Ultrasound finding of peripelvic urine extravasation in ureteropelvic junction obstruction.

Pylosinus backflow is usually associated with acute obstruction and occurs as the result of increased hydrostatic pressure in the renal pelvis. Microruptures or tears occur at the calyceal fornix. The extravasating urine may then be absorbed by lymphatics, rupture into veins, or dissect in the renal sinus. As extravasating urine extends from the peripelvic tissues into the perirenal space, it becomes easily visualized by ultrasound as a perirenal fluid collection. We describe the ultrasound finding of decreased peripelvic echogenicity in an infant with ureteropelvic junction obstruction and peripelvic urine extravasation. To our knowledge this finding on ultrasound has not been previously described. If not for the peripelvic decreased echogenicity, the resulting pelvic decompression may have otherwise mimicked a nonobstructed kidney.