Posterior Urethral Valves

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Posterior Urethral Valves

Posterior urethral valves (PUV) are the most common cause of bladder outlet obstruction in boys, with an incidence of 1 in 5,000 to 8,000 male births.1 Although the majority of boys with PUV are diagnosed antenatally, approximately one-third will be diagnosed during childhood or adolescence. PUV is the most common obstructive cause of end-stage renal disease (ESRD) in children, and is the etiology for approximately 35% of children who require renal transplantation.2

Embryology and Anatomy

At 5 to 6 weeks’ gestation, the orifice of the mesonephric duct migrates from an anterolateral position in the cloaca to Müller’s tubercle on the posterior wall of the urogenital sinus, occurring simultaneously with division of the cloaca. Remnants of the mesonephric duct normally remain as small distinct, paired lateral folds termed the inferior urethral crest and plicae colliculi. When the insertion of the mesonephric ducts into the cloaca is too anterior, normal migration of the ducts is impeded, and the ducts fuse anteriorly, resulting in abnormal ridges, which become the PUV. A smaller aperture between the leaflets causes more obstruction than those with a larger aperture and a less prominent anterior component.3

Three distinct types of PUV have been described. Type I is an obstructing membrane that radiates distally and anteriorly from the verumontanum toward the membranous urethra, fusing in the midline. Approximately 95% of PUV are type I, in which the valves are thought to be a single membranous structure with the opening positioned posteriorly near the verumontanum. Type III appears as a membranous diaphragm with a central opening at the verumontanum. The obstructing tissue also has been termed a congenital obstructing posterior urethral membrane.4 It is thought that instrumentation with a urethral catheter might disrupt the posterior aspect of the membrane, resulting in the appearance of a type I valve. Type II valves are prominent longitudinal folds of hypertrophied smooth muscle that radiate cranially from the verumontanum to the posterolateral bladder neck, but these are nonobstructive and clinically insignificant.

Antenatal Diagnosis, Management, and Outcomes

About 10% of antenatally diagnosed obstructive uropathy is due to PUV, and approximately two thirds of PUV are diagnosed antenatally. Typical findings on ultrasound (US) include bilateral hydroureteronephrosis, a distended bladder, and a dilated prostatic urethra, called a ‘keyhole’ sign.5 Discrete focal cysts in the renal parenchyma are diagnostic of renal dysplasia. Amniotic fluid volume is variable. Those with normal or slightly reduced amniotic fluid have a better prognosis. In contrast, oligohydramnios suggests significant obstructive uropathy, and pulmonary hypoplasia secondary to renal dysplasia is common. Oligohydramnios prevents normal lung development in utero. Pathologically, this process results in reduced branching of the bronchial tree and reduced numbers and size of alveoli.

The gestational age at which hydronephrosis is recognized influences prognosis. In one study, fetuses with normal appearing renal anatomy before 24 weeks were more likely to have normal renal function than were those with hydronephrosis before 24 weeks.6 One meta-analysis showed the best predictive value for postnatal renal function was the appearance of the fetal renal cortex.7 A more recent study showed that ultrasound parameters alone are not able to predict postnatal renal function.8 Prune-belly syndrome, urethral atresia, and bilateral high-grade vesicoureteral reflux (VUR) can have a similar prenatal sonographic appearance to PUV. Collectively, bladder outlet obstruction found prenatally can result in a significant (57%) incidence of renal failure by 2 years of age.9

In the fetus with suspected PUV and normal amniotic fluid volume, serial fetal sonograms are necessary to monitor the status of the hydronephrosis and amniotic fluid volume. If oligohydramnios develops, bladder drainage may help restore the amniotic fluid and allow normal pulmonary development. Before any intervention, a karyotype should be obtained to confirm the male gender and to detect chromosomal abnormalities, which occur in about 12%.10 Fetal renal function is assessed with serial urinary electrolytes and β2-microglobulin levels. Normally, fetal urine is hypotonic (favorable prognosis), with sodium less than 100 mEq/L, chloride less than 90 mEq/L, osmolality less than 210 mEq/L, and β2-microglobulin levels less than 6 mg/L.5 Elevated fetal urine electrolytes and β2-microglobulin levels are an indication of irreversible renal dysfunction. Sequential bladder aspiration every 48 to 72 hours should be performed because the initial urine sample may be stagnant and fresh urine more accurately reflects the function of the fetal kidneys.11,12

If fetal urine is hypotonic, and oligohydramnios is present, then fetal intervention to restore the amniotic fluid volume should be considered, with the goal of preventing life-threatening pulmonary hypoplasia. This procedure has been performed in the first trimester,13 although the majority of fetuses are diagnosed and treated in the second trimester. If the gestational age of the fetus is 32 weeks, early delivery is advisable. If the fetus is <32 weeks’ gestation, however, the urine may be diverted into the amniotic fluid with a percutaneously placed vesicoamniotic shunt (VAS). In a recent meta-analysis, antenatal bladder drainage appears to improve perinatal survival and relieve bladder outlet obstruction, especially in those fetuses with poor prognostic criteria.14 To date, there is no evidence that drainage of the obstructed fetal bladder will improve renal or bladder function.

