Urinary Tract | Lower urinary tract obstruction

A fetus developing inside the womb is surrounded by amniotic fluid. This fluid creates a protective layer, helps with the transfer of critical nutrients from mother to child, and allows the fetal lungs to grow. Around 16 weeks of pregnancy, the fetal kidneys begin to produce urine. This urine contributes significantly to the amniotic fluid levels in the womb. If the urine is unable to reach the amniotic fluid sac, typically because of a blockage somewhere along the urinary system, the levels of fluid around the fetus may become dangerously low. This can impact the fetal lung, kidney and limb development and lead to life-threatening complications.

Essential Information

  • Lower urinary tract obstruction (LUTO) is a condition in which the opening from the fetal bladder (the bladder outlet or urethra) is blocked.
  • As a consequence of this blockage, the fetus is unable to empty its bladder, and the bladder becomes distended.
  • The blockage can lead to urine backing up into the fetal kidneys, causing build-up of pressure and possible kidney damage. This blockage will also result in low levels of amniotic fluid, which can impact fetal lung development.
  • When amniotic fluid levels are severely low, fetal surgery may be required to relieve the blockage and allow the bladder to drain.

Overview

The urinary system is made up of two kidneys, two ureters (the tubes that connect the kidneys to the bladder), the bladder, and the urethra (a small opening in the bladder which allows the urine to drain outside the body).

Lower urinary tract obstruction (LUTO) is a condition in which the fetal bladder outlet (the urethra) is blocked. This can be due to small membranes (called “posterior urethral valves”) blocking the urethra or sometimes because the urethra never developed at all (called “urethral atresia”).

As a consequence of this blockage, the fetus is unable to empty its bladder, and the bladder becomes distended. The accumulating urine in the bladder causes back pressure into the kidneys and this can damage the kidney tissue. Often, this damage to the kidneys is not reversed after the baby is born, and these children are at high risk of moderate to severe kidney disease.

Additionally, as the fetus is not emptying the bladder, there is no urine being added to the amniotic fluid sac. The levels of amniotic fluid are therefore very low (called “oligohydramnios”) or sometimes absent (called “anhydramnios”). Amniotic fluid is very important for fetal lung development and to allow the fetus to move and stretch its arms and legs. A fetus with little amniotic fluid will often have underdeveloped lungs (called “pulmonary hypoplasia”). If severe, the lungs of these babies will be too small to allow for enough oxygen uptake at birth, and therefore, these babies are at high risk of dying soon after they are born. Similarly, in severe cases, the low amniotic fluid may lead to deformities of the fetal arms and legs. This combination of findings is sometimes called the ‘Potter sequence’.

Every fetus with suspected LUTO requires a detailed ultrasound to try to determine the exact nature of the problem causing the blockage, as well as to study the appearance of the lungs and kidneys and to rule out problems with other organs. Sometimes, a detailed heart ultrasound (called a fetal echocardiogram) may be required.

 

Treatment

In fetuses with little or no amniotic fluid, a prenatal intervention called a ‘bladder shunt’ may be considered. This procedure consists of inserting a special catheter guided by ultrasound into the fetal bladder. This catheter connects the bladder to the amniotic cavity, bypassing the urethra and allowing the urine to drain from the bladder.

Alternatively, a procedure called fetal cystoscopy may be performed. Here, a small camera is inserted through a small incision in the maternal abdominal wall into the uterus and the fetal bladder. If the obstruction at the level of the urethra is identified and if posterior urethral valves are present, they can sometimes be removed with laser energy.

After bladder shunting or a fetal cystoscopy, and if the kidneys are not too damaged, the amount of amniotic fluid may improve, thereby helping lung growth and possibly increasing the chance of survival after birth. The kidney damage that has occurred however is irreversible, and many of these infants will be born with moderate to severe kidney dysfunction. While surgery will likely still be required after birth, in-utero intervention may reduce the severity of the damage to the kidneys and lungs, and improve the survival rates for affected babies.

Referral Information (for physicians)

Inclusion criteria

  • Lower urinary tract obstruction with normal fetal kidney function on fetal urine testing.
  • Severe oligohydramnios or anhydramnios.

Exclusion criteria

  • Associated major genetic or anatomic anomalies
  • Poor fetal kidney function.

Practically

Procedures will typically be done at less than 16 weeks’ gestational age. Fetal vesicocentesis and genetic testing will be offered at Mount Sinai Hospital. These procedures typically require approximately 24 hours of hospitalization. Postoperatively, out-of-province patients will be referred back to their local care provider for further antenatal care and delivery. Local patients will be followed further at Mount Sinai Hospital and The Hospital for Sick Children (SickKids).