Answers

Short answers

1 The ultrasound image (fig 1) shows hydronephrosis of the upper pole of a duplex collecting system.

2 The ^99mTc-MAG3 diuresis renogram (fig 3) shows that the upper pole of the collecting system in the right duplex kidney is functioning but obstructed, as shown by the radioisotope still in the pelvicalyceal system of the right kidney at 25 minutes (red arrow) and by a flattened curve (yellow arrow). The unobstructed right lower pole with normal function is shown by radioisotope leaving the pelvicalyceal system at the same rate as in the normal left kidney. This is an important test because it shows residual function, which affects the choice of surgical intervention.

View larger version (25K): [in this window] [in a new window] [PowerPoint Slide for Teaching]   Fig 3 99mTc-MAG 3 diuresis renogram. Red arrow shows radioisotope is still present at 25 minutes in the upper pole of the right kidney; yellow arrow shows flattened curve of right kidney, which represents upper pole obstruction

3 Vesicoureteral reflux is a common complication of duplex kidneys, affecting the lower pole. Voiding cystourethrography is the investigation of choice.

Long answers
1 Prenatal hydronephrosis and duplex kidney
Infants referred for postnatal ultrasonography are those who, at the prenatal 18-20 weeks scan, are found to have a renal pelvic diameter of 5 mm or more.¹ A prenatal renal pelvic diameter below 12 mm is usually self limiting, but a greater measurement may indicate duplex kidneys, vesicoureteral reflux, obstruction of the ureteropelvic junction, and megaureters.^{1 2} The abnormalities in these postnatal ultrasound images relate to duplex kidneys.

Duplex kidneys, or more correctly "duplication of the ureters," is the most common anomaly of the urinary tract. The two ureters of a duplex kidney develop from separate ureteric buds, which have grown from a single Wolffian duct. Duplications may be discovered in childhood or (less commonly) in later life, or they may be occult.

Incomplete duplication is three times as common as complete duplication. Complete duplication occurs in about 1 in 500 people.³ In incomplete duplication, ureters unite at a variable distance from the kidney, and only one ureteral orifice is present. In complete duplication each ureter has an ipsilateral ureteral orifice, and complications are common. The ureter draining the lower pole enters the bladder at the trigone; the ureter draining the upper pole enters the bladder medial and inferior to this point (this is known as the Weigert-Meyer rule). Occasionally the ureter draining the upper pole ends ectopically outside the bladder. In males the ureter ends proximal to the external urinary sphincter—for example, in the prostatic urethra.⁴ An ectopic ureter is much more common in females, in whom it usually drains into the vesical neck or, distal to the sphincter, into the urethra, vagina, or uterus. Therefore urinary dribbling is common in females with this condition.

Ectopic ureteral insertion is a common complication of complete duplication. Another is an ectopic ureterocele, which is associated with upper pole hydronephrosis (fig 1). Complications affecting the lower pole are vesicoureteral reflux and ureteropelvic junction obstruction.³

2 Hydronephrosis of the upper pole in a duplex kidney
Hydronephrosis of the upper pole is almost eight times commoner in girls than boys. The most common cause of an upper pole renal mass in children is an obstructed, non-functioning upper pole collecting system in a duplex kidney.

One series found that only 35% of infants with hydronephrosis of the upper pole of a duplex collecting system have signs or symptoms related to the urinary tract. These are usually secondary to a urinary tract infection or less commonly secondary to an abdominal mass or a prolapsed ectopic ureterocele.⁵ Ureteroceles are congenital dilations of the intramural portion of the ureter. A large ureterocele may prolapse into or through the urethra, produce bladder obstruction, or even appear as an intralabial mass.³

3 Vesicoureteral reflux
Vesicoureteral reflux is a common complication of duplex kidneys and affects the lower pole. It occurs in up to 40% of children with postnatally confirmed hydronephrosis.⁷ Identification of high grade reflux to the lower pole moiety of a duplex system affects choice of surgical treatment.

