As hydronephrosis is detected in as many as 42,000 fetuses (1.4%), 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 cannot 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 on our website. 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.