What are some causes of pneumoperitoneum.
Perforation of a hollow abdominal viscus is the commonest cause of pneumoperitoneum. Other causes include postoperative intraperitoneal free gas, peritoneal dialysis, or dissection of gas from adjacent structures, e.g. pneumomediastinum or pneumothorax.
What would the best next test be?
Obviously CT would be the next best test. It may be possible to determine the source of free gas. In cases of perforation, even if the perforation cannot be directly visualised, the differential can be narrowed.
How would you protocol the CT?
CT domes to symphysis with IV contrast and water prep. Always worth getting a coronal reformat thrown in too.
If there was concern for perforation and the CT showed lots of gas and minimal free fluid, what would be most likely site of perforation be?
Broadly speaking, more gas than fluid suggests a lower GI perforation while more fluid than gas means an upper GI perforation is more likely. With this age-group, common causes for lower GI perforation include diverticular disease and tumour-related perforation.
A large amount of gas is seen under both diaphragmatic domes (yellow dotted lines). In the left upper quadrant gas is also seen in the stomach (red dotted line) and splenic flexure of the colon (blue dotted line).