DDX of appearances of pneumoperitoneum

Approach to the DDX of the different appearances of pneumoperitoneum:

  • On CXR upright + AXR upright: look for air beneath diaphragm
  • On AXR left lateral decubitus: look for air above the liver
  • On AXR cross-table lateral: look for tell-tale triangle sign of air between bowel loops and air superiorly between abdominal wall and viscera
  • On AXR supine: look for overall increased lucency of abdomen, American football sign of falciform ligament, Rigler’s sign (see air outlining both sides of bowel wall)
  • Note: in inflamed abdomen (due to perforation), pneumoperitoneum may be loculated and not be classically seen on upright / decubitus / cross-table lateral views, may appear as fixed air collection that does not appear to conform to bowel