retroperitoneal gas

Pneumoretroperitoneum is by definition presence of gas within the retroperitoneal space.

Pathology

Pneumoretroperitoneum is always abnormal and has a relatively small differential:

If localized, and especially in the presence of an air-fluid level, a retroperitoneal abscess should be suspected.

Radiographic features

Pneumoretroperitoneum is best appreciated by CT, however, can also be detected by plain abdominal radiograph and even by transabdominal ultrasound. Generally, the air is most commonly seen surrounding the kidneys in the right and left upper quadrants of the abdomen . There may also be a loss of the normal psoas muscle shadow .

Ultrasound

Interfaces between free air and soft tissues appear as echogenic lines with posterior reverberation artifacts and the obscuration of far-field structures. In pneumoretroperitoneum air will collect around the following structures :

  • right kidney
  • great vessels
    • the disappearance of the retroperitoneal inferior vena cava and abdominal aorta
  • head of the pancreas
  • gallbladder
    • retroperitoneal air will collect posteriorly 
  • duodenum

Differential diagnosis

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.

See also