What is meant by duplication of the collecting system, and what is its appearance on radiographic studies?
Duplication of the collecting system is a very common congenital anomaly and is a frequent finding on imaging studies. Embryologically, the ureter is derived from the mesonephric duct, and it ascends cephalad to meet with the primitive kidney, the metanephric blastema, to form the adult kidney and collecting system. If there are two buds developed from one mesonephric duct, there will be complete duplication of the collecting systems with two ureteral orifices in the bladder on the side of the duplication. In most completely duplicated systems, both ureters insert at the normal site in the bladder trigone, known as orthotopic insertion. However, there can be ectopic insertion of the ureters. The upper pole ureter usually inserts ectopically, below the bladder trigone, and is also obstructed, while the lower pole ureter inserts at the normal site in the bladder trigone but may have vesicoureteral reflux (VUR). This constellation of abnormalities is known as the Weigert-Meyer rule. The obstructed upper pole collecting system can cause mass effect on the lower pole ureter, and this appearance of the lower pole collecting system is referred to as a “drooping-lily” sign.
With partial or incomplete duplication, a single ureteral bud splits into two as it extends cephalad. It is important to determine where the two ureters join if urologic surgery is planned.
Duplication anomalies are well demonstrated on CTU, MRU, IVU, and retrograde pyelography. Awareness of these anomalies is important in patients undergoing urologic surgery, so that the surgeon can avoid inadvertent ureteral injury.