The term peripartum hydronephrosis refers to the dilatation of the renal collecting system. The collecting system refers to the collection of urine directly from the kidney tissue and its diversion from the ureters to the bladder structures. Hydronephrosis is also referred to as “kidney swelling”. In the past decade, the routine use of prenatal ultrasound has become more common, making it possible for urologists and pediatricians to be aware of possible kidney defects before birth.
Of the 3 million prenatal ultrasounds performed each year in the United States, hydronephrosis is the most commonly detected abnormality. Hydronephrosis is detected in up to 42,000 fetuses (1.4%). Therefore, obstetricians and pediatric urologists are often likely to encounter a diagnosis of prenatal hydronephrosis.
By using ultrasonography, dilatation of the renal collecting system can be observed, but obstruction cannot be confirmed. However, up to one half of these newborns do not have hydronephrosis on postnatal ultrasound. The remaining half of hydronephrosis had some diagnosis. Most (64%) were attributed to obstruction of the ureteropelvic junction (UPJ). The remaining 36% were secondary to vesicoureteral reflux, giant ureter, or posterior urethral valves. Ureteropelvic junction obstruction is a twisting or narrowing of the collecting system at the point where it begins to leave the kidney to form the ureter. This causes blockage of the urine flow and can lead to infection, scarring, and long-term kidney damage. Reflux is another important condition. In short, it is a condition that allows urine to flow backwards or reflux from the bladder up into the kidney. This can also lead to infection, scarring, and damage to the kidneys if the urine is infected with bacteria.
Developmental physiology.
At birth, there is a sudden increase in total cardiac output and renal vascular resistance. The kidney then enters a transitional physiological period. During the transition period, intravascular renal blood flow resistance decreases, total renal blood flow increases, and the glomerular filtration rate (GFR) doubles. The ability to concentrate urine increases exponentially during the first six months of life.
Since maturation and development are the main features of the neonatal period, any injury suffered by the kidney during this phase can be profoundly reflected in the final structure and function of the kidney. The most problematic sequelae caused by eventual obstructive nephropathy are defects in lung maturation and abnormal kidney development.
Prenatal imaging.
Despite the widespread use of ultrasound, there is debate in the field of maternal-fetal medicine regarding the required use of ultrasound in pregnancy (intrauterine). Clear indications for ultrasound include fundal height not consistent with gestational age, elevated maternal serum alpha-fetoprotein levels, and a previous history of pregnancy resulting in congenital anomalies. Regardless of these debates, certain basic details should be included when performing a pregnancy ultrasound. These include.
1. estimation of fetal size and maturity.
2. Amniotic fluid volume.
3. Standard fetal examinations of the head, spine, heart, lungs, extremities, and abdomen.
4. kidney assessment including location, size, and texture.
5.Appearance of the ureter and collecting system.
6. Bladder volume, wall thickness, and emptying.
7. Examination of other pelvic organs.
8.Appearance of external genitalia.
The fetal kidneys can be seen at 14 to 15 weeks of gestation. At 20 weeks of gestation, the internal structure of the kidney can be evaluated. Normal fetal ureters are rarely seen during ultrasonography. The actual incidence of genitourinary malformations during prenatal ultrasound is 0.2%.
Hydronephrosis is the most frequently detected abnormality on prenatal ultrasound. It accounts for approximately 50% of all prenatal detected defects. When prenatal hydronephrosis is detected by ultrasound, this finding does not confirm the presence of obstruction. This is attributed to the extremely elastic nature of the fetal kidney.
The etiology of fetal hydronephrosis varies.
The most common cause of fetal hydronephrosis is obstruction of the ureteropelvic junction (UPJ). This finding is characterized by varying degrees of dilatation of the renal pelvis. The most common cause of UPJ obstruction is an intrinsic narrowing of the proximal ureter. This narrowing can be compared to scarring or stenosis at the point where the ureter leaves the kidney. Sometimes, the ureter can be blocked by a band of tissue behind it or by a blood vessel that supplies the lower part of the kidney. This is called a “transverse vessel”.
When dilatation of the ureter is detected along with the renal pelvis, the likely diagnosis is vesicoureteral reflux or obstruction of the ureteral-vesical junction. Boys with significant ureteropelvic effusion, along with a large, thin-walled bladder and normal renal parenchyma, often have a condition of massive reflux called mega-cysto-ureteral disease.
