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.
Pathogenesis
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.
Between the fourth and sixth weeks of fetal development, the cloaca is
divided into the primitive urogenital sinus and the rectum, and the cloacal
membrane is likewise divided into the urogenital membrane and rectal membrane.
The primitive urogenital sinus has several distinct regions that give rise to
the different segments of the lower urinary tract. The dilated cranial region
becomes the urinary bladder; the neck just caudal to this region becomes the
prostatic and membranous urethra; and the most caudal region, known as the
definitive urogenital sinus, becomes the bulbous and spongy urethra.
As the urinary tract develops, swellings known as the cloacal folds
appear lateral to the cloacal membrane. These folds fuse above the cloaca to
form the genital tubercle. As the cloaca is divided, the portions of the
cloacal folds lateral to the urogenital membrane become known as the urogenital, or urethral, folds. After the urogenital membrane dissolves, the
urethral folds fuse in the midline, encasing the future bulbous and penile
urethra.
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.
If PUVs are not evident antenatally, their sequelae generally become
apparent in the first year of life, with the exact timing depending on the
severity of the obstruction.
Patients with the most severe obstructions present at birth with
pulmonary hypoplasia and Potter facies owing to oligohydramnios during
gestation (see Plate 2-8), as well as enlarged, poorly functional kidneys that
may be noted as palpable abdominal masses.
The remaining patients present in the weeks to months after birth with
failure to thrive, urinary tract infection, and often a poor urinary stream. On
further assessment, these patients are found to have renal insufficiency, which
results not only from obstruction-induced damage to the renal tubules, but also
from renal dysplasia secondary to a developmental obstruction to urine outflow.
The definitive diagnostic test in the infant is a voiding cystourethrogram
(VCUG), which reveals dilation of the bladder and prostatic urethra. The valves
them selves may be visible as urethral filling defects. The bladder wall may
appear trabeculated and contain diverticula, both of which reflect chronic
obstruction (see Plate 6-2). A variable degree of vesicoureteral reflux may also
be seen because even normally formed ureterovesical junctions may be unable to
tolerate the high pressure resulting from the outlet obstruction. In addition
to the VCUG, a renal scan may also be performed to assess the remaining level
of renal function and measure the severity of the obstruction.
Treatment
The first goal of treatment is urinary drainage. Although prenatal
interventions have been attempted in an effort to limit damage to the
developing kidneys, this is not a common practice, and thus most patients are
treated immediately after birth. In most cases, draining the bladder can be
accomplished simply by placing a urethral catheter. In the acute period,
patients may require a host of additional interventions to address the
pulmonary hypoplasia that can result from oligohydramnios, as well as the fluid
and electrolyte abnormalities that can occur secondary to renal insufficiency.
Once bladder drainage has been established, the patient can be monitored
for several weeks. After this period, the treatment of choice for full-term,
normal-sized infants is cystoscopic valve ablation using a laser, cold knife,
hook, or other device. It is important to permit urination immediately after
the ablation to prevent stricture formation. In a premature or low-birth-weight
neonate with a small caliber urethra that would not permit endoscopic ablation,
a cutaneous vesicostomy may be
performed as a temporizing measure.
Once the infant is older and the urethra is large enough to accommodate a
pediatric cystoscope, delayed valve ablation and eventual reversal of the
vesicostomy can be performed.
The long-term clinical syndrome that results from PUVs is known as the
“valve-bladder syndrome.” Affected patients have large bladder volumes and
subsequent overflow incontinence, which occur because bladder compliance and sensation decrease in response to longstanding obstruction. The large
bladder volumes perpetuate obstructive injury to the kidneys. The injured
kidneys, in turn, worsen the overfilling problem because their damaged collecting
ducts become unable to appropriately concentrate urine. Some of these changes
may be reversible if bladder pressure is regularly reduced, as with clean
intermittent catheterization.
