eMedicine Specialties > Radiology > Pediatrics

Posterior Urethral Valve: Imaging

Author: John S Wiener, MD, FACS, FAAP, Clinical Assistant Professor, Division of Urology, University of North Carolina at Chapel Hill; Associate Professor of Surgery and Associate Residency Program Director, Division of Urologic Surgery, Associate Professor of Pediatrics, Duke University School of Medicine
Coauthor(s): Anamaria Gaca, MD, Clinical Instructor, Division of Pediatric Radiology, Duke University Medical Center; Jeffrey Sekula, MD, Staff Physician, Chief Resident in Urology, Department of Urology, Duke University Medical Center
Contributor Information and Disclosures

Updated: Aug 7, 2007

Radiography

Findings

Plain radiographs do not add to the actual diagnosis of posterior urethral valves; however, chest radiography may be useful in the evaluation of pulmonary hypoplasia, and images of the kidneys, ureters, and bladder (KUB images) may show the ground-glass appearance of urinary ascites, if present.

VCUG is considered the diagnostic study of choice (see Images 1-5) for the evaluation of posterior urethral valves. The bladder is typically thickened with trabeculae and may exhibit vesicoureteral reflux or, less commonly, diverticula. The bladder neck is typically hypertrophic, leading to a lucent ring or collar. On voiding, the posterior urethra is dilated (ie, shield shaped), and valve leaflets may be seen as lucencies, giving the appearance of a spinning top. If leaflets are not visible, a commonly associated finding of posterior urethral bulging distally over the bulbar urethra may be noted. The anterior urethra is typically underfilled, and voiding is incomplete.

IVP is not routinely used in children because the contrast agent is poorly concentrated and visualized in newborn kidneys, particularly if renal function is diminished. Elevated serum creatinine levels may preclude the use of intravenous (IV) contrast material. IVP can show an absent kidney in the case of renal dysplasia or delayed renal function with persistent high intraluminal pressures. Hydroureteronephrosis may be seen. Delayed images may show bladder or urethral pathology, but the lower urinary tract is better visualized with VCUG.

Degree of Confidence

Again, VCUG is the criterion standard diagnostic test for posterior urethral valves. The constellation of bladder and urethral abnormalities with or without valve identification typically confirms the presence of valves. Cystourethroscopy with the patient under general anesthesia may be required to formally confirm the presence of posterior urethral valves at the time of intervention.

False Positives/Negatives

Regarding false-positive findings, any of the obstructive etiologies listed in the Differentials section above may show bladder and upper-tract changes that are typical of posterior urethral valves. However, most of the listed conditions have significant differences in imaging that separate them from posterior valves, such as the location of involvement (anterior valves, syringocele), associated anomalies (prune-belly syndrome), and degree of impairment (plicae colliculi). False-positive results should not be seen with functional disorders because the posterior urethra should not be dilated, except potentially in detrusor sphincter dyssynergy/dyssynergia.

False-negative results are rarely noted in cases of posterior urethral valve that have minimal anatomic obstruction and, thus, limited functional impairment and upper-tract changes. In boys with these findings, VCUG may not show valve leaflets. However, in a boy with the clinical stigmata of a posterior valve in whom definitive visualization of the valve is lacking, cystourethroscopy may be indicated to rule out urethral pathology. It is important to obtain voiding images in the lateral view with—and, ideally, without—the catheter in situ, because the images with the catheter removed optimally demonstrate the valves (see Images 3-5). It has long been believed that leaving the urethral catheter in place may hold the valves open and prevent proper visualization, but that does not happen in practice if good urethral distention is achieved.

In the postoperative setting after valve ablation, some posterior urethral dilatation usually persists, with secondary bladder changes. However, if incomplete valve ablation is suspected, then repeating cystourethroscopy rather than VCUG is recommended.

Computed Tomography

Findings

Rarely necessary in neonates, CT scans with IV contrast enhancement may reveal dysplastic and/or dilated kidneys with delayed renal function and excretion, hydroureter, dilated bladder with wall thickening, trabeculation, and diverticula. A dilated posterior urethra might be seen, although leaflets may be easily missed. Elevated serum creatinine levels generally preclude use of IV contrast material.

