A duplex kidney, also called a duplicated collecting system, is a common congenital anomaly where a single kidney has two separate systems for collecting and draining urine, instead of the typical single system. This variation is present from birth. Normally, a single ureter carries urine from each kidney to the bladder. A duplex kidney essentially doubles this plumbing within one organ. It is considered the most common structural variation of the urinary tract, found in approximately 0.8% to 1% of the population.
Anatomical Definition and Types of Duplication
The typical kidney has a single renal pelvis, the funnel-shaped structure that collects urine, leading into a single ureter. In a duplex kidney, the renal parenchyma is drained by two separate pyelocalyceal systems. The degree of duplication determines the categorization, ranging from a slight split to two completely separate drainage tubes.
The most minor variation is an incomplete duplication, often called a bifid ureter, where the two collecting systems merge below the kidney. They join to form a single ureter that enters the bladder through one opening. This partial duplication is the more common form of the anomaly.
A complete duplication involves two distinct collecting systems and two entirely separate ureters. Both ureters travel down to the bladder and connect through their own separate openings. This complete form of duplication is more frequently associated with clinical problems.
Clinical Presentation and Associated Complications
Many people with a duplex kidney remain asymptomatic, and the condition is often discovered incidentally during imaging for unrelated reasons. When symptoms occur, they are typically related to obstruction or the backflow of urine. Complications are most common in cases of complete duplication.
The Weigert-Meyer rule describes the predictable path of the two ureters into the bladder in complete duplication. The ureter from the upper part of the kidney usually inserts lower and more medially than the ureter from the lower part. This lower, medial insertion often has an abnormal opening, making the upper pole system prone to obstruction.
Conversely, the ureter draining the lower kidney segment inserts higher and more laterally, resulting in a shorter tunnel through the bladder wall. This shorter tunnel predisposes the lower pole ureter to vesicoureteral reflux (VUR), where urine flows backward. VUR increases the risk of recurrent urinary tract infections (UTIs). Recurrent UTIs and abdominal or flank pain from obstruction are common presenting symptoms.
A ureterocele is another complication, involving a ballooning of the distal end of the upper pole ureter as it enters the bladder. This swelling can block urine flow from that segment, leading to swelling and potential damage known as hydronephrosis. Rarely, the upper pole ureter may be ectopic, opening outside the bladder into the urethra or reproductive organs, which can cause continuous urinary incontinence in children.
Detection and Diagnostic Imaging
A duplex kidney is frequently first suspected during a prenatal ultrasound if the sonographer observes swelling or dilation of the collecting system, known as hydronephrosis. If noted, further imaging is usually conducted after birth to confirm the anatomy and assess kidney function.
For postnatal diagnosis, a renal and bladder ultrasound remains the standard initial test, as it is non-invasive and identifies the two separate collecting systems and any associated dilation. A specialized X-ray procedure called a Voiding Cystourethrogram (VCUG) is often utilized to confirm or rule out vesicoureteral reflux. The VCUG involves filling the bladder with contrast dye through a catheter and taking images while the patient urinates to visualize any backward flow of urine.
Specialized nuclear scans may be performed to fully map the urinary tract and assess the function of each segment. A Dimercaptosuccinic Acid (DMSA) scan measures overall function and looks for scarring in the kidney tissue. A MAG3 scan evaluates how well urine is draining and helps identify potential points of obstruction.
Management and Treatment Approaches
Management depends on whether the anomaly is causing symptoms or complications. If the duplex system is found incidentally and is asymptomatic, a strategy of watchful waiting and routine monitoring is employed. This typically involves periodic ultrasounds and urine tests.
If the patient experiences recurrent UTIs, prophylactic antibiotics may be prescribed to prevent infection while monitoring for surgical necessity. Surgical intervention is reserved for cases involving significant obstruction, severe reflux, or non-functional parts of the kidney. Procedures vary depending on the specific complication.
Ureteral reimplantation is a common procedure that corrects vesicoureteral reflux by creating a longer, more functional tunnel for the ureter through the bladder wall. For obstructions caused by a ureterocele, a surgeon may perform an incision to allow proper urine drainage. If a kidney segment is severely damaged or non-functional due to chronic issues, a partial nephrectomy (heminephrectomy) may be required to remove only the diseased portion, preserving the healthy part.