Idiopathic retroperitoneal fibrosis results from layers of fibrous tissue deposited along the posterior aspect of the peritoneal cavity. Although the extent and distribution of these fibrotic layers differs, the greatest amount of fibrosis is usually located anterior to the sacrum and the inferior lumbar spine. It frequently envelopes hollow structures such as the ureters, blood vessels, and lymphatics, and occasionally bowel loops. Retroperitoneal fibrosis occurs in association with many pathologic conditions including mediastinal fibrosis, sclerosing cholangitis, pseudotumor of the orbit, and Reidel's thyroiditis. Retroperitoneal inflammation, hemorrhage, and neoplastic involvement may cause retroperitoneal fibrosis. In addition, iatrogenic retroperitoneal fibrosis associated with the use of pharmacologic agents such as "SANSERT" has been reported. Clinical findings in retroperitoneal fibrosis include a history of backache, abdominal pain, and lower extremity edema. Laboratory findings include increased sedimentation rate and anemia. The typical radiographic findings include bilateral hydronephrosis, medial displacement, and compression of the ureters. Hydronephrosis is not always bilateral, and in some cases may be absent. Cross-sectional imaging techniques such as ultrasound, CT, and MRI can demonstrate characteristic retroperitoneal masses; however, needle or surgical biopsy is usually required to unequivocally establish the diagnosis of retroperitoneal fibrosis.
The sonographic appearance of retroperitoneal fibrosis is that of a homogeneous, hypoechoic, or anechoic mass usually associated with hydronephrosis. However, minor degrees of caliectasis in the presence of severe renal failure may be observed. Patency of compressed ureters may be demonstrated by retrograde passage of a catheter in these patients. The bulk of the retroperitoneal mass is located in the anterior retroperitoneum; however, the lateral aspects of the mass may extend into the perivertebral region. The anterior border of the mass is usually smooth with a relatively homogeneous internal consistency. However, the mass may occasionally be somewhat bulky and multilobulated. It is said that retroperitoneal fibrosis envelops the anterolateral aspects of the abdominal aorta but usually spares the posterior aspect of this vessel.
The differential diagnosis includes retroperitoneal lymphoma, sarcoma, hematoma, or abscess. Ultrasound is particularly useful in distinguishing retroperitoneal fibrosis surrounding a normal aorta from periaortic aneurysmal hemorrhage and/or peritoneal fibrosis.