Which radiographic diagnosis is most likely when there is a hyperlucent rim of air lateral to the cardiac border beneath the thymus with the thymus displaced superiorly?

• A. Pneumopericardium
• B. Pneumomediastinum
• C. Pneumothorax
• D. Asthma

Answer: (B)

Recognizing air in the mediastinum on a pediatric chest radiograph is key. A hyperlucent rim of air along the mediastinal border, especially just beneath the thymus, indicates free air within the mediastinal space. The thymus in infants sits in the superior mediastinum and can be pushed upward when air accumulates in that area, so seeing the thymus displaced superiorly with lateral air supports pneumomediastinum.

This differs from pneumopericardium, where air surrounds the heart itself and would create a radiolucent halo around the cardiac silhouette rather than a lateral border finding. It also differs from pneumothorax, which would show a pleural line with loss of lung markings peripherally and typically not specifically beneath the thymus. Asthma is a clinical diagnosis and isn’t identified by this mediastinal air pattern.

So the finding described is most consistent with pneumomediastinum.