Degree of Confidence

CT scans cannot reliably depict valves, although they should reveal the sequelae of bladder outlet obstruction.

False Positives/Negatives

False findings are identical to those for radiography. An exception is that plicae colliculi, which might be seen on VCUG studies, should not be noted on CT scans.

Magnetic Resonance Imaging

Findings

MRI findings are similar to those of CT scanning except that enhancement with IV gadolinium-based contrast agents may allow functional as well as anatomic assessment.

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have recently been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. As of late December 2006, the FDA had received reports of 90 such cases. Worldwide, over 200 cases have been reported, according to the FDA. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving  or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.

Degree of Confidence

The degree of confidence is similar to that with CT scanning.

False Positives/Negatives

False findings are similar to those observed with CT scanning.

Ultrasonography

Findings

A proper ultrasonographic study to evaluate the urinary tract must include images of both kidneys, the ureters, and the bladder. Hydroureteronephrosis with or without cortical thinning, a thick-walled bladder with trabeculation and diverticula, and a dilated posterior urethra with a hypertrophic bladder neck are usually seen. In the setting of renal dysplasia, the renal parenchyma is typically hyperechogenic with visible small cysts (<10 mm), but in the most mildly affected cases, renal ultrasound findings may be normal (see Images 6-10). Echogenic lines that are the actual valve leaflets might be seen. The combination of the dilated, thick-walled bladder and dilated posterior urethra has been described as a keyhole appearance (see Image 11).

The bladder may be of large or small volume, but it is invariably thick-walled. Urinary ascites or perinephric collections due to urinomas may also be seen, most commonly soon after birth, and are caused by rupture of the urinary tract, typically at the level of the calyces. A suggestive prenatal sonogram reveals a male fetus with bilateral hydroureteronephrosis; a dilated and thickened bladder with poor emptying; and, possibly, oligohydramnios (see Images 12-24). After the diagnosis has been established and initial management has begun, ultrasonography is useful to follow up on the degree of hydronephrosis and parenchymal integrity after treatment, as well as the adequacy of bladder evacuation with voiding and the resolution of any urinomas.

Degree of Confidence

The sum of the ultrasonographic findings with clinical correlates, combined with the rarity of other obstructive lesions, is highly indicative of posterior urethral valves. The valves typically cannot be seen on sonograms, and VCUG is required to make the definitive diagnosis.

False Positives/Negatives

False-positive results may be seen with other obstructive and functional disorders of bladder emptying. The ultrasonographic findings are the final common signs of most of the conditions in the differential diagnosis (see Differentials). False-negative results may be seen in mild cases without upper-tract abnormality and an essentially normal bladder. In a study by Williams et al, the reported sensitivity of renal and bladder ultrasonography for valves was 87% in patients younger than 4 years and 98% for those age 4 years or older.³

Nuclear Imaging

Findings

Nuclear cystography has no role in the diagnosis of posterior urethral valves because of the poor anatomic detail, but this modality can depict the presence of vesicoureteral reflux. However, grading of such reflux is not as accurate as with contrast VCUG.

Nuclear renography may be used to assess upper-tract consequences of bladder outlet obstruction. A urethral catheter must be placed before the study to eliminate the effect of a distended, high-pressure bladder on renal function and drainage. An absent or dysplastic kidney is seen as a photopenic area in the renal fossa. Delayed visualization of a renal unit with a slow rise to peak activity suggests altered renal function. Differential renal function is important to estimate relative renal impairment and is based on activity at 1-2 minutes after injection of a tracer. Hydronephrosis with ureteral dilatation can represent ureterovesical obstruction or chronic nonobstructive changes of posterior urethral valves; the latter should demonstrate washout after furosemide administration, if renal function is adequate (see Images 25-26).

Degree of Confidence

Nuclear medicine study is poor for the specific diagnosis of posterior urethral valves, but it is excellent for assessment of the upper-tract consequences (see Nuclear Medicine, Findings).

False Positives/Negatives

False-positive findings can result when a urethral catheter is not used. High bladder storage pressure or vesicoureteral reflux can prevent drainage from the kidney and ureter. Distal ureteral obstruction by ureterovesical junction obstruction or a ureterocele can mimic the changes of valves. False-negative findings can be seen in cases in which there is normal renal function and drainage.

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