Pneumoretroperitoneum is by definition presence of gas within the retroperitoneal space. It is always abnormal and has a relatively small differential:
perforated retroperitoneal hollow viscus
- ascending/descending colon
- rarely an intraperitoneal hollow viscus can perforate into the intramesenteric space and then track air to the retroperitoneal spaces
residual air from retroperitoneal surgery
- urological/adrenal 4
- spinal (anterolateral approach) 5
If localised, and especially in the presence of an air-fluid level, a retroperitoneal abscess should be suspected.
On plain radiography the differential is that of gas in other spaces which also projects over the abdomen. It thus includes:
CT has little difficulty in distinguishing these.
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