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KEY POINTS
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Key Points

  • Posterior urethral valves (PUVs) are the most common cause of bladder outlet obstruction in males.

  • PUVs account for 10% of all urologic anomalies detected by prenatal ultrasound.

  • PUVs have a broad range of severity from completely asymptomatic to complete obstruction, oligohydramnios, pulmonary hypoplasia, and renal dysplasia.

  • Oligohydramnios due to a PUV, if untreated, is associated with nearly uniform neonatal mortality due to pulmonary hypoplasia.

  • Sonographic prognostic criteria predicting poor outcome include presence of subcortical cysts and increased echogenicity of the kidneys, which are associated with renal dysplasia.

  • Fetal urine electrolytes (Na <100, Cl <90, Osm <210, β2-microglobulin <6) are associated with preserved renal function and favorable prognosis with treatment.

  • Fetal urine electrolytes are only valid for fetuses at 20 weeks of gestation or later.

  • Treatment options include vesicoamniotic shunt placement, fetoscopic ablation of PUV, and open fetal surgery for vesicostomy.

  • Fifty percent of successfully treated fetuses with PUV develop growth failure and chronic renal failure requiring dialysis and/or transplantation.

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CONDITION
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Unlike obstruction of the urinary tract at other levels, bladder outlet obstruction has the potential to affect the development of the whole urinary tract as well as the lungs. In males, the most common cause of bladder outlet obstruction is posterior urethral valves (PUVs) (Atwell, 1983). In contrast, in females the most common cause of bladder outlet obstruction is urethral atresia. As in many other congenital anomalies, there is a broad range of severity, with bladder outlet obstruction ranging from completely asymptomatic to the infant who presents at delivery with respiratory insufficiency due to pulmonary hypoplasia caused by long-standing oligohydramnios and renal failure from renal dysplasia.

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PUVs are thought to be embryologically derived from mullerian duct remnants or remnants of the cloacal membrane dating back to between the 7th and 11th weeks of gestation (Dewan et al., 1992, 1995). Young et al. (1919) described a classification scheme for PUVs. In type I, the valves are sail-like leaflets that arise from the crista-urethralis distal to the verumontanum. These valves may cover only the lower half of the urethra or may encircle the urethra and cause complete obstruction. Type II valves are nonobstructing folds of the superficial muscle and mucosa that extend from the verumontanum to the bladder neck (Cuckow, 1998). Type III valves usually cause a diaphragm-like obstruction at the level of the verumontanum, but can be seen at the level of the anterior urethra distal to the external urethral sphincter (Hendren, 1971).

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The characteristic prenatal presentation of bladder outlet obstruction is that of a dilated bladder and bilateral hydroureteronephrosis. The severity of prenatal bladder outlet obstruction ranges from mild to severe as is seen postnatally. At the better end of the spectrum, the fetus may have obstructive uropathy with preservation of amniotic fluid volume, minimal changes in the bladder and ureters, and no dysplastic changes in the kidneys. At the severe end of the spectrum are fetuses with profound oligohydramnios, distended bladder, ...

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