The Pediatric Page
"My Baby Has a Swollen Kidney" - December 2004
At California Pacific’s Ultrasound Department, the most common history for a parent’s request of a postnatal ultrasound study in an asymptomatic infant is, “My baby has a swollen kidney on a prenatal ultrasound study.”
Urinary tract abnormalities account for approximately 20% of all prenatally diagnosed abnormalities, with urinary tract dilatation as the most common. It occurs in about 1% of pregnancies. Fetal pyelectasis is the most commonly encountered form of urinary tract dilatation and is usually an incidental finding on the prenatal screening ultrasound. Although the threshold of fetal pyelectasis significance has been a controversial topic in literature, it has been proposed that for anteroposterior diameter of fetal renal pelvis (ADRP) = or > 5mm at any gestational age, a follow-up ultrasound and detailed postnatal evaluation be performed to detect and treat any significant uropathologies in order to prevent chronic renal damage1. The possible causes of urinary tract dilatation include: ureteropelvic junction obstruction, uretero-vesicle junction obstruction, posterior urethral valve, prune belly syndrome, multicystic dysplatic kidney and vesico-ureteral reflux. Extra-renal pelvis can also increase the antero-posterior diameter of the renal pelvis on the ultrasound study. As both pediatric radiologists and health care providers, we face the dilemma of helping diagnose significant urinary tract abnormalities without overusing imaging procedures.
What postnatal imaging studies should we use to evaluate prenatal pyelectasis?
Ultrasound is the initial postnatal study to evaluate the urinary tract. Besides measuring the antero-posterior diameter of the neonatal renal pelvis (ADRP) to assess interval change from prenatal studies, we also search for other sonographic abnormalities. These include: calyceal or ureteral dilatation; pelvic and/or ureteral wall thickening; renal parenchymal echogenicity, including corticomedullary junction differentiation; renal cysts; cortical scarring; small sized kidney and other signs of renal dysplasia.
Immediate ultrasound evaluation should be performed if there is a known prenatal diagnosis of moderate or severe renal pelvis dilatation, ADRP =/>10mm, since these children have a high incidence of obstructive uropathies that may require surgical intervention. If the pyelectasis is confirmed or has progressed from the prenatal study, the ultrasound study will be followed by voiding cystourethrogram (VCUG) and radionuclide Lasix renogram. Early diagnosis of urinary tract obstruction will allow for early surgical decompression if indicated.
If VCUG is negative for vesico-ureteral reflux, we perform a follow-up ultrasound when the child is at least 3 months. If this ultrasound shows the mild dilatation has stabilized, then that suggests no significant obstruction. To confirm this stability and/or regression, additional ultrasounds are performed when the child is 1 year and 2 years of age.
If the VCUG is abnormal, we grade the degree of vesico-ureteral reflux based on the level of reflux, degree of dilatation and tortuosity of the collecting system. Parenchymal DMSA scan is very sensitive in depicting renal cortical scarring and can be performed in our Nuclear Medicine Department if clinically indicated.
For mild prenatal pyelectasis, ADRP > 5mm but < 10mm, a radiologist should perform an initial sonographic at least 3 days after birth (between 5-7 days) to avoid false negative results due to relative neonatal dehydration and oliguria. If the dilatation is present but stable, we recommend a repeat ultrasound in 4-6 weeks. The goal of these postnatal ultrasound studies is to detect progressive dilatation which may reflect obstructive uropathies or severe vesico-ureteral reflux. If postnatal dilatation is 5 - 10mm, a VCUG should be performed. However, if the second post-natal ultrasound at 2-3 months of age shows ADRP >10mm, then further evaluation with Lasix renogram is needed.
If the initial postnatal ultrasound is normal and remains normal on the 6 week follow-up study, some studies suggest no further follow-up is needed. However, others suggest a follow-up ultrasound at 1 year of age.
Is VCUG needed if the postnatal ultrasound evaluation is normal?
Studies have shown that the incidence of vesico-ureteral reflux ranges from 13% to 38%. Since ultrasound is not sensitive in detecting vesico-ureteral reflux, some authors recommend VCUG in all infants with prenatal pyelectasis, despite normal postnatal sonographic studies. However, a recent Belgian study including 213 infants with prenatal mild to moderate pyelectasis,2 concluded that normal neonatal ultrasound “rarely leads to significant nephrouropathies.” (3% in this study). Therefore, further evaluation is not recommended by these authors.
It seems like there is no straightforward answer to this question. The decision may depend on the clinicians’ and parents’ comfort level in regards to following the infant clinically versus subjecting the infant to a VCUG. However, if a VCUG is not to be performed, the parents need to be informed about the possibility of undiagnosed vesico-ureteral reflux. If the infant has any episode of fever of unknown origin, he or she needs to be evaluated for possible urinary tract infection.
Despite numerous investigations, the relationship between prenatally diagnosed, asymptomatic vesico-ureteral reflux and urinary tract infection is unclear. The goal of our aggressive detection and treatment of asymptomatic reflux is to prevent potential urinary tract infection and resultant renal damage.
References:
- Ouzounian JG, Castro MA, Fresquez M, al-Sulyman OM, Kovacs BW. Prognostic Significance of Antenatally Detected Fetal Pyelectasis. Ultrasound Obstet Gynecol. 1996 Jun;7(6):424-8.
- Khalid Ismaili, Fred E. Avni, K. Martin Wissing, Michelle Hall.
Long–term Clinical Outcome of Infants with Mild and Moderate Fetal Pyelectasis: Validation of Neonatal Ultrasound as a Screening Tool to Detect Significant Nephrouropathies. The Journal of Pediatrics June 2004. - Jaswon MS, Dibble L, Puri S, Davis J, Young J, Dave R, Morgan. Prospective Study of Outcome in Antenatally Diagnosed Renal Pelvis Dilatation. Arch Dis Child Fetal Neonatal Ed 1999, 80:F135-F138.
- Tibballs JM, De Bruyn, R. Primary Vesicoureteric Reflux—How Useful is Postnatal Ultrasound? Archives of Disease in Childhood November 1996, 75 (5):444-447.
This information provided by California Pacific Medical Center’s Department of Radiology
Bernice Law, M.D.
Tel: (415) 600-1080