Posterior urethral valves (PUV) are obstructing membranous folds within the lumen of the posterior urethra that are the most common etiology of urinary tract obstruction in the newborn male, occurring in 1 in 5000 to 8000 pregnancies (figure 1) . PUV are also the most common cause of chronic renal disease due to urinary tract obstruction in children .
The diagnosis and presentation of patients with posterior urethral valves will be reviewed here. The management of these patients is discussed separately. (See "Management of posterior urethral valves".)
The pathogenesis of PUV appears to be due to an obstructing persistent urogenital membrane (figure 1). Although the exact mechanism resulting in this obstruction is not completely understood , it appears that the normal embryologic development of the male urethra between weeks 9 and 14 of gestation is disrupted.
Normal male urethral embryology — Between weeks four and six of gestation, the cloaca is divided into the anorectal canal and the urogenital sinus. Over the following weeks of gestation, the urogenital sinus is divided by the entrance of the mesonephric duct into the cephalad vesicourethral canal giving rise to the bladder and pelvic urethra. The caudal portion of the urogenital sinus (genital tubercule) forms the phallic urethra. As the genital tubercle elongates in the male fetus, the urethral groove and folds are created. As the penile urethra grows, it moves towards the urethral plate of the glans penis. Complete fusion of the entire urethra occurs at about 14 weeks gestation.
PUV embryology — Several disruptions of the male embryologic urethral development have been proposed as the mechanism resulting in PUV . These are based upon published anatomical descriptions in the literature and include the following :