Bladder Outlet Obstruction
Bladder outlet obstruction is a condition in which there is a blockage of urine flow anywhere along the urethra (tube that drains urine from the bladder and out the body). The most common causes of bladder outlet obstruction is urethral atresia for females and posterior urethral valves (PUV) for males (PUV).
Obstruction occurs when urethral valves are not positioned correctly and cause a blockage in urinary flow. The baby is not able to release urine into the amniotic fluid and amniotic fluid levels can become very low. Low amniotic fluid levels (oligohydramnios) impair the developing lungs and cause them to become hypoplastic (small). Severe and prolonged urinary obstruction also leads to kidney damage.
Babies with little or no amniotic fluid early in gestation (before 24 weeks) have severely under-developed lungs and little chance of survival. Those that maintain a moderate amount of amniotic fluid do better as their lungs are able to mature. However, the degree of kidney damage and function cannot be fully assessed until after birth.
Bladder outlet obstruction can be diagnosed prenatally on ultrasound. Serial ultrasounds are needed to assess renal function, measure amniotic fluid volume and assess the structure and function of the fetal urinary tract. A fetal MRI is obtained to better assess the fetal urinary tract and to rule out other structural anomalies. An echocardiogram is performed around 24 weeks gestation to assess the structure and function of the fetal heart and rule out other cardiac anomalies.
Serial fetal bladder taps help determine kidney function. A sample of fetal urine is obtained and urine electrolytes, protein levels and beta2 microglobulin (a marker of kidney damage) are measured. At this time, the fetal bladder is completely drained of urine. Determination of fetal chromosomes can be done at this point to rule out other chromosomal anomalies. This procedure is similar to an amniocentesis where a needle is placed through the mother’s abdomen and into the fetus’s bladder. Fetal urine specimens should be repeated in 24-48 hours and often requires a third specimen to determine the most accurate assessment of kidney function.
Fetal surgery may be an option for select patients with the goal of restoring adequate amniotic fluid levels to promote lung development and prevent ongoing kidney damage. Serial fetal bladder taps, analysis of fetal urinary electrolytes and beta2 microglobulin, and ultrasound of the fetal kidneys are required to select patients with reversible kidney function. Patients who have evidence of severe irreversible kidney damage are not candidates for fetal intervention.
Percutaneous vesicoamniotic shunting is the method most commonly used to relieve urinary tract obstruction. It involves the placement of a double pig-tailed catheter under ultrasound guidance and local anesthesia to allow drainage of fetal urine into the amniotic fluid.
Prenatal counseling with pediatric specialists, including fetal surgeons, urologists, nephrologists and neonatologists, may be helpful regarding options in treatment and long-term outcome.