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Posterior urethral valves are the most common cause of bladder outlet obstruction in males. An abnormal fold of tissue in the urethra blocks urine from flowing freely out of the bladder. If not corrected, this blockage can cause backup of urine in the bladder, the ureters and in the kidneys (hydronephrosis) and in severe cases, a deficiency of amniotic fluid (oligohydramnios) that can threaten lung development.
The effects in children born with bladder outlet obstruction range from completely asymptomatic to respiratory insufficiency at birth as a result of pulmonary hypoplasia (arrested development in the lungs) and kidney failure from renal dysplasia (abnormal development of the kidneys). Severe respiratory insufficiency at birth is a leading cause of neonatal death.
In most cases, bladder-outlet obstruction due to posterior urethral valves is a sporadic event. In rare cases, it is associated with a chromosomal abnormality, which increases the risk of a subsequent fetus being affected.
Posterior urethral valves account for 10 percent of all urologic anomalies detected by prenatal ultrasound. It is estimated, however, that prenatal ultrasound detects only 50 percent of new cases of posterior urethral valves, suggesting that the incidence of posterior urethral valves is 1 in 4,000 live births. This estimate does not take into account cases where pregnancy does not end in live birth or asymptomatic cases discovered later in life.
Posterior urethral valves can be diagnosed by ultrasound (sonogram) before birth. In mild cases, the fetus may have obstruction in the urinary tract, but with amniotic fluid volume maintained, minimal changes in the bladder and ureters, and no dysplasia in the kidneys. In severe cases, the fetus has profound oligohydramnios, distended bladder and ureters, and dysplastic changes in the kidneys.
Sonographic findings indicating bladder obstruction include a thickened bladder wall and if posterior urethral valves are the cause, the posterior urethra will also be dilated. Along with a detailed examination of the urinary tract itself, the sonographic evaluation should look for amniotic fluid volume and the presence or absence of ascites (fluid in the abdomen).
Oligohydramnios is a sign of high-grade obstruction and over time can lead to pulmonary hypoplasia as well as other deformations, such as club feet and "Potter facies," which results from facial decompression against the uterine wall. The increased incidence of these other malformations means that the fetus should also be scanned for them.
Ultrasonographic examination of the fetal kidneys may provide prognostic information. Fetuses with bilateral hydronephrosis and normal amniotic fluid may not need intervention and fetuses shown by ultrasound to have bilateral hydronephrosis, severe oligohydramnios, and severe dysplasia in the cortex of the kidney are unlikely to benefit from in utero therapy.
For fetuses in between these two extremes of not needing or not being likely to benefit from in utero therapy, prognostic criteria are the most important and sensitive means of assessing fetal renal function essential to making treatment decisions. In addition to the evaluation of the fetal urinary tract, an assessment of the overall growth and development of the fetus will be conducted.
The use of serial sampling of fetal urine and chromosomal analysis has significantly improved the predictive value of testing to identify fetuses with significant renal damage. Echocardiography should be performed to rule out structural heart disease. Consultations with a genetics counselor and pediatric specialists in urology, nephrology, and neonatology can be helpful in learning about treatment options and long-term outcome.
Once the diagnosis is established, fetal therapy can begin. Fetal surgery is usually done to help keep detected abnormalities from becoming life-threatening health issues once the baby is born.
Before invasive fetal therapy is undertaken, all test results and other assessments of the fetus and mother are reviewed to understand how a prenatal condition would progress if left untreated. Over the last decade, the ability has improved to differentiate fetuses that are most likely or least likely to benefit from in utero intervention. Proper selection of patients is important to avoid complications in fetuses likely to do well without intervention.
Fetuses most likely to benefit from fetal intervention are those with obstruction severe enough to compromise renal and pulmonary development, but not so severe that renal damage is irreversible even if the obstruction is relieved. Several methods have been suggested to assess the functional capacity of the kidneys in a fetus with obstructive uropathy, including the appearance of the kidneys on the sonogram, the volume of amniotic fluid, and various urinary electrolytes and proteins.
A retrospective reevaluation of 40 fetuses with bilateral hydronephrosis found significant differences in survival among fetuses assigned to the good prognosis group based on fetal urine electrolyte levels (sodium < 100 mEq/L, chloride < 90 mEq/L , osmolarity < 210 mOsm/L) and no sonographic evidence of dysplasia. The survival rate for this good prognosis group was 81 percent vs. 12.5 percent for those in the poor prognosis group not meeting these criteria. Intended as a means to select fetuses for in utero intervention, these prognostic criteria accurately reflect the fetuses who have sufficient renal function to have a favorable outcome if decompressed in utero. If the fetal obstruction is not relieved, the renal function is likely to deteriorate despite a favorable prognostic profile.
Using in utero intervention to restore amniotic fluid volume averts neonatal death from pulmonary hypoplasia. Regardless of whether fetuses were in the good or the poor prognosis group, not treating oligohydramnios was associated with a 100 percent mortality rate. Normal or restored amniotic fluid volume was associated with a 94 percent survival rate.
