Benign post-traumatic pseudopneumoperitoneum

Benign post-traumatic pseudopneumoperitoneum is the presence of ectopic gas typically between the 5^th-10^th intercostal spaces after high-energy trauma in the absence of other causes of pseudopneumoperitoneum such as pneumomediastinum ⁹. 

Epidemiology

Occurs with an incidence of 5% post-trauma (after excluding known causes for pseudopneumoperitoneum). The true incidence is unknown and difficult to determine but probably higher than this.

Clinical presentation

Typically younger patients sustaining high velocity-velocity trauma.

Pathology

Pseudopneumoperitoneum is a well recognised post-traumatic phenomenon in patients with pneumomediastinum and pulmonary interstitial emphysema ^1-4 where the gas is able to track down pressure gradients along tissue planes into the retroperitoneum (via the diaphragmatic apertures) and extraperitoneal space (via the foramen of Morgagni) ^1-7 by a process called Macklin effect ⁸.

A further relatively common, but under recognised cause for post-traumatic pseudopneumoperitoneum in trauma has recently been described ⁹. The finding is clinically benign, mutually exclusive from other causes of pseudopneumoperitoneum ⁹. 

Aetiology

The hypothesised mechanism for ectopic gas production mimicking pseudopneumoperitoneum is by vacuum phenomenon.  Rapid changes in local tissue pressure, acute fractures or acute soft tissue disruption can cause dissolution of nitrogen gas from the adjacent soft tissues resulting in small pockets of gas visible on CT ¹⁴. The association of benign post-traumatic pseudopneumoperitoneum with high velocity-velocity trauma supports this theory, by the fact that rapid changes in local tissue pressure from an external impact or local tissue disruption (diaphragmatic ligament attachment tears or costochondral microfractures) could create the required microenvironment for vacuum phenomenon.

Location

Between the 5^th-10^th intercostal spaces adjacent to the costochondral junctions and anterior diaphragmatic attachments.

Associations

There is a statistically significant association with high velocity-velocity trauma (with an incidence of 12.5% of all high velocity-velocity trauma) ⁹.

Radiographic features

CT

Small pockets of gas located anteriorly between the 5th and 10th intercostal spaces, near the costochondral junctions. The gas often strongly resembles pneumoperitoneum. On closer inspection, the gas may be appreciated external to the peritoneal surface. In many instances, however, the exact location of the gas is unable to be accurately resolved with CT.   

Treatment and prognosis

The finding is clinically benign but important to discriminate from true pneumoperitoneum in order to prevent clinical uncertainty. Ectopic gas occurring in the typical distribution for benign post-traumatic pseudopneumoperitoneum in a patient without ancillary imaging features for bowel injury should prompt the reader to consider this as a cause for the finding.