Original Research

JOURNAL OF ENDOUROLOGY Volume 29, Number X, XXXX 2015 ª Mary Ann Liebert, Inc. Pp. ---–--DOI: 10.1089/end.2014.0900

Single-Stage Bilateral Versus Unilateral Retrograde Intrarenal Surgery for Management of Renal Stones: A Matched-Pair Analysis Yonghan Peng, MD,1* Ling Li, MD,1* Wei Zhang, MD,1* Qi Chen, PhD,2 Min Liu, MD,1 Xiaolei Shi, MD,1 Xiaofeng Gao, MD, PhD,1 and Yinghao Sun, MD, PhD1

Abstract

Purpose: This study aimed to compare the effectiveness and safety of single-stage bilateral and unilateral retrograde intrarenal surgery (RIRS) for treatment of patients with renal stones. Patients and Methods: Patients undergoing single-stage bilateral RIRS for renal stones between January 2011 and April 2014 were retrospectively reviewed and matched to patients undergoing unilateral RIRS at a 1:1 ratio by the propensity score based on age, sex, body mass index, American Society of Anesthesiologists classification, stone burden, and stone location (involvement of the lower calix or not). Main outcome measures were stone-free rate per patient and per renal unit and RIRS-associated morbidity. Results: Of 60 patients treated by single-stage bilateral RIRS, 59 patients were matched with patients treated by unilateral RIRS. Stone-free rates per patient for bilateral RIRS and unilateral RIRS were 84.7% and 91.5%, respectively (P = 0.255). Median operative time (100 vs 65 min, P < 0.001) and postoperative hospitalization (2 vs 1 d, P = 0.011) was significantly longer for bilateral RIRS than unilateral RIRS. Median changes in hematocrit (1.3% vs 1.0%, P = 0.964) and serum creatinine level (3 vs 2 lmol/L, P = 0.716) were similar between bilateral and unilateral RIRS. The overall complication rate was slightly higher with bilateral RIRS (11.9% vs 8.5%, P = 0.542). No serious RIRS-associated morbidities occurred in either group. Conclusion: Single-stage bilateral RIRS was as effective and safe as unilateral RIRS among propensity scorematched patients. safety of B-RIRS and U-RIRS for the treatment of patients with nephrolithiasis via a matched-pair analysis in selected patients with comparable baseline and stone characteristics.

Introduction

B

ecause of major technical improvements in flexible ureteroscopy, retrograde intrarenal surgery (RIRS) has become more broadly applied in the treatment of patients with renal stones.1,2 RIRS is associated with a high stone-free rate and low complication risk, because stones are managed under direct vision through the natural tracts.3 Associated with a low risk of renal injury, RIRS has been performed safely even in patients with a bleeding diathesis or a solitary kidney, both of which make management with shockwave lithotripsy or percutaneous nephrolithotomy more technically challenging.4,5 Recent studies have shown that single-stage bilateral RIRS (B-RIRS) is feasible and safe for managing upper urinary stones.6–8 Controversy persists, however, regarding whether B-RIRS is associated with a higher procedural morbidity in comparison with unilateral RIRS (U-RIRS).8–10 The primary objective of this study was to compare the effectiveness and

Patients and Methods

The study protocol was approved by the Institutional Review Board at Changhai Hospital of the Second Military Medical University, Shanghai. Patients who underwent BRIRS or U-RIRS for the treatment of renal stones between January 2011 and April 2014 were retrospectively reviewed. B-RIRS patients were matched with those treated by U-RIRS at a 1:1 ratio according to age, sex, body mass index (BMI), American Society of Anesthesiologists (ASA) classification, stone burden, and stone location (involvement of the lower calix or not). The exclusion criteria for propensity score analysis of B-RIRS vs U-RIRS were as follows: Patients with bilateral renal stones and having received staged operations, those scheduled for B-RIRS and necessitating conversion to

1

Department of Urology, Changhai Hospital, Second Military Medical University, Shanghai, PR China. Department of Health Statistics, Faculty of Health Service, Second Military Medical University, Shanghai, PR China. *These authors contributed equally to this article.

