Imaging description

In the emergency setting, free intraperitoneal gas, or pneumoperitoneum, usually represents perforated bowel. Unless the patient has a history of recent abdominal surgery or penetrating abdominal injury, pneumoperitoneum usually indicates an emergent laparotomy.

However, accumulation of gas in the extraperitoneal spaces which lie adjacent to the peritoneal space may simulate pneumoperitoneum. Moreover, gas within bowel loops on plain radiography may simulate subdiaphragmatic air.

Gas beneath the right hemidiaphragm can be simulated by colonic or small bowel interposition between the liver and hemidiaphragm (Figure 51.1), and was first described by Chilaiditi in 1910 [1, 2].

Gas within the subperitoneal space, which is extraperitoneal, can originate within the mediastinum or rectum and spread throughout the abdomen, and simulate intraperitoneal air [3]. The subperitoneal space lies subjacent to the parietal peritoneum, whereas the peritoneal space lies between the visceral and parietal peritoneum [3].

There are areas of continuity between the extrapleural space in the chest and the subperitoneal space in the abdomen. The principal communications lie posteriorly, and are formed by the esophageal and aortic hiatus of the diaphragm. Additionally, two areas of continuity lie between the anterior attachments of the diaphragm to the lower sternum and ribs. These are termed the sternocostal triangle (or foramina of Morgagni) and contain connective tissues, the superior epigastric continuations of the internal thoracic (mammary) arteries, and lymphatics.