| Discussion: |
Duplex kidney is a developmental condition where there is incomplete fusion of the upper and lower poles of the kidneys. This can occur unilaterally or bilaterally. Complete duplication of the kidney and collecting systems down to the bladder are more commonly seen. Many duplex kidneys are incidental findings and are not the cause of symptoms although duplex kidneys are more prone to urinary infection, reflux and obstruction. The ureter from the upper element of a duplex kidney is often ectopically inserted in boys into the posterior urethra or in girls distal to the bladder neck or in the vagina. Ectopic insertion into the bladder of the upper element of a duplex kidney may also be subject to ureterocele formation . Upper element of duplex kidneys are more prone to obstruction and lower elements to reflux and scarring. The lower element of a duplex kidney when complete has its ureter inserted into the trigone of the bladder. The course through the bladder wall is however more perpendicular to the bladder wall than usual making it prone to reflux. Pelviureteric junction obstruction of the lower element of a duplex kidney is well described although uncommon.
A number of modalities may be used to image duplex kidneys. Ultrasound is sensitive to the diagnosis of duplex kidneys when there is dilatation of one or other elements. In uncomplicated duplex kidney there is splitting of the renal sinus echo. It is also sensitive in diagnosing dilated ureters and ureteroceles both in antenatal and postnatal life. An intravenous urography (IVU) gives elegant demonstration of the kidneys and their collecting systems and if function is preserved in both elements is a very sensitive method of demonstrating the collecting systems and ureters. Micturating cystourethrography will demonstrate the collecting system if there is reflux. Technetium-99m-DMSA scanning will often confirm the diagnosis of duplex kidney where function is discrepant between the two elements. A normally functioning duplex kidney, however, cannot be differentiated from a normal one on DMSA although it can be suspected if the kidney is elongated and/or differential function is increased on that side. Ultrasound also has a significant false negative rate in the diagnosis of uncomplicated duplex kidney.