In theory, VAS does not allow the bladder to cycle. Consequently, when counseling expectant parents, they need to understand that their newborn may have limited renal function or ESRD, even if the drainage procedure is successful. VAS can have complications in up to 45%.5,15 The shunts become obstructed or displaced in 25% of cases, necessitating additional procedures that increase morbidity to the mother and fetus, and there is a 5% procedure-related chance of fetal loss. In addition, omental or bowel herniation through the fetal abdominal wall can occur.

Despite adequate bladder drainage, renal function may be so limited that the amniotic fluid volume remains low. In one study of high-risk fetuses identified in the first trimester with severe bilateral hydroureteronephrosis, bladder distention, and oligohydramnios managed with a VAS, a 60% overall survival rate and a 33% incidence of renal failure were found.16 In a review of 14 fetuses with proven PUV and favorable fetal urinary electrolytes undergoing VAS at a mean gestational age of 22.5 weeks, six deaths occurred before term delivery. Of the surviving eight neonates, three had ESRD, and the other five had an elevated serum creatinine.17 In a more optimistic study of 20 boys with ‘lower urinary tract obstruction’ managed by VAS, the overall 1-year survival was 91%. In this study, the mean birth weight was 2574 g, 40% had acceptable renal function, 20% had mild renal insufficiency, and 30% required dialysis.18 Of this group, seven had PUV and seven had prune-belly syndrome, a nonobstructive condition. This lack of evidence regarding fetal drainage is the impetus for the study on Percutaneous Shunting in Lower Urinary Tract Obstruction (PLUTO) trial, which is an international randomized controlled trial comparing the effect of VAS versus no intervention. This study is evaluating prenatal and perinatal mortality, renal function, and other variables.19

Fetal ultrasound is useful in the diagnosis of lower urinary tract obstruction (LUTO). Sonographic features of LUTO include bilateral hydronephrosis, a thick wall dilated bladder, and dilated upper urethra. Unfortunately, there are a number of etiologies for this sonographic appearance, including urethral atresia, prune-belly syndrome, as well as PUV. The first two causes would not necessarily be an indication for either VAS or in utero endoscopic ablation. Percutaneous endoscopic ablation is now being performed in a few centers in the USA.20–22 One meta-analysis showed a perinatal survival advantage using endoscopic ablation compared to observation, but there was no significant improvement in survival with ablation when compared to VAS.23 Two small studies have evaluated the role of fetal cystoscopy to improve the accuracy of the prenatal ultrasound findings regarding a definite etiology.24,25 In addition, if PUV is identified at cystoscopy, then ablation can be considered.

Clinical Presentation

Neonates with PUV not diagnosed prenatally can present with symptoms of delayed voiding or a reduced urinary stream.15 Also, respiratory distress secondary to pulmonary hypoplasia may be the primary manifestation of PUV. Other postnatal signs and symptoms include an abdominal mass, failure to thrive, lethargy, poor feeding, urinary tract infection (UTI), and urinary ascites. Physical examination in the newborn typically discloses a palpable walnut-sized bladder, secondary to the hypertrophic detrusor muscle. Urinary ascites can result in significant abdominal distention. Older boys can have persistent diurnal incontinence or abdominal distention.

Radiographic Evaluation

Significant bilateral hydroureteronephrosis and a thick-walled, distended bladder are seen on ultrasound. Corticomedullary differentiation is a favorable prognostic sign regarding renal function (Fig. 57-1). Conversely, echogenic kidneys or subcortical cysts and the loss of corticomedullary differentiation are unfavorable signs. Suprapubic or perineal ultrasound may demonstrate a dilated prostatic urethra, which is pathognomonic for PUV.

The voiding cystourethrogram (VCUG) is the only radiographic study that definitively establishes the diagnosis of PUV (Fig. 57-2). The valves appear as a defined lucency in the distal prostatic urethra. The posterior urethra is dilated and elongated. The bladder is trabeculated due to muscular hypertrophy with a clear delineation of the bladder neck. Unilateral VUR is present in 25% and bilateral VUR in 25% of infants with PUV.

Renal nuclear scintigraphy with a technetium-99m–labeled dimercaptosuccinic acid (99mTc-DMSA) is performed if imaging studies show thin or abnormal parenchyma in either kidney on ultrasound and/or high-grade VUR. The study should be delayed until 6 to 8 weeks of age to allow maturation of renal function. This study is effective in establishing baseline differential renal function. However, if renal function is poor, visualization of the kidneys will be suboptimal. An alternative to renal scintigraphy is dynamic contrast-enhanced magnetic resonance urography (MRU). This study provides high-resolution renal images and assessment of differential renal function, but requires an anesthetic.26

Initial Management

The initial treatment of neonates suspected of having PUV is to decompress the bladder with a 5 French or 8 French feeding tube. The catheter can be difficult to pass because of the valvular obstruction as well as the significant dilation of the prostatic urethra and hypertrophy of the bladder neck. The catheter tends to coil in the prostatic urethra. A Coudé tip catheter may help overcome this problem. Ultrasound can confirm placement of the catheter within the bladder. Insertion of a Foley catheter is discouraged because the inflated balloon can obstruct the ureteral orifices when the thick-walled bladder is decompressed, and can cause bladder spasm that obstruct the intramural ureters.

