Normal Anatomy and Kidney Function

Before discussing fetal (unborn) and newborn urinary tract abnormalities, one should have some idea of the normal anatomy and kidney function. Most of us are blessed with two kidneys. Each kidney is about the size of our fist. They are located just in front of the lower ribs in the back. The kidneys are made up of a parenchyma (meaty portion) and a central urine collection area called the renal pelvis. The renal pelvis connects to the ureter at the ureteropelvic junction. The ureter is a long, thin, hollow tube which is muscular and propels the urine by a "milking action" into the bladder. The bladder is located just behind the pubic bone in the lower abdomen and is purely a storage unit that contracts occasionally, propelling the urine out through the urethra. Please see the accompanying drawing of the urinary system.

The function of the kidneys is to filter the waste products of metabolism from the blood. The blood flows in and out of the kidneys and extra water and impurities from metabolism are taken from the blood and passed into the renal pelvis so that urine can then pass through the ureter and bladder and out of the body. If the kidneys do not function properly, the waste products build up in our blood stream. That is called renal failure and can eventually lead to a comatose state and death in extreme cases.

It is known that a fetus begins to produce urine by nine weeks of gestation. Much of the amniotic fluid that the fetus swims in is the baby's own urine.

Fetal Ultrasound

Many mothers are now having ultrasounds performed to study the fetus. Fetal ultrasound is a remarkable technology that has allowed us to have an early look, or preview, of the developing baby's health. By 16 to 20 weeks of gestation, the kidneys can usually be identified as well as the bladder. Kidney and bladder abnormalities can be seen in 1 out of 500 to 1 out of 1,000 fetuses studied by ultrasound. Thus, if the baby has a serious problem with the kidney or bladder, it can be identified long before birth.

Hydronephrosis (a swelling of the renal pelvis and/or ureter) is the most common fetal ultrasound finding. Other things can also be determined such as the size, shape and number of kidneys and as to whether the bladder fills and empties well. An important role of the ultrasound is to be certain there is sufficient amniotic fluid for the baby to swim and grow in. If the kidneys are unable to produce enough urine, then the amniotic fluid volume is low and this leads to poor lung development.

My Role as Consultant

At times, I am asked to see a pregnant mother because of abnormal findings on the fetal ultrasound and, at other times, I am asked to see the newborn infant after birth because of a known abnormal fetal ultrasound. As I review the problem, there are certain questions that I need to find the answers to. Does the fetus have hydronephrosis of one kidney or both kidneys? Are there two kidneys? If the hydronephrosis is on one side, does the other kidney appear normal? What is the degree of hydronephrosis and is it becoming progressively worse or was it transient on serial ultrasounds? Are the ureters dilated on one or both sides? Is the bladder always full or does it empty at times? Does the bladder wall look thickened? Is there normal amniotic fluid or is there excessive or too little amniotic fluid? Are there other associated potentially lethal or disabling abnormalities? What is the fetus gestational age? What is the sex of the baby?

Risk vs. Benefits

Ultrasound usually provides an excellent anatomic delineation of the fetal urinary tract, but ultrasound can only tell us the shape of things and cannot tell us about the function of the kidneys. The function must be inferred and sometimes that can be dangerous and misleading. At times, we might consider intervening in the fetal development in hopes of improving the kidney function and/or lung development. However, there are great risks in fetal intervention and these risks must be weighed against the potential benefits.

Examples of Urinary Problems that can be Determined by Ultrasound:

1. Absence of one or both kidneys.

2. Multicystic dysplastic kidney on one side.

3. Hydronephrosis.

   With blockage:

   1. Ureteropelvic Junction Obstruction. This is a partial blockage that occurs between the renal pelvis and the ureter. It is the most common cause of fetal hydronephrosis. It can be seen on one side or both sides and can be mild, moderate, or severe in degree.

   2. Bladder Outlet Obstruction. If there is a blockage at the outlet of the bladder that keeps urine from leaving the bladder, urine will build up and create a back pressure and hydronephrosis. A common example that is seen in boys posterior urethral valves. These are obstructing valves located just below the prostate gland.

   3. Megaureters. Megaureters are large dilated ureters that are often caused by a point of obstruction where the ureter enters the bladder. This can occur on one side or both.

   No blockage

   1. Full bladder. Baby may have a transiently dilated urinary collecting system because of a full bladder and this dilation goes away when the baby empties his bladder. This would be seen on repeat ultrasounds.

   2. Hormones (progestational) that are high in the mothers blood because of the pregnancy can cause the ureters to be dilated. This resolves after birth.

   3. Vesicoureteral reflux. Vesicoureteral reflux occurs when urine goes backwards from the bladder towards the kidney. This is a frequent finding and, if severe enough, can cause a dilated collecting system (hydronephrosis).

   4. Prune belly syndrome. This is a syndrome seen in boys consisting of a lack of abdominal musculature, undescended testicles and dilated ureters, but there is no obstruction. It is quite rare.

   5. Transient obstruction. There may have been obstruction early in the fetal life that was seen on ultrasound that resolves because blockages may resolve or improve on their own.

Errors in Diagnosis

Fetal ultrasound studies are not always accurate. During some of the stages that the unborn child goes through, he/she produces very large quantities of urine that can make the renal ultrasound appear to be abnormal. It is also thought that some of the pregnancy hormones (progestational hormones) can also cause a relaxation of the muscles of the ureter and this can give a dilated appearance. There are other disease entities that can produce abnormal kidneys that are not necessarily associated with hydronephrosis and obstruction such as multicystic kidneys, vesicoureteral reflux, prune belly system, renal cysts, non obstructed large ureters, etc.

Nevertheless, if serial fetal ultrasound shows continued dilation, there is a high likelihood that a ureteropelvic junction obstruction exists.

Currently, there is very little reason to intervene with the development of the fetus or to induce labor and delivery early. Only in the rare cases of both kidneys showing increasing hydronephrosis and decreasing amniotic fluid, would fetal intervention and/or early delivery be considered.

Evaluation of the Newborn Child

After delivery of an infant with apparent hydronephrosis, respiratory status is evaluated to be certain that the lungs are properly developed. Urine output is monitored and baseline kidney blood tests may be determined. A urinalysis and, often, a urine culture is performed.

An ultrasound of the newborn is performed, but, if this is done in the first 48 hours, it may not truly reflect what is really happening to the baby as babies have very low urine production during the first two days of life. We often have to repeat the ultrasound after 48 hours of life. If the ultrasound shows hydronephrosis, then a bladder x-ray (voiding cystourethrogram) may be performed to rule-out reflux of urine from the bladder to the kidney.

After one month of age, a nuclear renal scan may be performed to determine the function of the kidney or kidneys.

If the testing points to a significant obstruction with back pressure and hydronephrosis, then we would usually try to repair the obstruction in the first three or four months of life. If the tests are not clear cut, we would continue to follow the infant and repeat some of the tests periodically to determine whether the kidneys are getting better, worse, or staying stable.

If your child's work-up shows something other than obstruction of the ureteropelvic junction, then he may need close follow-up or medications or surgery as appropriate for the problem.