Vesicoureteral reflux is more common in patients with ectopic ureterocele (63%) than in those without (33%), probably because of distortion of the orifice of the ipsilateral lower pole by the ureterocele.⁵ Voiding cystourethrography, which can identify vesicoureteral reflux, is indicated if a dilated ureter behind the bladder is identified on the ultrasound scan.¹

Cystourethrography is the most sensitive test for ureteroceles. The ureterocele produces a filling defect in the contrast filled bladder. This is best seen at the start of bladder filling as later on the larger volume of contrast either side of the ureterocele will be too dense to allow the ureterocele to be distinguished from the remaining bladder. It is also easier to see on oblique views because of the posterior and inferior location of the ureterocele.

Management
Detection of hydronephrosis on antenatal ultrasound and subsequent detection of a dilated upper pole of a duplex collecting system may decrease the proportion of neonates presenting with urinary tract infection and urosepsis, because prophylactic antibiotics can be started at birth and continued until surgical correction.⁵ But prophylactic antibiotics may be associated with increased risk of resistant infections.⁸

The decision about surgical approach is based on the results of the postnatal uroradiological studies clarifying the status of the upper pole, the status of the lower pole, and the status of the opposite kidney, plus the presence of reflux and the characteristics of the ureterocele. Prenatal detection means that infection can be prevented, obstruction can be relieved, and dilated non-functioning renal units can be removed before the infant becomes ill.⁵

Initial treatment consists of incising the ureterocele to enhance drainage of the upper tract, and may result in upper pole reflux. Definitive treatment may be determined by the degree of function of the upper pole. When there is sufficient function or parenchyma, the upper pole is retained and an ipsilateral upper to lower ureterostomy [is cut ok?]or ureteropyelostomy is performed. When there is extremely poor function or thin, dysplastic parenchyma, a partial nephrectomy and ureterectomy are performed.³

Imaging and management remain controversial, and few long term follow-up studies of the disease, treatment, and outcome have been performed.¹

Postnatal ultrasound

Ultrasound examination can be diagnostic but, in the context of a unilateral abnormality that has been detected antenatally, should not be carried out before 72 hours and, to maximise hyrdonephrosis, should be carried out with the neonate prone.¹ Longitudinal sections are particularly valuable, since they depict on a single section the localised hydronephrosis as well as the normal calyceal echo pattern of the lower pole. The ultrasound image in figure 1 shows hydronephrosis of the upper pole of a duplex collecting system. The identification of a cystic mass in the upper pole that lacks the typical configuration of a simple cyst should raise the question of an obstructed ectopic ureter.⁴ This obstruction may be secondary to a ureterocele. One study of infants with hydronephrosis of the upper pole of a duplex collecting system found that half had ectopic ureteroceles.⁵ A ureterocele may be visualised with ultrasound both prenatally and postnatally and may have a varied appearance.

Quantitative tests (scintigraphy: ^99mTc-DTPA or ^99mTc-MAG3 with frusemide) or qualitative tests (excretory urography) can confirm the diagnosis and evaluate the function of the upper pole (and the lower pole and other kidney). The obstructed dysplastic upper pole has poor function and might not be visualised.^{5 6} The ^99mTc-MAG3 diuresis renogram in figure 3 shows that the upper pole of a right duplex collecting system is functioning but obstructed, as evidenced qualitatively by failure of radioisotope to have left the pelvicalyceal system at 25 minutes (red arrow) and quantitatively by a flattened curve (yellow arrow). The unobstructed right lower pole with normal function is shown by radioisotope leaving the pelvicalyceal system at the same rate as the normal left kidney. Residual function is important in deciding on the choice of surgical intervention.

Excretory urography is more than just a useful functional test. Structural signs—beloved of postgraduate examiners—may indicate upper pole hydronephrosis. A hydronephrotic upper pole may compress the lower pole. During excretory urography the abnormally oriented "kidney" (lower pole) has fewer calices than a normal contralateral kidney and has a laterally displaced and tortuous ureter. The abnormal axis of the lower pole segment is sometimes referred to as the "drooping lily sign" (fig 4).³ Excretory urography is contraindicated in neonates as it may cause complications.¹

View larger version (34K): [in this window] [in a new window] [PowerPoint Slide for Teaching]   Fig 4 Completely duplicated ureters with complications: blue arrow points to hydronephrotic upper pole, yellow arrow points to ureterocele, and red arrow points to "drooping lily sign"