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

Pathology

Pneumoretroperitoneum is always abnormal and has a relatively small differential:

• perforated retroperitoneal hollow viscus

  • duodenum

    • peptic ulcer disease ¹²

    • blunt or penetrating abdominal trauma ¹²

    • endoscopy +/- biopsy (rare) ³

    • ERCP: especially when a sphincterotomy is performed; incidence 0.5-2% ^3,12

  • ascending/descending colon

    • colorectal carcinoma (CRC)

    • diverticulitis

    • endoscopy +/- biopsy

    • ischaemic colitis

  • rectum

    • surgery, e.g. transanal excision of rectal carcinoma ²

    • foreign body insertion

    • endoscopy

    • trauma

  • 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.

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 ⁶. There may also be a loss of the normal psoas muscle shadow ⁶.

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 ¹⁰:

• right kidney

  • referred to as the veiled right kidney sign ⁷

    • will not change appearance with patient re-positioning, as opposed to the free air in pneumoperitoneum ¹¹

• great vessels

  • the disappearance of the retroperitoneal inferior vena cava and abdominal aorta ⁹

• 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:

• pseudopneumoretroperitoneum

• pneumoperitoneum

• subcutaneous emphysema

CT has little difficulty in distinguishing these.

See also

• abnormal intra-abdominal gas