In the lateral decubitus chest radiograph, how can a small pneumothorax be identified?

• A. Cardiac silhouette enlarges
• B. Air will layer dependently beneath the chest wall
• C. Lateral costophrenic angle blunts
• D. Diaphragm becomes elevated

Answer: (B)

In a lateral decubitus view, a small pneumothorax is most readily seen when air collects against the chest wall on the dependent side, producing a distinct pleural line where the visceral pleura is separated from the parietal pleura. The air layers beneath the chest wall in this position, so you look for that sharp edge with absence of markings beyond it to identify the pneumothorax.

Other findings listed are not the telltale signs of a small pneumothorax on this projection: enlargement of the cardiac silhouette isn’t specific to pneumothorax, blunting of the lateral costophrenic angle suggests effusion, and diaphragmatic elevation is nonspecific.