Duplex collecting systems can be unilateral or bilateral and can be associated with a variety of congenital genitourinary tract abnormalities. Most patients are asymptomatic, with genitourinary tract abnormalities being detected incidentally on imaging studies performed for other reasons. Symptomatic patients usually have complete ureteric duplication in which the ureters are prone to developing obstruction, reflux, and infection.
The Committee on Terminology, Nomenclature, and Classification of the Section on Urology of the American Academy of Pediatrics suggests the use of the following terms in reference to duplex collecting systems3 :
•Duplex kidney - The duplex kidney has a single renal parenchyma that is drained by 2 pyelocaliceal systems.
•Upper or lower pole - The poles represent one component of a duplex kidney.
•Duplex system - The kidney has 2 pyelocaliceal systems and is associated with a single ureter or with a bifid ureter (a partial duplication) or, in the case of a complete duplication, with 2 ureters (double ureters) that drain separately into the urinary bladder.
•Bifid system - Two pyelocaliceal systems join at the ureteropelvic junction (bifid pelvis), or 2 ureters join before draining into the urinary bladder (bifid ureters).
•Double ureters - Two ureters open separately into the renal pelvis superiorly and drain separately into the bladder or genital tract.
•Upper and lower pole ureters - Upper and lower pole ureters drain a duplex kidney's upper and lower poles, respectively.
For further reading:
http://emedicine.medscape.com/article/378075-imaging
http://www.sonoworld.com/fetus/page.aspx?id=2660
