles owing to ball valve effect secondary to bronchial obstruction.

Current most favored explanation for pneumatoceles is that they are subpleural collection of air formed by dissection of air from ruptured bronchi or alveoli. The dissection occurs through the interstitium of the lung. On radiographs, pneumatoceles appear as thin walled air containing lesion - they may show air fluid level.

Depending on size, it shows mass effect / mediastinal shift. They usually appear during the first week of pneumonia & almost always are gone within 6 weeks. The pneumatocele occurs when child is getting better & its presence does not denote any dire prognostic consequences. Approximately 10% of pneumatocele rupture & cause pneumothorax.