The developmental abnormality that gives rise to PUVs remains uncertain, at least in part because of the ongoing debate over the precise morphology of this defect. It has been hypothesized, however, that PUVs represent a remnant of an incompletely dissolved urogenital membrane or, alternatively, overdevelopment of a portion of the urethral folds.

PRESENTATION AND DIAGNOSIS

The majority of PUVs are noted on prenatal ultrasonography. Although the valves themselves are too small to be seen, the anatomic sequelae of the chronic outflow obstruction are evident, including dilation of the prostatic urethra and bladder, thickening of the bladder wall, and bilateral hydroureteronephrosis. The dilated prostatic urethra can be seen caudal to the dilated bladder, a phenomenon known as the “keyhole” sign. The renal parenchyma itself may also appear abnormal, generally manifest as increased echogenicity. If the obstruction is severe, oligohydramnios may be seen. Of note, many of these features may not be prominent if ultrasound is performed before 24 weeks of gestation.