Bladder outlet obstruction (BOO) is a blockage of the bladder.It is sometimes also referred to as fetal lower urinary tract obstruction (LUTO). This blockage prevents the urine flow into the amniotic fluid. Because of this, the amniotic fluid level can become very low. Low amniotic fluid levels (oligohydramnios) impair the developing lungs causing them to become very small (hypoplastic). Babies with little or no amniotic fluid early in gestation (before 24 weeks) may have severely underdeveloped lungs. As a result, these babies have a poor chance of survival. Babies that have a moderate amount of fluid do better as their lungs are able to mature.
If the blockage persists, kidneys will become damaged. The degree of kidney damage can sometimes be determined before birth.
Bladder outlet obstruction occurs in about 1 in 5,000 live births. The most common cause in a male fetus is abnormally formed tissue called posterior urethral valves.
Diagnosis of bladder outlet obstruction
Ultrasound images are used to assess the size of the bladder, and changes in the appearance of the kidneys. It also is used to check fluid variations around the baby.
Bladder outlet obstruction can be seen in babies with trisomy 13, 18, and 21. A chromosome test is recommended if there is a suspicion of an underlying problem. A fetal echocardiogram is recommended if a heart defect is suspected.
Fetal treatment of bladder outlet obstruction
Fetal treatment is available for select patients whose kidney damage is believed to be reversible. The goal is to restore amniotic fluid levels to promote lung development and prevent further kidney damage.
Regular fetal bladder taps are used to determine which babies will benefit from treatment. A needle is used to collect the fetal urine and it is sent to the lab for analysis. The analysis looks at urine electrolytes and beta2 microglobulin. Both are used to determine the degree of kidney function. Patients whose test results suggest severe irreversible kidney damage are not candidates for fetal therapy.
Fetal therapy involves placing a tiny flexible tube (shunt) from the fetal bladder into the amniotic fluid. This is called a vesicoamniotic shunt. This tube allows urine to bypass the blockage. There are limitations to this therapy. The tube can move out of place, become occluded, or it can not drain the bladder well.
In select patients, a fetoscopic procedure may be used to destroy the abnormal tissue causing the blockage.
Management of pregnancy
Ongoing ultrasounds will be used to monitor fluid levels throughout the pregnancy. For those babies with a shunt in place, ultrasound is used to check tube position. A second shunt may be recommended if the first one is out of place and not working properly.
Delivery of a baby with bladder outlet obstruction
Babies born with a bladder outlet obstruction need access to a team of pediatric specialists at birth. Delivery should be planned at a hospital with access to pediatric nephrology and urology.The birth plan for severe cases of bladder outlet obstruction where there is prolonged low fluid and suspected small lungs should be discussed in advance.
It is also advised that delivery be planned at a hospital that is prepared for high-risk births including a newborn intensive care unit and pediatric surgical services. This will simplify communication between obstetrical, neonatal and pediatric surgical teams as well as limit the separation between mother and baby.
University of Michigan is one of only a few centers nationwide at which the birth center is co-located within a comprehensive children’s hospital. This unique setting allows for seamless integration between our private-room birth center, state-of-the-art newborn intensive care unit (NICU), and access to around-the-clock pediatric surgical services in one convenient location.
Postnatal treatment of bladder outlet obstruction
After delivery, the baby will undergo a full evaluation. It is important to know the degree of kidney function and the area where the blockage occurred to create the best treatment plan. The neonatologists will work with the other pediatric specialists, including surgery, urology and nephrology, during this process.
A renal ultrasound is used for the initial assessment. Babies with severe problems should have an ultrasound within the first day of life. The amount of urine the baby makes is measured.; Babies with severe obstruction will have an operation. Blood tests are done to assess kidney function beginning at 24 hours of age.
The long-term health of these babies is dependent on many factors. Things to consider include the amount of kidney damage, the degree of lung development, and if any other anomalies are present.; Gradual kidney failure can occur even with successful fetal therapy These babies may still need dialysis or a kidney transplant. Your team of doctors will discuss possible outcomes with you.
Future pregnancy risk
Most cases of bladder outlet obstruction that are related to posterior urethral valves are a random event. There is very little risk of it happening again.
Those cases caused by a chromosomal abnormality, or by a rare syndrome, have a higher risk of happening again. Your doctor and a genetic counselor will discuss the risk in your family.