Figure 21a. Pneumomediastinum in a 5-year-old boy with a history of trauma who developed swelling of the neck, face, and thorax. Posteroanterior chest radiograph (a) and lateral radiograph of the neck (b) show pneumomediastinum and air in the neck and soft tissues. Note the air in the carotid space (arrows in a) and retropharyngeal space (arrowhead in b).