In the United States, 3 million maternal ultrasounds are performed annually with hydronephrosis being the most commonly detected anomaly. It is detected in as many as 42,000 fetuses (1.4%). Thus obstetricians and pediatric urologists alike commonly encounter the diagnosis of prenatal hydronephrosis.
With the use of sonography, dilation of the renal collecting system can be observed, but obstruction can not be confirmed. However, up to one half of these neonates do not have hydronephrosis on the postnatal ultrasound. The remaining one half with hydronephrosis have a range of diagnoses. The majority (64%) are attributed to ureteropelvic junction (UPJ) obstruction. The remaining 36% are secondary to vesicoureteral reflux, megaureter, or posterior urethral valves. UPJ obstruction refers to a kink or stricture of the collecting system as it begins to leave the kidney to form the ureter. This causes blockage of urine flow and can possibly lead to infections, scarring, and long term damage of the kidney. Reflux is another important condition that is described elsewhere in our web site. In short, this refers to a condition that allows for the backflow or reflux of urine up into the kidney from the bladder. If the urine is infected with bacteria, this can also lead to infection, scarring, and damage to the kidneys.
At birth, a sudden increase in total cardiac output and renal vascular resistance occurs. Subsequently the kidney enters a period of transitional physiology. During the transitional phase, resistance to flow decreases in the renal vasculature, total renal blood flow increases, and the glomerular filtration rate (GFR) doubles. Over the first six months of life the urinary concentrating ability improves exponentially.
Since maturation and development are the key features in the neonatal period, any insults suffered by the kidney during this phase can be profoundly reflected in the ultimate structure and function of the kidney. Ultimately the most morbid sequelae that can result from obstructive uropathy are defects in lung maturation and renal maldevelopment.
Despite widespread use of ultrasound, a debate exists in the field of maternal fetal medicine over the required use of gestational (in utero) ultrasound. Clear indications for sonography include discrepancies in fundal height for gestational age, elevated levels of maternal serum alpha fetal protein, and a history of previous pregnancies resulting in congenital anomalies. Regardless of the controversy, when a gestational ultrasound is performed certain basic details must be covered in the examination. These include:
- Estimation of fetal size and maturity
- Amniotic fluid volume
- Standard fetal survey of head, spine, heart, lungs, limbs, and abdomen
- Assessment of kidneys including position, size, and texture
- Appearance of ureters and collecting system
- Bladder volume, wall thickness, and emptying
- Examination of other pelvic organs
- Appearance of external genitalia
Fetal kidneys can be visualized by the 14th to 15th week of gestation. By the 20th week of gestation, the internal architecture of the kidneys can be assessed. A normal fetal ureter is rarely visualized during ultrasonography. The actual incidence of genitourinary abnormalities on prenatal ultrasound is .2%.
Hydronephrosis is the most common abnormality detected on prenatal ultrasonography. It accounts for about 50% of all prenatally detected defects. When prenatal hydronephrosis is discovered on ultrasound, the finding does not confirm the presence of obstruction. This is due to the extremely elastic nature of the fetal kidney.
Different Causes of Fetal Hydronephrosis
The most common cause of fetal hydronephrosis is ureteropelvic junction (UPJ) obstruction. This finding is characterized by varying degrees of renal pelvic dilation. The most common cause of UPJ obstruction is an intrinsic narrowing of the proximal ureter. This narrowing can be likened to a scar or stricture in the ureter as it leaves the kidney. Sometimes, the ureter can be obstructed behind a band of tissue or a blood vessel supplying the lower segment of the kidney. This entity is known as a "crossing vessel".
When a dilated ureter is detected along with the renal pelvis, the working diagnosis is vesicoureteral reflux or uretero-vesical junction obstruction. Males with significant hydroureteronephrosis occurring simultaneously with a large thin walled bladder and normal renal parenchyma will often have a condition of massive reflux known as megacystis-megaureter.
Prenatal findings of hydroureteronephrosis can be associated with other diagnoses such as posterior urethral valves, prune-belly syndrome, or duplication anomalies such as ectopic ureter or ureterocele.
