Pneumoretroperitoneum

Pneumoretroperitoneum, as its name spells out, is gas in the retroperitoneal space. In the bad old days of radiology, insufflation of the retroperitoneal space was an important method of examining retroperitoneal organs such as the pancreas. In modern times, pneumoretroperitoneum is usually due to penetrating trauma or perforation of the retroperitoneal portions of the gastrointestinal tract (duodenum, ascending colon, descending colon, and rectum).

The differential diagnosis for the site of injury can be narrowed or rearranged by noting the location of the gas. Gas in the perirenal space is most commonly due to renal infection. Indeed, focal pneumoretroperitoneum anywhere can be sign of a retroperitoneal abscess.

Gas in the right anterior pararenal space is usually due to pathology in the descending portion of the duodenum, while gas in the left anterior pararenal space is more commonly due to pathology in the descending or sigmoid colon. Bilateral anterior pararenal space gas can be due to sigmoid or rectal perforation or complicated pancreatitis.

Gas in the posterior pararenal space bilaterally can be due to rectal perforation or dissecting gas from above the diaphragm. On the other hand, pneumomediastinum can also be seen secondarily when gas dissects superiorly through the aortic hiatus.

Finally, when gas is isolated to the left posterior pararenal space, one can consider pathology related to the sigmoid colon.

Radiographic findings of pneumoretroperitoneum include gas along the psoas margins and around the kidneys and other retroperitoneal organs. However, CT nicely shows the gas confined to the retroperitoneal space. A CT finding related to colonic perforation is the so-called dirty mass, a focal collection of extraluminal fecal matter close to the perforation site.

In the case above, we see gas dissecting along the aortic hiatus and outlining the proximal portions of the psoas muscles. The CT shows gas predominantly in the right anterior pararenal space, but also seen on the left. A stent is present in the descending colon in this patient with carcinoma of the colon.

References

• Dalrymple NC, Leyendecker JR, Oliphant M. Problem Solving in Abdominal Imaging. Elsevier Health Sciences, 2009. Page 153.
• Saeki M, Hoshikawa Y, Miyazaki O, Nakayama F, Okamoto E, Ishikawa T. Computed tomographic analysis of colonic perforation: "dirty mass," a new computed tomographic finding. Emerg Radiol 1998; 5: 140-145.