Objective sonographic measurements of renal pelvic diameter and renal parenchymal thickness can identify renal hypofunction and poor drainage in patients with antenatally detected unilateral ureteropelvic junction obstruction

Introduction

Hydronephrosis grading systems risk stratify patients with potential ureteropelvic junction obstruction, but only some criteria are measured objectively. Most notably, there is no consensus definition of renal parenchymal thinning.

Objectives

The objective of this study was to assess the association between sonographic measures of renal length, renal pelvic diameter, and renal parenchymal thickness and the outcomes of a)renal hypofunction(differential renal function{DRF} <40%) and b)high-risk renal drainage(T1/2 > 40 min).

Study design

An institutional database of patients who had diuretic renograms(DR) for unilateral hydronephrosis was reviewed. Only infants with Society for Fetal Urology(SFU) grades 3/4 hydronephrosis without hydroureter on postnatal sonogram and had a DR within 120 days were included. The following measurement variables were analyzed: anterior posterior renal pelvic diameter(APRPD), renal length(RL), renal parenchymal thickness(PT), minimal renal parenchymal thickness(MPT = shortest distance from mid-pole calyx to parenchymal edge), and renal pyramidal thickness(PyrT). RL, PT, MPT, PyrT measurements were expressed as ratios (hydronephrotic kidney/contralateral kidney). Multivariate logistic regression was performed for each outcome by comparing three separate renal measurement models. Model 1: RLR, APRPD, MPTR; Model 2: RLR, APRPD, PTR, Model 3: RLR, APRPD, PyrTR. Individual performance of variables from the best performing model were assessed via ROC curve analysis.

Results

196 patients were included (107 with SFU grade 3, 89 with SFU grade 4) hydronephrosis. Median patient age was 29[IQR 16,47.2] days. 10% had hypofunction, and 20% had T1/2 > 40 min 90% with hypofunction and 87% with high-risk drainage had SFU4 hydronephrosis. Model 1 exhibited the best performance, but on multivariate analysis, only APRPD and MPTR were independently associated with both outcomes. No other measure of parenchymal thickness reached statistical significance. The odds of hypofunction and high-risk drainage increase 10% per 1 mm increase in APRPD(aOR 1.1 [CI 1.03–1.2], p = 0.005; aOR 1.1 [CI 1.03–1.2], p = 0.003). For every 0.1unit increase in MPTR the odds of hypofunction decrease by 40%(aOR 0.6 [CI 0.4–0.9], p = 0.019); and the odds of high-risk drainage decrease by 30%(aOR 0.7 [CI 0.5–0.9], p = 0.011). Optimal statistical cut-points of APRPD >16 mm and/or MPTR <0.36 identified patients at risk for obstructive parameters on DR.

Discussion and conclusion

Of the sonographic hydronephrosis measurement variables analyzed, only APRPD and MPTR were independently associated with objective definitions of obstruction based on renal function and drainage categories. Patients who maintain APRPD <16 mm and/or MPTR >0.36 can potentially be monitored with renal sonograms as there is >90% chance that they will not have DRF<40% or T1/2 > 40 min.

Summary Table. Receiver operator curve analysis of the ability of the test variables Minimal Parenchymal Thickness Ratio and APRPD to predict the outcomes of renal hypofunction and high-risk renal drainage.

Empty Cell	Cut-point value	ROC AUC	Sensitivity (%)	Specificity (%)	Positive Predictive Value (%)	Negative Predictive Value (%)
Renal Hypofunction (RDF <40%)
APRPD	>16.9 mm	0.853	90	71	26	98
MPTR	<0.36	0.803	85	63	21	97
High-Risk drainage (T1/2 > 40 min)
APRPD	>18.1 mm	0.787	77	80	48	93
MPTR	<0.36	0.820	82	68	39	94