Questions remain about whether fetuses with bladder obstruction should be treated before oligohydramnios develops. Waiting until infancy or childhood to treat obstruction may not prevent the progression to end-stage renal failure.
Up to now, fetal therapy has been aimed at restoring amniotic fluid volume to allow pulmonary development and prevent death from pulmonary hypoplasia. Using fetal urinary β2-microglobulin as an additional prognostic predictor could allow selection of fetuses with a good prognostic profile but at risk for ongoing renal damage and make the treatment goal preservation of renal function. This would significantly expand the indications for fetal intervention in obstructive uropathy, but more data on β2-microglobulin are needed.
A retrospective review of the fetal surgery database at the University of California, San Francisco, from 1981 to 1999 identified 40 patients evaluated for fetal intervention and 36 fetuses that had surgery, including 14 with confirmed posterior urethral valves. The patients all had favorable urinary electrolytes. Six patients died before term delivery and the remaining eight were followed for a mean follow-up of 11.6 years. Five developed chronic renal disease; two have had renal transplantation, and one was waiting at the time of the report. Five of the 8 have had urinary diversion.
The authors concluded that fetal intervention for posterior urethral valves carries a considerable risk to the fetus, with a fetal mortality rate of 43 percent. Because long-term outcomes may not change the prognosis of renal function or be a predictor of whether urinary diversion will be needed later, the authors conclude that it should be performed only for carefully selected patients who have severe oligohydramnios and “normal” looking kidneys.
Clinical experience suggests that the consequences of bladder obstruction to the lungs and the kidneys will vary with the severity of the obstruction. The broad spectrum of severity presents a challenge: selecting which patients have severe enough obstruction to warrant prenatal intervention, which should be delivered early to prevent ongoing renal damage, and which fetuses should go to term and be managed postnatally.
In utero interventions are intended to decompress the urinary tract, restore amniotic fluid dynamics, and prevent neonatal death due to pulmonary hypoplasia and renal dysplasia. Available fetal interventions are vesicoamniotic fetal shunting, open fetal surgery, and fetoscopic surgery.
Vesicoamniotic shunting drains urine from the bladder into the amniotic space. This procedure has the advantage of bypassing the obstruction, but has risk of associated complications.
Obstructive uropathy has been treated in utero by vesicoamniotic shunting since the early 1980s. Since then, advances in diagnosis, technique, shunt design and patient selection have contributed to the enthusiasm for treating fetal obstructive uropathy. This enthusiasm, however, resulted in vesicoamniotic shunting becoming widely implemented before stringent selection criteria for treatment were developed and the effectiveness of the procedure established.
It is unclear how effective prenatal decompression is in treating obstructive uropathy. In a large study using shunts, the survival rate among fetuses with posterior urethral valves was 60 percent.
An International Fetal Surgery Registry was established to help define the risks and benefits of fetal intervention for obstructive uropathy, but insight into the effectiveness of the procedure was limited by the number of patients, the lack of standard criteria, and limited follow-up.
Among the first 87 reports of vesicoamniotic shunting for hydronephrosis, the overall survival rate was 40.2 percent and 47.3 percent among the 74 fetuses not aborted. Survival depended on the underlying cause, but was highest (68 percent) among fetuses with posterior urethral valves. Despite limitations, the registry data did establish that treating obstructive uropathy was feasible and helped to define the role of intervention in fetal obstructive uropathy.
Several factors have limited the success of vesicoamniotic shunting inappropriate for long-term decompression of the urinary tract early in gestation and serve as the impetus for developing open fetal surgical and fetoscopic techniques to treat obstructive uropathy in utero and reintroduction of open fetal surgery for vesicostomy.
The maternal morbidity associated with vesicoamniotic shunting has been reported to be minimal, but the incidence of chorioamnionitis, inflammation of the fetal membranes, has been as high as 14 percent. These cases of chorioamnionitis occurred, however, before the routine use of preventive antibiotics and when long-term catheterization rather than aspiration was used for fetal urine sampling. In addition, there have been reports of shunt-induced abdominal wall defects with herniation of the bowel through stab wounds and maternal ascites through leakage of amniotic fluid into the maternal peritoneal cavity.
Long-term shunt success is variable, largely due to shunt obstruction or displacement. Functional shunt failure has been reported to occur in 40 percent to 50 percent of cases after successful placement of the shunt in the fetal abdomen or amniotic space.
Despite the ease with which vesicoamniotic shunts can be placed, the most serious limitation is the poor renal outcome associated with vesicoamniotic shunting.
Survival following vesicoamniotic shunting for bladder outlet obstruction ranges from 40-85%. Even among successfully treated cases, vesicoamniotic shunts do not completely decompress the urinary tract.
Despite restoring amniotic fluid with good pulmonary outcome, 50% of survivors go on to develop renal failure requiring dialysis and renal transplantation.