The diagnosis of posterior urethral valves is reached when a male fetus has the following findings: bilateral hydroureteronephrosis; thick-walled, moderately dilated bladder; dilated posterior urethra, and kidney architectural changes and diminished volumes of amniotic fluid.
Other genitourinary diagnoses that can be made prenatally include renal dysplasia, multicystic dysplasia, renal agenesis, renal hypoplasia, renal cystic disease, bladder/cloacal exstrophy, and external genital anomalies.
In Utero Treatment
Management of prenatal anomalies has changed over the past 15 years and is presently a matter of debate. Once a diagnosis of prenatal hydronephrosis is made, serial ultrasounds are often needed. More invasive diagnostic tests may be required such as amniocentesis, PUBS, and chorionic villous sampling.
It is difficult to estimate fetal renal function based on ultrasonography. The appearance of the kidney tissue can provide important insight, but no foolproof method of predicting renal damage is currently available. One potentially useful technique is the aspiration of fetal bladder urine. Here urine is removed directly from the fetal bladder.
Many different types of therapeutic fetal interventions have been performed over the past 25 years with varying degrees of success using intravenous sedation, local anesthesia, and ultrasound guidance.
However, at this time there is no clear data that supports the routing use of fetal intervention. A plan must be worked out for each individual case. An explanation of potential risks, benefits, and shortcomings of prenatal interventions must be discussed with the patient and family before any therapeutic interventions.
Postnatal Management of Hydronephrosis
The first important step in managing patients with prenatal findings of decreased amniotic fluid, genital malformations, or other serious anomalies is to have the child treated in a tertiary care center for delivery and management of neonatal issues. Upon delivery, a directed physical examination is important to rule out associated anomalies. Often, the urologist will order a X-ray test called a VCUG which involves instilling dye into the bladder and taking xrays while the child empties the bladder. This test helps to look for bladder abnormalities and the presence of urinary reflux.
In most cases, postnatal evaluation can be performed on an outpatient basis. Children with moderate degrees of prenatal hydronephrosis can be discharged home on amoxicillin with no further inpatient evaluation. Generally an ultrasound is not performed in the first 2 days of life as the neonate is usually dehydrated creating the potential to underestimate hydronephrosis. The first ultrasound can be performed after 3-4 weeks on an elective basis while the child continues antibiotic prophylaxis.
We tend to follow the guidelines espoused by Mandell and Retik in reference to the role of post-natal voiding cystogurethrography (VCUG). When the fetal renal pelvic diameter is greater than 8mm in the presence of caliectasis or ureterectasis, then a VCUG is performed. The VCUG should be performed even if postnatal sonography reveals resolution of the hydronephrosis. The controversial aspect of management is whether all children with any degree of hydronephrosis should have a VCUG performed. When VCUG is performed, an experienced pediatric radiologist should perform and interpret the studies with the urologic team. When the VCUG does not reveal reflux on the first filling cycle, repeat cycles are necessary if reflux is strongly suspected or if the voiding is incomplete.
When the postnatal ultrasound reveals severe pelvic dilation (over 1.2 cm) and caliectasis, diuretic renography is needed to assess the degree of renal obstruction and to assess renal function. Ideally, the renography is performed after the infant is 1 month of age; 99mTc MAG-3 is the radionuclide agent of choice.
The actual surgical and medical management of post-natal urinary tract obstruction due to UPJ is a controversial entity. Safe effective interventions have been developed, but controversy arises regarding the necessity and timing of surgical intervention. Much of this debate stems from the inability of sonography and renography to accurately assess the degree of obstruction and renal function. Generally, about 25% of children who present with diminished function or poor kidney drainage will ultimately require a surgical intervention. Different approaches to management range from aggressive observation with serial studies to repair of any obstruction when delayed drainage is seen on nuclear studies.
The finer aspects of UPJ obstruction management are still evolving. However, due to increased awareness and establishment of treatment algorithms, a reduction in episodes of infection and improvement in quality of life has been observed.