Ureteropelvic junction obstruction associated with extrarenal pelvis: A potential cause of cystic abdominal mass anterior to a normal-appearing kidney in the newborn.

An extrarenal pelvis is associated with the absence of central sinus echoes on sonography. However, central sinus echoes are normally inapparent in some newborns. Furthermore, true absence of a central sinus may cause calices to simulate normal renal pyramids so that the kidney appears normal in a fetus or newborn. This case illustrates the potential for an obstructed extrarenal pelvis in a fetus or newborn to distend so that it is mistaken on sonography for a cystic anterior abdominal mass unrelated to the kidney. However, the absence of central sinus echoes should suggest the diagnosis of an obstructed extrarenal pelvis if the adjacent cystic abdominal mass is positioned to obstruct an intrarenal pelvis. Copyright 1999 John Wiley & Sons, Inc.