Prenatal findings of ureteropelvic effusion may be associated with other diagnoses such as posterior urethral valves, Merganser’s abdomen syndrome, or duplicated malformations such as ectopic ureteral openings or ureteral cysts.
The diagnosis of posterior urethral valves can be made when the male fetus has the following findings: bilateral ureteropelvic effusion; thick-walled, moderately dilated bladder; dilated posterior urethra and changes in the renal structural system, and decreased amniotic fluid volume.
Other genitourinary diagnoses that can be done prenatally include renal dysplasia, polycystic dysplasia, renal dysplasia, renal hypoplasia, cystic renal disease, bladder/cloacal ectopia, and external genital anomalies.
Intrauterine treatment.
The management of prenatal anomalies has evolved over the past 15 years and remains a controversial issue. Once a diagnosis of prenatal hydronephrosis is made, a series of ultrasounds is often required. More invasive diagnostic tests such as amniocentesis, transumbilical cord blood sampling, and chorionic villus sampling may be required.
It is difficult to estimate fetal kidney function based on ultrasound. The appearance of renal tissue can provide an important reference, but there is no simple and reliable method to predict renal damage. One technique that may be useful is intra-fetal bladder urine puncture aspiration. This removes urine directly from the fetal bladder.
Over the past 25 years, many different types of therapeutic fetal interventions have been performed through the use of intravenous sedation, local anesthesia, and ultrasound guidance with varying degrees of success.
However, at this time, there are no clear data to support the use of a pathway for fetal interventions. A plan must be developed for each individual case. Prior to any therapeutic intervention, a discussion must be held with the patient and their family to explain the potential risks, benefits, and shortcomings of prenatal interventions.
Postnatal management of hydronephrosis.
The first important management step for patients with prenatal findings of reduced amniotic fluid, genital anomalies or other severe malformations is to have the child placed in a tertiary care center responsible for delivery and management of neonatal problems. Once delivered, a directed physical examination is important to rule out associated malformations. Usually, a urologist will prescribe an X-ray called VCUG, which involves filling the bladder with contrast and taking X-rays while the child is emptying the bladder. This test helps to detect bladder abnormalities and the presence of urinary reflux.
In most cases, postnatal evaluation can be performed on an outpatient basis. Children with moderate prenatal hydronephrosis can be discharged home on amoxicillin without staying in the hospital for further evaluation. Ultrasound should generally not be performed in the first 2 days of life, as this is when newborns are usually dehydrated and thus may underestimate hydronephrosis. On a selective basis, the first ultrasound can be performed after 3-4 weeks while the child continues antibiotic prophylaxis.
Based on the role of postnatal voiding cystourethrography (VCUG), we tend to follow the guidelines advocated by Mandell and Retik. VCUG is performed when the fetal renal pelvis is greater than 8 mm in diameter in the presence of a dilated calyx or dilated ureter.VCUG should be performed even if postnatal ultrasound reveals resolution of the hydronephrosis.The controversial point of management is whether a child with any degree of hydronephrosis should undergo VCUG.When performing VCUG, an experienced pediatric radiologist should work with the urology team to perform and interpret the examination. When the first round of fluid-filled VCUG does not show reflux, repeating several rounds is necessary if reflux or incomplete voiding is strongly suspected.
When postnatal ultrasound shows severe pelvic dilatation (more than 1.2 cm) and calyces dilatation, a diuretic nephrogram is required to assess the degree of renal obstruction and to evaluate renal function. Nephrograms are best 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 postnatal urinary tract obstruction caused by the pyeloureteral junction is a controversial issue. Safe and effective interventions have been developed, but controversy has arisen regarding the need for and timing of surgical intervention. Many of these controversies stem from the inability of both ultrasound and nephrography to accurately assess the extent of obstruction and renal function. Typically, approximately 25% of children presenting with diminished function or poor renal drainage will eventually require surgical intervention. The management varies widely from active observation with serial testing to surgery for repair of any obstruction when delayed drainage is identified on nuclear examination.
More refined aspects of management of obstruction at the pelvic ureteral junction are still evolving. However, as a result of increased awareness and the establishment of treatment procedures, a reduction in infection episodes and an improvement in quality of life have been observed.