Amoxicillin or cephalexin prophylaxis should be initiated. Electrolytes, BUN (blood urea nitrogen), creatinine, and fluid status should be monitored carefully. The serum creatinine concentration at birth reflects maternal renal function. With satisfactory newborn renal function, the creatinine value should gradually decrease to 0.3–0.4 mg/dL. However, with limited renal function, the creatinine will remain the same or increase, even with bladder decompression. Metabolic acidosis and hyperkalemia are common complications if renal function is impaired. Consultation with a pediatric nephrologist is invaluable because renal tubular acidosis (RTA), renal insufficiency, ESRD, and somatic growth abnormalities are common and need long-term monitoring. Neonates with respiratory distress may require immediate pulmonary resuscitation with endotracheal intubation and positive-pressure ventilation. If urinary ascites is present, paracentesis may be necessary to correct the fluid and electrolyte imbalance.

Primary Valve Ablation

Endoscopic valve ablation is performed after the neonate is stabilized. Well-lubricated infant urethral sounds should be passed to gently dilate the meatus and glandular urethra. The neonatal male urethra usually accepts a 9.5 French endoscope. Overly aggressive dilation of the urethra in order to pass a larger endoscope may lead to urethral trauma with subsequent stricture formation, and should be avoided. Vigorous dilation may also result in iatrogenic hypospadias due to splitting of the glans to the subcoronal level.27

An 8 French or 9.5 French cystoscope typically is used with a Bugbee electrode on low cutting current inserted through the operating channel. The valve leaflets should be incised using a low cutting current at the 5 and 7 o’clock positions (Fig. 57-3). Incision at the 12 o’clock position, where the valve leaflets fuse, also may be helpful. An alternative technique employs the Nd : YAG laser.28 In a premature or small neonate, a cystoscope as small as 6.9 French can be used, although visualization of the PUV may be suboptimal. If urethral bleeding develops, coagulation should be performed carefully because injury to the urethra can occur with overzealous cautery. Following valve ablation, a pediatric feeding tube is left for one to two days.

A VCUG and renal ultrasound should be obtained two to four weeks after ablation to confirm satisfactory valve disruption and assess the upper urinary tracts. In addition, renal function should be monitored carefully. Valve ablation is successful in more than 90% of patients. The most common complication is incomplete valve ablation in which case repeat cystoscopy and valve incision is necessary. Urethral stricture is uncommon if small endoscopes are used.

Temporary Urinary Diversion

An alternative to primary valve ablation is cutaneous vesicostomy (Fig. 57-4). This approach is appropriate in a small or premature neonate when the pediatric cystoscope is too large for the urethra or if severe hydroureteronephrosis, urinary ascites, or high-grade VUR and poor renal function are present. In these cases, optimal upper urinary tract drainage is necessary to maintain existing renal function. The most popular technique was described by Blocksom and popularized by Duckett.29 A small transverse incision is performed midway between the umbilicus and pubic symphysis, and the dome of the bladder is brought to the skin. The vesicostomy should calibrate to 24–26 French to avoid stenosis. Daily dilation of the stoma with a plastic medicine dropper helps prevent stomal contraction. The vesicostomy allows urine to drain directly into the diaper, obviating the need for a collection device. Complications include stomal stenosis, if the stomal size is less than 24–26 French, and prolapse, if the anterior wall of the bladder is exteriorized rather than the bladder dome.

Valve ablation should not be performed at the time of vesicostomy because the urethra will remain dry and stricture is likely. A vesicostomy allows the bladder to cycle and grow at low pressures, and does not reduce bladder capacity. These neonates should be maintained on antibiotic prophylaxis.

In the past, after insertion of a urinary catheter into the bladder, proximal diversion with cutaneous pyelostomy or cutaneous ureterostomy was advocated for neonates and infants with severe hydronephrosis and a persistently elevated creatinine.30 Theoretically, proximal diversion provides better renal drainage than a vesicostomy, particularly with ureterovesical obstruction, and optimizes the potential for renal function and somatic growth. However, proximal drainage has not been shown to prevent ESRD because at least 85% of these patients have renal dysplasia.31 In addition, by diverting the urine away from the bladder, regular cyclical bladder filling and contraction does not occur which results in a smaller, less compliant bladder.32

Currently, proximal diversion is reserved for the rare case in which valve ablation or vesicostomy fails to improve upper tract drainage. When needed, the preferred method of proximal diversion is the Sober-en-T ureterostomy in which the proximal ureter is divided and exteriorized on the abdominal wall (Fig. 57-5

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