The shunts themselves have a narrow lumen and are four inches long resulting in significant assistance to flow of urine. As a result, significant intravesical pressure must be generated to force urine through the shunt into the amniotic cavity. The high bladder pressure gets transmitted to the kidney if the ureterovesical junction is incompetent. This pressure on the developing kidney results in ongoing dysplastic changes, setting the stage for postnatal renal failure, dialysis and kidney transplantation.
Awareness of the poor long-term outcomes with vesicoamniotic shunts has driven the development of fetoscopic procedure to eliminate the valves causing bladder outlet obstruction and open fetal surgical procedure to completely decompress the urinary tract.
In fetoscopic surgery, surgeons use a fiberoptic endoscope to enter the uterus and correct defects through small surgical openings. This minimally invasive procedure is in its infancy and is limited to very few patients. Posterior urethral valves are among the fetal malformations treatable by fetoscopic techniques.
Several small reports over the past decade have shown the feasibility of fetoscopic cystoscopic laser treatment (using a fetoscope to inspect and pass instruments into the bladder and urethra) can be used to treat bladder obstruction due to posterior urethral valves.
A 1995 report of percutaneous fetal cystoscopy on a male fetus confirmed posterior urethral valves as shown by sonography and percutaneous endoscopic fulguration (electrodessication) of the valves was successfully performed at 22 weeks. While the authors said the case illustrates the feasibility of using endoscopic procedures to diagnose and treat congenital anomalies in the fetus, they called for more studies to establish the actual value, risks, and limitations of the approach.
In a 2000 case report, an infant had successful in utero endoscopic ablation of posterior urethral valves and survived the neonatal period. In a 2003 study from England, among 13 fetuses examined by fetoscope, 10 were considered treatable by saline hydroablation and / or guide-wire passage, including 5 fetuses with confirmed posterior urethral valves normal fetal urinary electrolytes and 2 had acceptable renal function at follow-up.
The authors concluded that guide-wire passage or hydro-ablation may allow ablation of posterior urethral valves in utero with relatively little trauma and without chronic bladder decompression associated with vesicoamniotic shunting, but that technical limitations need to be overcome, possibly by flexible or angled fetoscopes, before this form of treatment can be formally evaluated.
Fetoscopic procedures have benefited from recent advances in optics and instrumentation. The fetoscopic approach, with its small uterine puncture sites, could reduce the risks of preterm labor, hemorrhage, amniotic fluid leak, and uterine rupture and eliminate the need for cesarean delivery following open fetal surgery.
Developing an approach in an 18- to 20-week old fetus, however, has been problematic. Several attempts at laser ablation of posterior urethral valves were technical successes, but postoperative obstetrical complications resulted in no long-term survivors from this early experience.
Despite a compelling rationale for fetal cystoscopic treatment of posterior urethral valves and advances in fetoscopic techniques, this approach has yet to be shown to be a safe and effective treatment.
The progressive angulation that occurs as gestation progresses limits our ability to fetoscopically visualize the posterior urethral valves. After 20 weeks gestation, this angle become too acute for current instrumentation. As a result, we have reserved the fetoscopic approach to patients presenting prior to 20 weeks gestation with posterior urethral valves, oligohydramnios and favorable prognostic profile based on fetal urine electrolytes and imaging studies (U/S and MRI).
The first successful treatment of a structural anomaly by open fetal surgery was performed in 1981 for bladder outlet obstruction. This initial case was treated by bilateral ureterostomies. Subsequent open fetal surgical approaches for PUV was by vesicostomy.
In this initial experience, Crombleholme et al report survival of 3 of 4 fetuses with one survivor requiring transplantation. This experience was prior to the development of fetal urinary electrolyte criteria to select patients with a favorable prognosis for intervention. At the same time, the first case treated by vesicoamniotic shunt was reported. This minimally invasive approach was rapidly disseminated and became the accepted standard of care prior to rigorous evaluation of efficacy of vesicoamniotic shunts.
The poor long-term renal outcomes with vesicoamniotic shunts prompted the Fetal Care Center of Cincinnati to re-introduce open fetal surgery for vesicostomy in cases of PUV. This is the most definitive means of decompressing the urinary tract to prevent ongoing injury to the developing kidney. In fetal vesicostomy, the fetal abdomen is opened below the umbilical cord insertion. The bladder is opened and sutured to the fetal skin, allowing complete urinary diversion eliminating high bladder pressure seen with vesicoamniotic shunts.
While this is a much more invasive approach with greater potential risk for the mother and fetus, our preliminary experience has been encouraging. Normal creatinine and normal bladder function have been observed postnatally following open fetal surgery for visicostomy. The baby is usually born prematurely, however, and the mother will need to deliver by Cesarean section due to the uterine scar created by the hysterotomy.
Open fetal surgery for vesicostomy is a therapeutic innovation we are now offering to select fetuses with PUV and favorable prognosis profile with oligohydramnios. Long-term follow-up studies will be needed to prove this approach is superior to vesicoamniotic shunting and that the improved outcome is substantial enough to warrant the potential increased fetal and maternal risks.
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