2

1

2

U-RIRS at operation, or those with a solitary kidney or complicating renal malformation. All patients were evaluated by routine hematology examination, clinical biochemistry tests, urinalysis, urine microorganism culture (1 week before operation), and intravenous urography as well as noncontrast enhanced CT as routine to exclude possible renal malformation and evaluate stone burden. In that the cumulative stone diameter has been reported to be less predictive of stone volume in patients with a high stone burden,11 stone volume was calculated using the following formula: Stone volume per stone = length · width · height · p · 1/6, and total stone volume was calculated as the sum of the volume of all stones. Any patient with a positive culture was given sensitive antimicrobial therapy, and all patients received intravenous antimicrobial prophylaxis 30 minutes before ureteroscopy. Usually, a patient with a negative urine culture was given a second-generation cephalosporin, while for the patient with positive urine culture, the sensitive antimicrobials was used. Before operation, all patients signed informed consent forms. Operative technique

All operations were performed by an assigned endourologic team led by two board-certified endourologic surgeons (XG and YS). A unit would have stent placement using ureteroscopy for passive dilation because of complicating ureteral narrowing or stricture. Under general anesthesia with endotracheal intubation, patients were positioned in the standard lithotomy position. Laterality of initial treatment depended on hydronephrosis severity and stone burden. The kidney with a more serious hydronephrosis would be managed in the initial session and that with a smaller stone burden would be operated on in a patient without hydronephrosis. A ureteral access sheath (Cook Medical, Bloomington, IN; inner diameter, 9.5/12F; outer diameter, 11.5/14F) was advanced through the working guidewire to introduce a 7.5F Flex-X2 flexible ureteroscope (Karl Storz, Tuttlingen, Germany) into the renal pelvis. The sheath was positioned in the upper ureteral segment near the ureteropelvic junction. For multiple or lower-pole stones, if possible, a 2.2F NGage nitinol stone extractor (Cook Medical) or a 2.4/1.7F NCompass nitinol stone extractor (Cook Medical) was used to basket stones into the more accessible renal calix. After the stones were fragmented with a 200-lm holmium laser fiber (VersaPulse 100 PowerSuite; Coherent Medical Group, Santa Clara, CA), the stone basket was used to remove fragments, if any. The 6F or 7F Double-J stent was placed on completion of stone elimination and removed (both sides for B-RIRS) 1 month after surgery because of intraoperative use of the access sheath. Main outcome measures

All patients were scheduled for a follow-up visit at least 3 months after operation. Plain abdominal radiography of kidneys, ureters, and bladder (KUB) was performed on the first day after operation and the first day after stent removal. Further follow-up was performed with renal ultrasonography at 6 weeks and 3 months after operation. Postoperative stone-free status was defined as the complete absence of any stone fragment on KUB radiography 1 month after the operation. Postoperative complications were classified using the modified Clavien

PENG ET AL.

grading system,11 and a higher grade was used for analysis if a patient experienced more than one episode of complications.12 Statistical analysis

Categoric variables were presented as the frequency and percentage, and compared using the chi-square test or Fisher exact test. Continuous variables were shown as the median and interquartile range, and compared using the nonparametric Mann-Whitney test. To adjust the inherent differences of baseline characteristics between patients who underwent B-RIRS and U-RIRS, a propensity score matched analysis was performed. The propensity score was generated using a multivariable logistic model13 including the following variables: Age, sex, BMI, ASA score, stone burden, and location of stone (involvement of the lower calix or not). Patients undergoing B-RIRS and U-RIRS were matched in the propensity score at a ratio of 1:1 with a caliper of 0.01; covariate balance and surgical outcomes were compared between the matched groups. A two-sided P value less than 0.05 was considered statistically significant. All data processing and statistical analyses were performed using SAS version 9.2 (SAS Institute Inc, Cary, NC). Results

Among all the patients who were scheduled for B-RIRS, B-RIRS was converted to U-RIRS with contralateral ureteral stent placement in two patients and to bilateral ureteral stent placement in five patients, because of complicating ureteral narrowing or stricture. Overall, B-RIRS and U-RIRS had been accomplished in 60 and 404 patients, respectively. Finally, two matched groups with 59 patients in each were included for analysis. The two groups were comparable in terms of demographic characteristics at the baseline (all P values > 0.05; Table 1). The two groups had a similar percentage of patients with positive urine culture (B-RIRS vs U-RIRS 18.6% [11/59] vs 11.9% [7/59], P = 0.306), including grampositive bacterial infection, gram-negative bacterial infection, and fungal infection in 5, 10, and 3 patients, respectively. A Double-J stent was used preoperatively in 7 and 12 patients (11.9% vs 20.3%, P = 0.210) of the B-RIRS and U-RIRS groups, respectively. In the B-RIRS group, bilateral Double-J stents were placed in five patients as passive ureteral dilation because of complicating narrow ureters and in two patients because of bilateral ureteral obstruction with complicating upper ureteral stones. In the U-RIRS group, the Double-J stent was placed in 10 patients as passive ureteral dilation and in 2 patients because of ureteral obstruction. The observed treatment outcomes are described in Table 2, and the median follow-up duration was 7 (range 3–12) months. The B-RIRS group had a significantly longer operative time (100 vs 65 min, P < 0.001) and postoperative hospital stay (2 vs 1 d, P = 0.011). None of the patients experienced massive bleeding or needed blood transfusion at operation. The B-RIRS group had a greater but clinically insignificant decline in hematocrit from the baseline compared with the U-RIRS group (1.3% vs 1.0%, P = 0.964). Serum creatinine levels were similarly reduced in both groups (3 vs 2 lM, P = 0.716). At the follow-up visit 1 month after the single-session RIRS, the two groups had similar stone-free rates per renal unit (89.0% vs 91.5%, P = 0.598) and per patient (84.7% vs

