Fetal urinary obstructions — LUTO

Pathology description

Lower Urinary Tract Obstruction (LUTO) is a malformation of male fetuses that consists of the obstruction of the urethra at the level of its exit from the bladder. The urethra is the final tube that connects the urinary bladder to the outside through the penis, and allows urine to exit. The obstruction of the urethra can be complete or partial. 

Obstruction of the fetal lower urinary tract occurs in isolation in 1 in 3000 pregnancies. 

Why does it occur?

The cause of urinary obstruction is unknown and can occur in several forms, as a genetic syndrome or associated with other malformations. But in most cases it presents as an isolated disease with no other problems. 

Obstruction can occur for different reasons, but the most common is the existence of a membrane that obstructs the exit of the urethra, known as posterior urethral leaflets. Less frequently, in only 20% of cases, there is a severe narrowing of a path of the urethra (urethral stenosis), its final part (megalourethra) or even a total or partial absence of the urethra (urethral atresia). Finally, obstruction of the urethra may be due to a malformation at the entrance of the ureters to the bladder (ureterocele). 

What consequences does lower urinary tract obstruction have for fetal development?

If the obstruction is complete or very severe, the baby does not manage to urinate and the first effect will be on the bladder, which fills up as the kidneys continue to produce urine but cannot be emptied. This leads to an exaggerated growth of the bladder, which first struggles to overcome the obstruction by making its wall more muscular and thicker (bladder hypertrophy).

If the obstruction persists, the bladder will continue to grow, swelling in a balloon-like way, and may become so large that it occupies most of the fetal abdomen. This is called bladder distention and has two important consequences on the development of the fetus: 

• There will be a decrease, or even absence, of the amniotic fluid that normally surrounds the fetus, a situation known as oligohydramnios. This occurs because the amniotic fluid is formed mainly from fetal urine and, when the fetus stops urinating, it disappears. Amniotic fluid is essential for the normal development of fetal lungs, and its absence produces an alteration in lung development, defined in its most severe degree as pulmonary hypoplasia, which are lungs of small size that cannot be used to breathe after birth. 
• The distended bladder will produce a very significant increase in pressure due to the accumulation of urine that cannot be eliminated by the urethra as it normally would, so the pressure is transmitted upwards, dilating the tubes that connect the kidney with the bladder (the ureters). Finally, the pressure will also affect the kidneys, which can lead to kidney damage and affect their function. 

In the most severe cases, which are those in which the obstruction is severe or total and becomes evident already before 20-22 weeks of gestation, the baby will die after birth from pulmonary hypoplasia in more than 90% of cases.

If the baby survives he will have severe renal failure and, therefore, will need dialysis and to enter a kidney transplant programme in early childhood, usually already in the first year of life in more than 50% of cases. In some cases, the fetus can die even in utero, because the distended bladder causes great abdominal pressure and can alter heart function. 

Treatment

In many cases, if the obstruction is removed in fetal life, the serious problem of oligohydramnios can be solved, thereby reducing or eliminating the risk of pulmonary hypoplasia. In addition, this decompression relieves the pressure on the kidneys. 

Fetuses with urethral leaflet obstruction are normal fetuses in most cases, so if the only problem they have is this one, it makes sense to try fetal therapy to reverse the serious problem of urinary obstruction. The fundamental requirements for a fetus to benefit from fetal therapy are: 

• It being a serious problem of obstruction: oligohydramnios exists already before 20 weeks of pregnancy. If the amount of amniotic fluid is normal there is no point in performing fetal therapy. 
• It being an isolated problem: there are no other alterations, neither genetic nor in any other organ. 
• Kidney function being preserved: Treatment will be done at a time when the kidneys are still functioning normally and it makes sense to try to prevent them from being irreparably damaged. Therefore, it is very important to evaluate renal function, which can be studied both in the urine and in fetal blood. 

The main objective of the treatment is to enable the fetus to empty the bladder and thus normalize the amount of amniotic fluid, avoiding pulmonary hypoplasia and ensuring that the baby can survive after birth. Bladder decompression also helps improve kidney function, although it is not always fully achieved.

There are two types of fetal interventions to achieve these goals: 

• Rupture (ablation) of the posterior urethra leaflets by fetal surgery (fetal cystoscopy). This is a treatment that is performed by fetal endoscopy. The introduction of a fine instrument (a tube 2–3 mm in diameter) into the bladder allows the visualization and breaking of the urethral leaflets, so that the urine is allowed to exit. 
• Placement of a small tube that directly connects the bladder with the amniotic cavity (vesico-amniotic shunt) and allows the exit of urine through the abdomen. It is placed through the fetal abdominal wall by ultrasound control, and should remain until the end of gestation. 

Both interventions are performed with local anaesthesia and sedation of the pregnant woman, and with fetal anaesthesia. They are not very aggressive interventions for pregnant woman, but are of great technical difficulty. 

In our centre the first-line intervention is fetal cystoscopy. Although it is a much more difficult and sophisticated technique than the placement of a drain, it is also not very invasive for the pregnant woman and fetus. It allows, in case of success, a permanent solution in a single intervention and enables the fetus to urinate through the natural route.