Subpulmonary pneumothorax.

Seven cases of subpulmonary pneumothorax are presented: four due to penetrating injury, two to blunt trauma and one to osteosarcoma metastasis. The typical and diagnostic appearance is a basal band of radiolucency bounded above by the thin hair-line of visceral pleura paralleling the dome of the hemi-diaphragm. When partially clotted blood is also present, the appearance becomes less typical and has to be differentiated from traumatic diaphragmatic herniation of bowel and from traumatic pneumatocoele by barium studies and by decubitus radiographs respectively. It is the bridge-like disposition of the pleural cavity between the dome of the hemi-diaphragm and the hollowed concavity of the lung base which allows pneumothorax to collect in it. It is rarely seen because blebs and bullae which are the commonest causes of pneumothorax are most often located in the upper zones.