Figure 1: Abdominal xray showing free air in the retroperitoneal space.

(Click on image to enlarge)



The retroperitoneum is the posterior aspect of the abdominal cavity, the chief structure within being the kidney.


The retroperitoneum is chiefly composed of 3 further spaces: – the perirenal space along with both the anterior and posterior renal spaces.


It is far more uncommon for a pneumoretroperitoneum to occur in this than the pneumoperitoneum.


When this does occur, the plain radiograph findings are different, but also very distinct in appearance (Figures 1 & 2).


The air outlines the kidney – giving a reniform appearance and also tracks along the psoas muscle, giving one of the body’s few natural straight lines on imaging (Figure 3).


If a CT is undertaken to further investigate the cause of the perforation, it illustrates the gas distribution exquisitely and shows the confinement of gas within the retroperitoneum (Figure 4).

Figure 2: Red arrows indicating site of free air in the retroperitoneal space.

(Click on image to enlarge)

Figure 3: Annotated abdominal xray showing free air in the retroperitoneal space outlining the left kidney and psoas muscle clearly.

(Click on image to enlarge)

Figure 4: CT scan of the abdomen showing dark areas of free air anterior and posterior to the right kidney, anterior to the left kidney and in the mesentry, indicating free bowel perforation.

(Click on image to enlarge) 


Causes of a pneumoretroperitoneum include:

-          Duodenal perforation (D1-2)

-          Post-operative – especially renal and rectal surgery

-          Emphysematous pyelonephritis

-          Selective colonic perforations

-          Iatrogenic following endoscopy ( as in this case )

-          Trauma




Images and text contributed by

Dr Ian Bickle, Department of Radiology,RIPAS Hospital.

All images are copyrighted and property of RIPAS Hospital.