Lower moiety hydronephrosis in duplicated kidneys.

Because of the high incidence of lower pole reflux in complete duplications, decreased, delayed, or absent function in a hydronephrotic lower pole moiety is usually initially attributed to reflux nephropathy. Obstruction occurs commonly in the upper pole moiety due to an ectopic ureteral orifice or ureterocele. However, obstruction is not uncommon in the lower pole segment. Here, the etiologies are more numerous and, on occasion, result in confusing presentations. Eleven cases of obstruction involving the lower pole moiety are discussed. Pathologic entities involved include ureteropelvic junction obstructions in both complete and incomplete duplications, calculi and epithelial tumors obstructing the lower system at various levels, and an ectopic upper pole ureterocele compressing and obstructing the ureteral orifice to the lower pole.