Congenital Urinary Tract Obstruction
The urinary tract consists of two kidneys, two ureters (the tube that connects the kidney to the bladder), the bladder and the urethra. Urine flows from the kidneys, through the ureters into the bladder. Passing through the urethra, urine empties into the amniotic cavity. Fetal urine is the main component of amniotic fluid.
Urinary tract obstructions are caused by a narrowing at some point in the urinary tract that slows or stops the flow of urine. If one ureter is blocked, the kidney will not be able to produce urine and may become enlarged (hydronephrosis), or even damaged. If both ureters are blocked, or if the blockage is in the urethra, the fetus is unable to discharge (or "void") urine and cannot produce amniotic fluid, which can lead to underdeveloped lungs.
How common is it?
Congenital urinary tract obstruction occurs in one in 5,000 to 7,000 births, most commonly in males.
How is it diagnosed?
It's normally diagnosed through ultrasound in the middle of the second trimester. Ultrasound is a noninvasive test that allows us to assess overall fetal growth and development, the severity of the obstruction and the condition of the kidneys.
If an obstruction is detected, it's essential to assess the kidneys for functionality and damage. This also helps to determine whether a fetus may benefit from fetal intervention.
An additional test to monitor kidney function involves extracting a sample of fetal urine and analyzing the electrolytes and protein levels. This procedure is performed exactly like amniocentesis-a fine needle is inserted into the fetal bladder under ultrasound guidance. For the most accurate assessment of kidney function, a urine sample is taken twice, since the first sample has been in the bladder for a long time and may not provide the most accurate information.
What can happen before birth?
When the obstruction occurs in both kidneys or low in the urinary tract, it can damage or hinder the development of the kidneys (renal dysplasia) and the lungs. Fetal lungs need sufficient amniotic fluid to grow. If there is little or no fluid (oligohydramnios), the lungs cannot expand and may not fully develop (pulmonary hypoplasia).
In males, severe lower urinary tract obstructions are usually due to posterior urethral valves. The male urethra (the tube that connects the bladder to the tip of the penis) has a few folds that can become large enough to block the passage of urine. However, most of these blockages are incomplete and can be treated after birth.
What can be done before birth?
Most urinary tract problems of the fetus are minor: either a partial blockage on one side or an enlarged ureter or renal pelvis (the part of the kidney where urine collects before flowing into the ureter). As long as there is sufficient amniotic fluid, fetal intervention is unnecessary in these cases. Serial ultrasound will be used to monitor fetal development and amniotic fluid levels, and to help plan the delivery and care after birth.
Fetal intervention may be considered only when the risk of progressive damage to the kidneys or, more importantly, to the lungs, is high-for example, in cases of oligohydramnios.
Using local anesthesia and ultrasound guidance, a small tube (catheter) will be inserted through the abdominal wall of the fetus, into the bladder. The catheter is called a "double pigtail" because of its shape: both ends of the tube are curled to make sure that it stays in place; one end in the bladder and the other in the amniotic cavity. This allows urine to bypass the blockage in the urinary tract and empty into the amniotic fluid.
There is a small risk that this procedure may cause uterine contractions or rupture the membranes, which could lead to labor and premature delivery. Additionally, it can be difficult to place the catheter in the right place and it can become dislodged later-often because the fetus pulls it out. Furthermore, not all fetuses with urinary tract obstruction and oligohydramnios benefit from this procedure. If the kidneys have suffered too much damage, they cannot produce any urine. For these fetuses, there is little that can be done. Often they die at birth of respiratory problems caused be severely underdeveloped lungs.
Nevertheless, this procedure is the most effective in restoring amniotic fluid and bypassing a blockage, thereby helping lung development and often preventing further damage to the kidneys.
What are my delivery options?
Unless there are signs that the fetus is in trouble, pre-term delivery or Cesarean section is not necessary. Cesarean section may be necessary for obstetrical reasons, however. It is recommended that mothers deliver in a hospital that has immediate access to a specialized neonatal intensive care unit (NICU), with a pediatric surgical specialist present.
What will happen at birth?
Although some babies will need an operation after birth, it's usually not urgent. Some may only require antibiotics to prevent urinary infections. Most babies do not require intensive care, and can be evaluated in the nursery or even after they have left the hospital.
What is the long-term outcome?
After birth, urologists and nephrologists (kidney specialists) evaluate kidney and bladder function to determine what may occur as the baby grows. Postnatal treatment options depend on the type of obstruction. For posterior urethral valves, endoscopic resection is a minimally invasive technique performed within the first weeks of life, along with the removal of the shunt. Urologic surgeons attach specialized surgical instruments to a tool with a light and camera (endoscope) and insert it into the urethra to remove the tissue (valves) causing the obstruction.
In more complicated cases, a vesicostomy - an opening below the belly button that allows the bladder to drain directly into a diaper-diverts urine until the baby is healthy enough to undergo valve resection or urethral reconstruction.
Left: Bladder obstruction: the bladder (*) is very distended, and there is no amniotic fluid around the fetus.
Right: Urinary obstruction: the kidney is very distended (arrow), and loops of very dilated ureter (*) are visible.