Percent improvement in renal pelvis antero-posterior diameter (PI-APD): Prospective validation and further exploration of cut-off values that predict success after pediatric pyeloplasty supporting safe monitoring with ultrasound alone

INTRODUCTION: Renograms are frequently obtained post-pyeloplasty in patients with residual hydronephrosis to confirm adequate drainage. Recent evidence suggests that percent improvement in antero-posterior diameter (PI-APD) ≥38 is predictive of success. We sought to further explore PI-APD ranges that would allow identification of patients who would benefit from ultrasound (US) monitoring alone vs. post-operative renal scan, and those more likely to develop recurrent ureteropelvic junction obstruction (rUPJO). METHODS: A single-center prospectively-collected pyeloplasty database (2008-2015) was queried (n = 151). Only patients with both pre- and post-operative APD measurements were included (n = 138). PI-APD was divided into 3 categories: <20%; 20-39%; ≥40%. The following variables were collected post-operatively: patients monitored with US alone, renogram and US, rUPJO and minimal or resolved hydronephrosis (SFU ≤2; UTD ≤1; APD ≤15 mm). RESULTS: Mean age at first and last follow-up were 4.8 (median 4.0; range 0-60) months and 26.6 (median 20.5; range 1-77) months, respectively. Of 138 patients, 84 (61%) had US alone, 54 (39%) had a renogram and US post-operatively, and 6 (4%) developed rUPJO. Of 84 patients who had US alone, 71 (84%; p < 0.01) demonstrated ≥40% PI-APD. Of 54 patients with renogram and US 46 (85%; p < 0.01) had ≥40 PI-APD. Of the 6 patients who developed rUPJO, all were in the <20 PI-APD group (100%; p < 0.01). Resolution of hydronephrosis according to SFU, UTD and APD occurred in 96/138 (70%), 89/138 (64%) and 113/138 (82%) patients respectively. Of these, 87 (91%), 81 (91%), and 108 (95%) occurred in >40% PI-APD group. CONCLUSION: ≥40% PI-APD at the first post-operative visit strongly predicts pyeloplasty success, as up to 82% of these patients showed resolved hydronephrosis and 61% underwent non-invasive monitoring by US alone. Our data suggests that up to 85% of renograms may have been unnecessary. Finally, <20% PI-APD permitted identification of all rUPJO cases. Stratification of patients based in PI-APD is a promising strategy for further minimizing radiation exposure while safely detecting children at risk for rUPJO.