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Saturday, July 4, 2009
Posterior Urethral Valve (PUV)
10 Month old child came with repeated UTIs. MCUG (Micturiting cystourethrography) shows irregular outlines bladder mucosa with multiple small diverticula. There is vesicoureteral reflux on the left side. The posterior urethra is dilted with hypertrophy of bladder neck and lucency between the bladder neck and posterior urethra giving the typical appearance of spinning top. Features suggestive of posterior urethral valve.
A posterior urethral valve is an abnormal congenital obstructing membrane that is located within the posterior male urethra; this valve is the most common cause of bladder outlet obstruction in male children. A posterior urethral valve occurs in approximately 1 in 5000 live male births.
There are 3 types of posterior urethral valves.The type I valve is a bicuspid valve that radiates distally from the posterior edge of the verumontanum to the anterior proximal membranous urethra. It account for 95% of all valves. The type II valve is no longer considered an obstructing valve; rather, it is thought to be a sequela of voiding dysfunction. It was described as a hypertrophic band of superficial muscle that runs along the posterolateral wall of the urethra from the ureteral orifice to the verumontanum. Therefore, these valves can be differentiated from type I and type III valves by their location proximal to the verumontanum. The type III valve is a circumferential membrane or diaphragm that is located at the membranous urethra, which is thought to result from the incomplete regression of the urogenital membrane during embryogenesis. There is a central aperture, and the central portions of the ring may prolapse into the more distal urethra during voiding, which results in a wind-sock appearance on VCUG. Type III valves account for almost 5% of all valves.
VCUG (Voiding cystourethrography) is considered the diagnostic study of choice 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.
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. 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. 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.