EMBRYOLOGY OF POSTERIOR URETHRAL VALVES

Three discrete embryologic defects are postulated in the etiology (1).

1. The most common type (Type I in the Young classification).
• Pleats of exaggerated plicae colliculi that project from the lateral wall in the region of the verumontanum.
• Type I valves are thought to arise from the abnormal origin of Wolffian ducts in the cloaca, which leave crescent-shaped remnants along their path of migration to their final site in the verumontanum (2).
• Other workers postulate that Type I valves are caused by abnormal fusion of the urethrovaginal folds and the possible failure of Cowper's gland to migrate (3).
• The resultant valves can fold over to obstruct the urethra and may be held in this position by the increased pressure in the bladder.
2. Less commonly the posterior urethral obstruction may be caused by a true transverse membrane of varying thickness occluding the urethra just distal to the verumontanum (Type III in the Young's classification). This membrane is probably a remnant of the urogenital membrane and may be solid or fenestrated by single or multiple apertures of varying size.
3. Rarely, the posterior urethral valve may consist of mucosal folds that spiral longitudinally along the urethra (Type II in Young's classification). This valve is not thought to cause any fetal obstructive uropathy.

REFERENCES

1. Young HH, Frontz WA, Baldwin JC. Congenital obstruction of posterior urethra. J Urol 1919;3:289.
2. Stephen FD. Urethral obstruction in childhood: The use of urethrography in diagnosis. In: Douglas SF (ed). Congenital malformations of the rectum, anus and genitourinary tract. Edinburgh: Livingstone 1963:209-245.
3. King LR. Posterior urethra. In: Kelalis PP, King LR, Belman AB (eds). Clinical Pediatric Urology. 2nd ed. Philadelphia: WB Saunders 1985:527-558.