POSTERIOR URETHRAL VALVES
Posterior urethral valves (PUVs) are abnormal mucosal folds in the distal urethra that arise during fetal development and interfere with the normal outﬂow of urine. They are the most common cause of congenital urinary tract obstruction, occurring in 1 in 8000 to 1 in 25,000 live births, and are seen only in males. Even if treated early on, the obstruction associated with PUVs frequently causes severe, often permanent urinary tract abnormalities.
The traditional classiﬁcation system describes two major types of valves, which vary both in morphology and relative frequency. Type I valves, said to account for more than 95% of cases, begin as a mucosal ridge from the seminal colliculus, which extends distally and divides into two ﬂaps that fuse with the walls of the membranous urethra. There is typically incomplete fusion of the ﬂaps with the anterior wall of the urethra, and there is a small opening in the membrane near the posterior wall of the urethra, adjacent to the seminal colliculus. Type III valves, in contrast, are said to account for about 5% of cases and resemble disklike membranes that span the entire circumference of the membranous urethra and contain a small central opening. (Type II valves, extending from the seminal colliculus toward the bladder neck, are no longer thought to be actual valves but rather bladder neck hypertrophy, which accompanies any distal urethral obstruction.) More recent work, however, suggests that in fact all PUVs are membranous, originally resembling type III valves, and that type I valves are actually an artifact of urethral instrumentation, which divides the single membrane into two ﬂaps.
The male urethra is divided into four portions, the precursors of which become evident early in development. The segments include the prostatic urethra, which extends from the bladder neck to the urogenital diaphragm; the membranous urethra, which traverses the diaphragm; the bulbous urethra, which extends from the urogenital diaphragm to the penoscrotal junction; and the spongy (penile) urethra, which continues through the penile shaft until the urethral meatus.