Anomalies of the Urinary Tract Renal Agenesis

Renal agenesis may result from aplasia of the Wolffian duct or absence of the ureteral bud with consecutive lack of metanephrogenic tissue induction.

Unilateral renal agenesis is rather common, occurring in about 1 in 1,000 newborn infants [1]. There is a strong association of renal anomalies with Mullerian duct anomalies. Therefore, in females, the urinary tract and the reproductive system should always be evaluated as a unit.

Renal agenesis may also be mimicked by a multicystic dysplastic malformation of the kidney later in life since the multiple cysts seen in the affected infant disappear with time. In 10% of cases unilateral renal agenesis is associated with aplasia of the ipsilateral adrenal gland. In all other cases a characteristic elongated feature of the adrenal gland is found. Bilateral renal agenesis (Potter syndrome) is incompatible with life and most infants die shortly after birth.

Crossed Renal Ectopia

In crossed renal ectopia one kidney is displaced across the midline and lies on the opposite side of the ureteral insertion into the bladder. Four types of renal crossed ec-topia are described (with fusion, without fusion, solitary, bilateral), with the crossed-fused ectopia being the most common.

Standard as well as colour Doppler ultrasound techniques document the location of ureteral orifices through the ureteral jet phenomenon.

Megaureter

The diameter of a normal ureter is rarely more than 5 mm and, therefore, the ureter is not visualised on pre- and postnatal ultrasound. Megaureter as a general term describes a dilated ureter with or without associated dilatation of the upper collecting system. Ureteral dilatation may be in response to anatomical or functional obstruction or vesicoureteral reflux, but may also occur in their absence.

Primary megaureter corresponds to an obstructive dilatation of the ureter above an adynamic segment at the ureterovesical junction. It is caused by hypoplasia and atrophy of muscle fibres in the distal segment of the ureter associated with an increase in collagen fibres in the pathologic segment and muscular hypertrophy of the proximal ureter.

Ultrasound demonstrates the dilated ureter and an increased peristalsis.

Secondary megaureter occurs as a result of reflux, ure-thral obstructions or bladder abnormalities such as posterior urethral valves or a neuropathic bladder.

Ureteral Duplication

Ureteral duplication (Fig. 1) is the most frequent renal anomaly. The duplications vary from partial to complete separation of the collecting system. Incomplete duplication is thought to result from early branching of a single ureteral bud, resulting in a common distal stem with two ureteral segments impinging on the metanephric blastema.

Complete ureteral duplication results from the formation of two ureteral buds on the same side. During absorption and migration of the distal portion of the mesonephric duct into the developing bladder, the orifices of the ureter draining the lower segment of the kidney migrates more cephalad and lateral than the orifice of the ureter draining the upper segment of the kidney.

Collecting Duct Malformation

Fig. 1. Complete ureteropelvic duplication on the left with obstructive upper-pole ureterocele. Urinary tract infection in the upper pole collecting system in a 21-day-old male. a Longitudinal, b transverse pelvic scan. The dilated upper pole ureter (U) and the ectopic ureterocele (C) are full of debris (purulent exudate), B, bladder. (Reprinted with permission from [2])

Fig. 1. Complete ureteropelvic duplication on the left with obstructive upper-pole ureterocele. Urinary tract infection in the upper pole collecting system in a 21-day-old male. a Longitudinal, b transverse pelvic scan. The dilated upper pole ureter (U) and the ectopic ureterocele (C) are full of debris (purulent exudate), B, bladder. (Reprinted with permission from [2])

This embryological relationship is known as the Weigert-Meyer-law.

When the area of the ureter to the lower segment buds is displaced closer to the urogenital sinus than normal, the orifice migrates more lateral and cephalad and the lower segment is more likely to be associated with vesi-coureteral reflux.

When the area of the ureter to the upper segment, on the other hand, is displaced more proximally on the mesonephric duct than normal, the ureteral orifice will be ectopic in location and may open at the level of the bladder neck or even more distally into the urethra, vestibule or vagina.

Vesicoureteral reflux is the most common abnormality seen in association with complete ureteral duplica tion and commonly affects the lower segment due to the fact that the ureter to the lower segment has a more laterally placed orifice and a shortened submucosal tunnel. The ureter from the upper pole may function normally or may be obstructed, but reflux into the upper pole is rare.

Sonography clearly demonstrates malformations of the kidney, such as a dilated ureter due to either stenosis or reflux and ballooning of the submucosal segment of the upper ureter in the bladder (ureterocele). The ureteral jet sign locates the orthotopic as well as the ectopic ureteral orifice.

Ureteral Ectopia in Single and Duplex Collecting Systems

A ureter that opens anywhere but into the trigone of the bladder is considered ectopic (Fig. 2). About 20% of ec-topic insertions of the ureter drain a kidney with a single collecting system. The remainder are associated with the affected ureter draining the upper pole of a duplex kidney.

In males, the ectopic ureter may empty into the lower bladder, posterior urethra, seminal vesicle, vas defer-ens or ejaculatory duct. In females it may insert into the lower bladder, urethra, vestibule, vagina, uterus or remnants of the Gartner's duct. Ureteral ectopia is more common in females and affected girls are frequently found to be incontinent due to the ectopic ureter terminating at a level distal to the continence mechanisms of the bladder neck and external sphincter. Ectopic ureters may show reflux, but obstruction is more common. When the collecting system is dilated the lack of an in-travesical ureterocele suggests an insertion of the ec-topic ureter on another side, such as the bladder neck or the proximal urethra. Ultrasound documents a dilated ureter on a lower and more medial level than expected and the exact location of the orifice may be documented via the ureteral jet sign.

Ureterocele in Single and Duplex Collecting Systems

Ureteroceles represent cystic dilatation of the intravesical segment of the ureter as a response to obstruction, and may be associated with single or duplex ureters (Fig. 1). They are called orthotopic or simple when found with a single collecting system, and ectopic when found with a duplicated collecting system. Ectopic ureteroceles are positioned beyond the confines of the trigone and frequently represent the distal portion of the upper pole ureter. On ultrasound the ureterocele appears as a cystic structure within the bladder. The ureteral jet sign documents the orifice of simple ureteroceles opposite the bladder wall. In ectopic ureteroceles the orifice is commonly located next to the bladder wall.

Posterior Urethral Valves (Case no. 1 on CD-ROM)

Posterior urethral valves are the commonest urethral abnormality occurring in boys. A thick membrane is found

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