Retroperitoneal Detail

Over the last few days, I’ve talked about the different appearances of poor peritoneal detail. To recap, detail can be focally or diffusely poor, or mottled. Diffuse causes are usually lack of fat or peritoneal effusion, while local or mottled poor detail can be caused by inflammation, small amount of fluid, or carcinomatosis.

The retroperitoneal space is a bit neglected on radiographs. Like the peritoneal space, it’s a potential space that is mostly filled with fat. It contains the kidneys, adrenals, ureters, and most of the bladder. In cats especially, the retroperitonel space is a depot for fat. It makes the kidneys very visible with its nice fat-soft tissue interface. Ureters are rarely seen, but can be visible if they are dilated, or if the animal is pu/pd.

Poor detail in the retroperitoneal space is almost always streaky or mottled. You will see bands of soft tissue opacity running roughly parallel to the spine. I think it’s because the fluid or inflammation tracks along the planes of fat, since the retroperitoneal space is less distensible than the abdomen. What are the causes of poor retroperitoneal detail? Just think of the organs that reside in the space. Hemorrhage can arise from trauma or mass lesions, and urine from ruptured kidney or ureter. Inflammatory conditions such as peritonitis are less likely to cause poor retroperitoneal detail, though you can see localized fluid and hyperechoic fat on ultrasound.

Another differential to keep in mind is ruptured urethra. Since the retroperitoneal space encompasses the bladder, urethral tears in cats can cause urine to track retrograde into the retroperitoneal space. A urethrogram will make this diagnosis.

So keep an eye out, and be alert for this radiographic sign. Once you see it, you’ll recognize it again, and it is usually an important finding.