Posterior Urethral Valves Workup

Updated: Apr 02, 2020
  • Author: Martin David Bomalaski, MD, FAAP; Chief Editor: Marc Cendron, MD  more...
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Workup

Laboratory Studies

For the first 24 hours after birth, the infant's serum chemistries are the same as the mother's. Therefore, serum values for creatinine and blood urea nitrogen (BUN) should be obtained at least 24 hours after birth. In utero, the placenta functions as the major blood filter for the fetus, with waste passed on to the mother. Observing serial serum chemistries for several days to weeks is important to determine the true status of the newborn's renal function.

The normal newborn kidney is still undergoing maturation at birth, and the infant's glomerular filtration rate (GFR) continues to improve during the first several months of life. Because of renal immaturity at birth, the newborn is unable to concentrate urine and is susceptible to dehydration. This defect is exacerbated by renal dysplasia such as that found with posterior urethral valves (PUVs).

As renal maturation continues, creatinine clearance normally improves. If significant renal dysplasia or damage has occurred, the serum creatinine fails to reach a normal level during the first year of life. Serum creatinine levels higher than 0.8 mg/dL during the first year of life have been demonstrated to be associated with poor long-term renal function; thus, such levels are considered a negative prognostic indicator.

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Imaging Studies

Ultrasonography

Antenatal ultrasonography (US) has been found to be reasonably accurate in distinguishing PUV from vesicoureteral reflux (VUR). [15]

Every child with antenatal hydronephrosis should be assessed with renal and bladder US in the immediate postnatal period, [16] with a focus on the appearance of the renal parenchyma, any evidence of renal collecting system dilatation, the thickness of the bladder wall, and the presence or absence of ascites. The quantity (total area) and quality (corticomedullary differentiation and renal echogenicity) of the renal parenchyma on initial postnatal US have prognostic value for determining the future risk of stage 5 chronic kidney disease. [17]

Because newborns commonly have relative hypovolemia during the first few days of life, it is recommended to perform US after the first week of life if findings from the first US examination were normal in a child with previously diagnosed antenatal hydronephrosis before making a final determination that the hydronephrosis has resolved (see Presentation, History).

Contrast-enhanced serial voiding urosonography has been sugegsted as a useful complementary test in pediatric patients with PUVs. [18]  

Voiding cystourethrography

The key to the workup of any child with antenatal hydronephrosis is voiding cystourethrography (VCUG). VCUG should be performed under fluoroscopy, with imaging of the posterior urethra, especially during the voiding phase. Cycling the bladder during the study several times improves the sensitivity of the evaluation.

The diagnosis of PUV is indicated by visualization of the valve leaflets. Other clues to the diagnosis are a thickened trabeculated bladder, a dilated or elongated posterior urethra, and a hypertrophied bladder neck (see the first image below). Diverticula, cellules, VUR, and reflux into the ejaculatory ducts secondary to elevated bladder and urethral pressures may also be present (see the second image below).

Note hypertrophied bladder neck and dilated poster Note hypertrophied bladder neck and dilated posterior urethra proximal to valve narrowing.
Note irregular trabeculated bladder and high-grade Note irregular trabeculated bladder and high-grade vesicoureteral reflux.

VCUG may miss late-presenting cases of PUV. [19]

Renal scintigraphy

Renal scintigraphy, though not necessary in every child, may be helpful in some cases. It should not be performed in the neonatal period, because renal immaturity does not allow for accurate estimation of renal function. If renal dysplasia is suspected, nuclear imaging can determine relative renal function. In some cases, children with a very thickened bladder wall may have secondary ureterovesical junction obstruction due to bladder hypertrophy.

Tc-dimercaptosuccinic acid (DMSA), glucoheptonate, and mercaptoacetyltriglycine (MAG-3) renal scintigraphy are cortical imaging studies that provide information about relative renal function (each kidney relative to the other) and intrarenal function (eg, photopenic areas within the kidney indicate scarring or dysplasia). Additionally, the MAG-3 renal scan with furosemide provides information about renal drainage and possible obstruction.

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Other Tests

Urodynamic evaluation provides information about bladder storage and emptying. In the older child, the mature bladder should store urine at a low pressure and then completely empty at appropriate pressures. A newborn's bladder may not empty completely under normal circumstances.

The term valve bladder is used to describe patients with PUV and a fibrotic noncompliant bladder. These patients are at risk of developing hydroureteronephrosis, progressive renal deterioration, recurrent infections, and urinary incontinence.

Patients with PUV require periodic urodynamic testing throughout childhood because bladder compliance may further deteriorate over time.

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Procedures

Cystoscopy

Cystoscopy serves both diagnostic and therapeutic functions in these infants. Appropriately-sized cystoscopes (< 8 French) are needed to avoid injury to the urethra.

Diagnostic

Confirmation with cystoscopy is required in every child in whom PUV is suggested after VCUG. In some, the filling defect observed on VCUG may represent only external sphincter contraction during voiding; in others, the valve leaflets are confirmed.

Therapeutic (ie, transurethral incision of PUVs)

Multiple techniques are described for PUV ablation. Disruption of the obstructing membrane by blind passage of a valve hook is now only of historic interest. Currently, valves are disrupted under direct vision by cystoscopy using an endoscopic loop, Bugbee electrocauterization, or laser fulguration. In extremely small infants (< 2 kg), a 2-French Fogarty catheter may be passed either under fluoroscopic or direct vision for valve disruption. [20] This is performed in the least traumatic fashion possible to avoid secondary urethral stricture or injury to the urethral sphincter mechanism.

Vesicostomy

In some patients, the urethra may be too small for the available cystoscopic instrumentation. Fortunately, because of continued advancements in pediatric endoscopic equipment, this is an uncommon occurrence. When this situation arises, a temporary vesicostomy may be performed.

Outpatient intermittent catheterization via a sensate and dilated posterior urethra and bladder neck may not be feasible in all patients. A minivesicostomy in the subinguinal region can allow continued, intermittent passage of a catheter when the urethra is not available. [21]

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