Background: The Society for Fetal Urology (SFU) grading system risk stratifies patients with potential ureteropelvic junction obstruction, but grading is qualitative and subject to variability. Most notably, there is no consensus definition of renal parenchymal thinning. Understanding when to continue monitoring patients with SFU grade 3 and 4 hydronephrosis with ultrasound versus when to obtain a diuretic renogram (DR) could be improved with identifying quantitative measures that are significantly associated with decreased drainage and hypofunction.
Objective: Determine the association between the renal sonographic measurements: renal length ratio [RLR], renal parenchymal thickness ratio [MPTR], and intrarenal anterior posterior renal pelvis diameter [APRPD] with a) high-risk drainage (T1/2 > 40 minutes), and b) renal hypofunction (differential renal function [DRF] <40%).
Methods: Using a large single institution hydronephrosis registry (2000-2016), infants with antenatal unilateral SFU grade 3 or 4 hydronephrosis who had a DR within 120 days of initial screening sonogram were included. Ultrasound measurements were obtained de novo by the study radiologist (AK, HA). Measurements included intrarenal APRPD, and RLR (hydronephrotic kidney length/contralateral kidney length), MPTR (hydronephrotic kidney minimal parenchymal thickness/contralateral kidney parenchymal thickness). Minimal parenchymal thickness was measured by identifying the shortest distance from mid-pole calyx to parenchymal edge. Descriptive statistics were calculated. An adjusted analysis was performed for each outcome to determine whether any of the screening ultrasound measurements were associated. If significant, test characteristics were then conducted to determine the optimal numeric cut-off for identifying the outcomes for each measurement.
Results: 196 patients, 107 had SFU3 and 89 SFU4 hydronephrosis. Median T1/2 in SFU3 was 6 [IQR 4.0, 11.5] vs. 26 [IQR 10.0,99.0] minutes for SFU4 (p<0.001). 87% of those with high-risk drainage were SFU4. Median DRF in SFU3 was 50% vs. 45% in SFU4 (p<0.001). The majority with hypofunction (90%) were SFU4. On unadjusted analysis, all renal measurement variables were associated with both renal hypofunction and high-risk renal drainage. However, on multivariate analysis only APRPD and MPTR were independently associated with both outcomes. With every 1-mm increase in APRPD, the odds of hypofunction and high-risk drainage increased by 10% (aOR 1.1 [CI 1.03-1.2], p=0.005; aOR 1.1 [CI 1.03-1.2], p=0.003), respectively. For every 0.1-unit increase in MPTR, the odds of hypofunction decrease by 40% and the odds of high-risk drainage decrease by 30% (aOR 0.6 [CI 0.4-0.9], p=0.019; aOR 0.7 [CI 0.5-0.9], p=0.011, respectively). The optimal cut-points associated with both outcomes were >16mm for APRPD and <0.36 for PTR, all test characteristics are reported in Table 1.
Conclusion: By applying sonographic measurements of APRPD and MPTR to patients with SFU grade 3 and 4 hydronephrosis, we can identify patients with a higher likelihood of high-risk drainage and hypofunction. This can aid in stratifying those patients who would benefit from DR, versus lower risk patients who may continue with ultrasound monitoring only.
