Background: The sonographic finding of an anechoic retroperitoneal abnormality suggests a fluid collection (e.g., abscess, urinoma, hematoma), Our study was performed to evaluate cases in which this sonographic finding appeared to be a manifestation of systemic edema. Subjects and Methods: Inpatient sonograms performed over a 40-month period were reviewed for the presence of anechoic areas suggestive of fluid collection in the retroperitoneum of the flank, Records of patients with such findings were reviewed for evidence of retroperitoneal abscess, urinoma, or hemorrhage, as well as for the presence and cause(s) of peripheral edema. Results: Of the 29 patients identified with sonographic findings suspicious for retroperitoneal fluid collection, 13 (45%) had no cause for and no clinical evidence of focal retroperitoneal collection, All 13 patients had peripheral edema attributable to hypoalbuminemia, congestive heart failure, overhydration, cirrhosis, and/or the systemic inflammatory response (multiple organ failure) syndrome. Resolution of the retroperitoneal abnormality following therapy for congestive heart failure was documented in one case, and CT scan confirmed retroperitoneal edema in another. Conclusion: Anechoic regions that represent edema can be seen on sonograms of the retroperitoneum in patients with conditions that cause edema in other regions. The possibility of edema mimicking a fluid collection should be particularly considered prior to invasive procedures in the retroperitoneum.
