Visible muscle of the diaphragm: sign of extraperitoneal air.

Epiphrenic air collections may be difficult to compartmentalize. The air may be in the peritoneal cavity, extraperitoneal or extrapleural spaces, or in a pleural cavity. Extraperitoneal and extrapleural air usually comes from a pneumomediastinum, but it may arise from an intraabdominal source, such as a ruptured second part of the duodenum. Pneumoperitoneum is frequently the only radiographic sign of a perforated hollow viscus. The radiologist is confronted with an important differentiation. A pneumoperitoneum may indicate a need for abdominal surgery and a pneumothorax may require surgical aspiration; but extraperitoneal and extrapleural air require no specific treatment, assuming that the source of the air can be ascertained and the quantity does not compromise respiration. A new sign is described to assist in the recognition of extraperitoneal and extrapleural air. When the muscle bundles of the diaphragm are outlined by air, the air must be between the parietal peritoneum and parietal pleura and thus be extraperitoneal and/or extrapleural in location. This sign was observed in five patients.