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Bladder Outlet Obstruction (BOO)


What is Bladder Outlet Obstruction?
Bladder Outlet Obstruction is a blockage in the fetus’ bladder that restricts or prevents the flow of urine out of the body. In most boys with this condition there is a thin membrane, a posterior urethral valve, which blocks the urine. In rare cases, the urethra has not formed properly and the urine cannot exit the body, a condition known as urethral atresia. This can cause Bladder Outlet Obstruction in both boys and girls

Bladder Outlet Obstruction varies in severity from very severe, causing serious kidney problems and requiring surgery, to mild, requiring minimal treatment after birth.  Bladder Outlet Obstruction is not related to anything the mother did or did not do during pregnancy.

How is Bladder Outlet Obstruction diagnosed?
Bladder Outlet Obstruction can be diagnosed as early as 16 weeks, but it is often found during a focused ultrasound at 20 to 24 weeks of gestation.  In analyzing the ultrasound, the physicians at the St. Louis Fetal Care Institute will look at the size of the fetal bladder, the size of the developing kidneys, and the amount of amniotic fluid to determine whether there is an obstruction, and how severe it is.

How is Bladder Outlet Obstruction managed during pregnancy?
After diagnosing Bladder Outlet Obstruction, our team assesses the severity of the obstruction.  Mild cases may only require observation during pregnancy and further evaluation after birth.  In severe cases, however, fetal surgery may be the best option.

Typically, the earlier the obstruction is diagnosed, the more severe the condition because there is a higher chance of severe kidney and bladder dysfunction, as well as poor lung development.  The kidneys and lungs are linked in development through the amniotic fluid.  The amniotic fluid exerts pressure on the lungs, promoting growth.  Most of the amniotic fluid is composed of fetal urine.  When the urine cannot get out of the bladder, the amount of amniotic fluid begins to decline and there is no pressure on the lungs, leading to poor lung growth.

What is the fetal surgery for Bladder Outlet Obstruction, and how does it work?
Before considering surgery, the Fetal Care Institute team must examine the degree of kidney damage by doing an ultrasound or a fetal urine analysis. To perform the fetal urine analysis, the surgeon places a very small needle through the mother’s abdomen and into the fetus’ bladder—much like an amniocentesis, to collect a urine sample. The fetal urine is analyzed to help determine the kidney function.

Once the kidneys have been evaluated, our team will help you make the best decision for your baby and whether fetal intervention is an option. Those most likely to benefit from fetal surgery are those with an obstruction severe enough to damage kidney and lung development, but not so severe that the kidney damage is irreversible. Fetal surgery aims to restore the normal flow of urine into the amniotic fluid, allowing the lungs and kidneys to develop.

The most common fetal procedure established for bladder outlet obstruction is the placement of a vesicoamniotic shunt.  This shunt is a small flexible tube that is placed through the fetal skin, into the baby's bladder and allows urine to pass into the amniotic cavity (the bag of water).  The shunt can increase the potential for normal pressure in the fetal kidneys and bladder as well as normal fluid in the amniotic cavity.  This establishment of normal amniotic fluid allows for the best chance of normal lung development. Because the shunt is placed into the front of the baby's body, the baby may pull the shunt out of the bladder.  Other complications include the chance of infections, fetal bleeding and uterine contractions. 

With recent advancements in fetal medicine, new procedures have been developed over the past 15-20 years. One of these procedures is fetal cystoscopy (this is when a small fiberoptic scope is inserted into the fetal bladder) to remove the obstruction. The St. Louis Fetal Care Institute at SSM Cardinal Glennon Children's Medical Center is one of the few centers with the experience, and ability, to perform this fetal intervention. During the minimally invasive fetal cystoscopy procedure, the surgeon makes a pencil-tip-sized incision in the mother’s abdomen and inserts a very small fetoscope directly into the fetal bladder. If a posterior urethral valve is found, a laser is used to remove the valve. If a urethral atresia is present a small tubular shunt is placed to allow fluid to drain from the bladder into the amniotic cavity. Removing the obstruction improves the odds that the baby will have healthy kidneys, functional lungs, and need less intervention after birth.

How does Bladder Outlet Obstruction affect my baby after delivery?
After delivery, your newborn may be started on antibiotics to prevent a urinary tract infection.  We also do kidney ultrasounds and scans to ensure that the kidneys are functioning well. In some cases, a pediatric urologist will perform a third test called a Voiding Cystourethrogram (VCUG) to ensure that the urine is not backing up from the bladder into the kidneys.

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