The urinary tract consists of two kidneys (where urine is made), two ureters (tubes which lead the urine into the bladder), the bladder, and the urethra (the tube which leads the urine from the bladder to the outside of the body). The urine should flow from the kidney, through the ureter, to the bladder, and out of the fetus through the urethra to the amniotic fluid. There are many causes of urinary tract obstruction in the fetus. Most are caused by a narrowing at some point in the urinary tract. This narrowing can slow down or stop the flow of urine, and this in turn can interfere with the development of both the kidneys and the lungs.
Amniotic fluid (fetal urine) is crucial in the development of the fetal lungs. If there is not enough amniotic fluid, the lungs of the fetus do not grow. As a result, fetal urinary tract obstruction can produce pulmonary hypoplasia (small lungs) and renal dysplasia (destruction of the kidneys). A low amniotic fluid level, or no amniotic fluid, can signal a blockage at some point in the urinary tract to the flow of urine.
Outcome is related to the type of obstruction (where it is in the urinary tract), the severity of the obstruction, and the affect on renal function and amniotic fluid volume. Fetuses who have an obstruction or abnormality in one kidney and have a normal kidney on the other side will do very well. Individuals with one good kidney can support a normal life.
On the other end of the spectrum are fetuses with severe obstruction to both kidneys. This can be at the level of the bladder outlet (urethra), the bladder inlet (ureterovesical junction) or the kidney outlet (ureteropelvic junction). If the obstruction prevents urine from getting out into the amniotic space around the fetus, and the amniotic fluid goes away (oligohydramnios), the fetus’s lungs will not develop. The small lungs may prevent survival after birth. The obstruction also damages the kidneys, leading to kidney failure after birth. Many of these babies will not survive.
However, most fetuses with urinary tract obstruction fall between these two extremes and their outcome depends on the severity of the obstruction and how it progresses throughout the pregnancy. For all types of urinary tract obstuction, fetuses that lose all their amniotic fluid (oligohydramnios) before 18-24 weeks will not have big enough lungs to survive. Fetuses who maintain amniotic fluid volume throughout pregnancy will have large enough lungs to survive at birth, but may still develop renal failure after birth due to the damage to kidney function before birth. In addition to urologic procedures to relieve obstruction and sometimes reconstruct the urinary drainage system, these babies may develop renal failure over months or years and sometimes require kidney transplantation. Fetuses who maintain normal amniotic fluid and have only minor damage to the kidney will do well and may not need anything done after birth.
Fortunately, the severity of damage to both the lungs and kidneys can often be determined by ultrasound and, sometimes, by sampling and analyzing the fetal urine. In most cases, serial ultrasound observation is all that is needed. This applies to all unilateral lesions in which there is one normal kidney. These fetuses can be safely followed by serial ultrasounds to make sure they maintain amniotic fluid volume and to help plan the delivery and care after birth.
Fetuses with obstruction to both kidneys must be closely followed for changes in amniotic fluid volume, further dilation of the urinary tract, and the sonographic appearance of the kidneys themselves (increased echogenicity or brightness suggests ongoing damage to the kidney itself). If amniotic fluid volume is maintained, the kidney echogenicity does not deteriorate, and the system does not become more dilated, then these fetuses can be followed to normal delivery near term and treatment after birth. Some may require several surgeries after birth, but most will do well.
Fetuses with obstruction to both kidneys, dilated bladder and ureters, and increasing echogenicity of the kidney itself, are on the severe end of the spectrum and some will not survive. Very severely affected fetuses, who lose amniotic fluid before 18 weeks of gestation and have very echogenic, dysplastic, or multicystic kidneys, often cannot be saved. When these symptoms take place fetal urine will show a salt content which demonstrates that the kidneys cannot function properly.
Fetuses with obstruction to both kidneys that has not caused increased echogenicity or dysplasia and who have maintained the kidneys’ ability to extract salt from the urine, may be saved by fetal intervention to relieve the urinary tract obstruction. These fetuses should be carefully studied for the degree of renal dysplasia or echogenicity, for the degree of dilation of the urinary tract and the anatomy of the obstruction, either at the bladder outlet or above the bladder, and for the amniotic fluid volume. Those who are candidates for fetal intervention can be accurately assessed for the degree of renal functional damage by sampling and testing the fetal urine 2–3 times over 3–5 days to measure fetal urine electrolytes and beta-2-microglobulin. These tests are quite accurate in predicting how well the fetal kidneys are functioning, the degree of impairment, and the potential to recover.
Baby Sarah Elizabeth has a very successful recovery from an SCT which is removed while she is still a fetus.