Supernumerary Kidneys Do Exist!

Abstract

AJR 2016; 206:W48 0361–803X/16/2062–W48 © American Roentgen Ray Society Supernumerary Kidneys Do Exist! We read with great interest the article “MDCT and MR Urogram Spectrum of Congenital Anomalies of the Kidney and Urinary Tract Diagnosed in Adulthood” by Dr. Surabhi and colleagues [1] in the September 2015 issue of the AJR. We would like to add an important differential diagnosis to the authors’ list of congenital anomalies of the kidney and urinary tract: anomalies in renal number. Anomalies in renal number include the development of supernumerary kidneys and renal agenesis. Renal agenesis is well-described in the article by Dr. Surabhi and colleagues. We would like to add few important comments about supernumerary kidneys. Supernumerary kidneys are said to be present when the total number of kidneys exceeds two; therefore, supernumerary kidneys can be defined by either the presence of more than two separate kidneys or the observation of an extra kidney coexisting with a horseshoe kidney. A supernumerary kidney can develop from secondary outpouching of the Wolffian duct or from branching from the initial ureteral bud; both means of development induce growth of the metanephric anlage, even though the kidneys have separated entirely [2]. It is necessary to differentiate a supernumerary kidney from a duplex kidney. A supernumerary kidney has a completely separate or partially fused renal parenchyma, a separate renal capsule with a greater number of calyces, and a separate arterial supply and venous drainage. On the other hand, for a duplex kidney, both poles are attached with a single renal capsule, the number of calyces does not exceed the number of calyces of the opposite kidney, and the vascular supply is the same as that of the opposite kidney [2]. Two types of supernumerary kidney exist: the type that is drained by a bifid ureter and the type that is drained by a separate ureter. When a bifid system is present, the supernumerary kidney lies caudally; however, when a separate ureter is seen, the supernumerary kidney is located cranially in relation to the normal kidney. If both a horseshoe kidney and a supernumerary kidney coexist, then the half of the horseshoe kidney located on the side of the body containing the supernumerary kidney is always small. We also would like to highlight an important sign (other than the drooping lily sign) seen in the collecting system of a duplex kidney. This sign is known as the “nubbin sign,” a term that refers to the tiny nonfunctioning lower pole of a duplex kidney that is seen on urography images and that may be confused with a focal renal mass [3].