Predictability of Vesicoureteral Reflux Using Interlobar Renal Arterial Resistive Indices in Sonographically Normal Pediatric Kidneys-Preliminary Results ()
1. Introduction
Urinary tract infections are very common in the pediatric population, especially in girls. Most children with vesicoureteral reflux and urinary tract infections have an excellent prognosis with resolution of reflux by 4 years of age. The greatest risk of renal involvement in urinary tract infection is in newborn boys and pre-school age girls [1].
According to the guidelines of the American Academy of Pediatrics (AAP), a sonogram should be performed on all infants and young children with fever and their first documented UTI; prior to 2011, according to the AAP guidelines of 1999, voiding cystourethrography (VCUG) or radionuclide cystography (RCN) were recommended. New AAP guidelines [2] have recently been proposed for children with urinary tract infections (UTI). It is now recommended that VCUG or RCN not be done after the first febrile UTI. This is based on the fact that “the risks, costs, and discomfort of the VCUG are hard to justify, because there is no evidence that patients benefit from having their VUR diagnosed” [2].
Regardless of the new AAP recommendations, it has been shown that a normal renal sonogram does not exclude the presence of vesicoureteral reflux, even in children aged 5 years or older. Therefore, if reflux is considered, a VCUG must be performed even in older children, regardless of the sonographic findings [3].
There are some studies regarding the effects of reflux on renal blood flow. Paltiel [4] studied the effect of chronic, low pressure reflux on renal growth and function in a porcine model. The results of that study showed that such reflux, in pigs, led to mild chronic interstitial inflammation and fibrosis after 1 year. However, the imaging findings were normal.
Helin [5] studied blood flow parameters, including renal vascular resistance, in piglets with unilateral ureteric pressure elevation, as may happen in vesicouretero reflux. In that study no significant changes in renal vascular resistance was found between normal kidneys and those in which there was elevated ureteric pressure.
Resistive index (RI) is defined, in Doppler ultrasound, as the ratio of the peak systolic velocity minus the end diastolic velocity/peak systolic velocity. RI is widely used a measurement of resistance of arterial flow, and eliminates the need for precise angle-measurements of blood flow. Most ultrasound machines can automatically calculate the RI within an arterial structure automatically, as illustrated in Figure 1.
Radmayr [6] studied the relationship for resistive index (RI), reflux and scarring. Patients with VUR grades IV and V demonstrated a statistically significant increase in RI compared to those with low grade or no reflux. The author concluded that a possible cause of these elevated RIs may be the presence of renal scarring, found in many children with higher grades of reflux. The author also recommended routine determination of RI during renal ultrasound examinations to detect renal injury and scarring in patients with VUR.
Our study was performed in effort to evaluate the following research questions:
1) Is there an alteration of the resistive indices of the interlobar arteries, in children with reflux and sonographically normal appearing kidneys?
2) If such an alteration in RI exists, can these measurements be used to predict the presence or absence of vesicoureteral reflux?
3) Is there a correlation between resistive index and the grade of reflux?
Given the known pathophysiology of reflux and renal blood flow in animals with reflux, our hypothesis was that resistive index would not be a positive predictor of reflux in children, and therefore, not a useful tool to predict the presence or absence of reflux in normal appearing kidneys on sonography.
2. Materials and Methods
After obtaining IRB approval, we investigated 35 children with a history of UTI with grayscale and color Doppler US followed by a standard voiding cystouretherogram to assess the presence of vesicoureteral reflux. Ages ranged from newborn - 12 years of age. There were 14 males and 21 females. All renal units (individual kidneys) were morphologically normal without evidence of hydronephrosis or renal scarring. Resistive index (RI) was measured at the level of the interlobar arteries. The arcuate arteries were not selected due to technical difficulties in obtaining Doppler measurements in such small structures, which relies on patient cooperation. Three measurements of the interlobar RIs were obtained at the level of the upper, mid, and lower poles of each kidney.
All sonograms were performed on the Siemens Antares unit. Each sonogram was performed by one of two experienced ultrasound technicians. All VCUGs were performed fluoroscopically and the results of each study were tabulated by one of five experienced pediatric radiologists (with a range of 5 - 20 years of experience). Reflux grade was assigned according to the international reflux grading system (Grades I - V). RI values were then
Figure 1. Spectral Doppler ultrasound of the interlobar arteries of the kidney, demonstrating the calculation of the resistive index. In this case, the RI is 0.51.
tabulated with VCUG results.
3. Results
A tabulation of the results of this study is presented in Table 1. A total of 67 kidney units were assessed. 3 kidney units yielded no diagnostic results due to technical factors (crying, agitation). 46 kidney units (69%) were not associated with any degree of reflux. 17 kidney units
(25%) were associated with low-mid grade reflux (grades I - III). 4 kidney units (6%) were associated with high grade reflux (grades IV - V).
The mean RI values obtained in the three positions of each kidney are summarized in Table 2. The overall mean RI was 0.68.
Table 3 demonstrates the number and percentages of infants with each grade of reflux in both kidneys. Here,