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Table 1. Baseline Characteristics and Propensity-Score Matching of Patients Scheduled for Unilateral Retrograde Intrarenal Surgery or Bilateral Retrograde Intrarenal Surgery (n = 464) Unmatched Variables

B-RIRS (n = 60)

Age, years, mean (SD) 47.5 (11.6) Sex, n (%) Male 43 (71.7) Female 17 (28.3) BMI, kg/m2, median (Q1-Q3) 24.7 (22.8–26.3) ASA, n (%) Class 1 and 2 56 (93.3) Class 3 and 4 4 (6.7) Stone burden, cm3, median (Q1-Q3) 1.6 (0.8–2.4) Stone localization, n (%) Lower calices 35 (58.3) Upper and middle calices 25 (41.7)

Matched

U-RIRS (n = 404)

P value

48.2 (12.7)

0.680 47.7 0.526 273 (67.6) 42 131 (32.4) 17 24.3 (21.4–26.4) 0.342 24.8 0.773 380 (94.1) 55 24 (5.9) 4 0.9 (0.5–1.8) 0.004 1.3 0.088 188 (46.5) 34 216 (53.5) 25

B-RIRS (n = 59) (11.5)

U-RIRS (n = 59) 46.4 (12.9)

P value 0.559 0.398

(71.2) 46 (78.0) (28.8) 13 (22.0) (22.9–26.3) 25.0 (22.8–26.4) 0.770 1.000 (93.2) 56 (94.9) (6.8) 3 (5.1) (0.7–2.3) 0.8 (0.5–1.8) 0.063 0.852 (57.6) 35 (59.3) (42.4) 24 (40.7)

B-RIRS = bilateral retrograde intrarenal surgery; U-RIRS = unilateral retrograde intrarenal surgery; SD = standard deviation; BMI = body mass index; ASA = Amican Society of Anesthesiologists.

91.5%, P = 0.255). A second-look RIRS for elimination of residual stones was needed in three patients with B-RIRS and one patient with U-RIRS (5.1% vs 1.7%, P = 0.619). Stone analysis was comparable between the B-RIRS and U-RIRS groups, and the predominant stone composition was calcium oxalate in both groups. The two groups had similar overall postoperative complications rates (11.9% [7/59] vs 8.5% [5/59], P = 0.542), and no observed complications were classified as grade III or IV (Table 3). Pyrexia was the most frequent grade I complication, with incidence rates of 5.1% [3/59] and 3.4% [2/59] in the B-RIRS and U-RIRS groups (P = 1.000), respectively. Late-onset hematuria (3.4% [2/59] vs 5.1% [3/59], P = 1.000) and urinary sepsis (3.4% [2/59] vs 0.0% [0/59], P = 0.496) were the main grade II complications and occurred at similar frequencies between the two groups. CT scan identified blood clots in the renal pelvis but excluded the presence of any hematoma in patients who experienced hematuria; the clots resolved spontaneously without medical/surgical intervention. No anuria occurred after stent removal, and no ureteral strictures occurred during the follow-up period. Discussion

Continued refinement of endourologic techniques and miniaturization of the ureteronephroscope have improved the

effectiveness and safety of RIRS for treating patients with nephrolithiasis, especially in cases of multiple intrarenal calculosis, with favorable navigation and visualization of the ureter and renal pelvis.14 Use of a flexible ureteroscope allows synchronous entry to both ureters and renal pelvises in serial attempts without necessitating an additional invasive access.4,15 Incorporation of laser lithotripsy further improves the effectiveness and safety of stone removal, even in patients receiving anticoagulation therapy and minimizes the risk of residual stones.4 To the best of our knowledge, the present work was the first case-control study evaluating the effectiveness and safety of B-RIRS compared with U-RIRS for the treatment of patients with nephrolithiasis with respect to stone elimination and safety profile. Comparative studies have been reported for evaluating the efficacy and safety of single-stage bilateral and unilateral ureteroscopy but not in a match-paired setting. Thus, controversy persisted regarding whether the bilateral procedure would lead to a higher procedural morbidity compared with the unilateral approach. Variation in patients’ characteristics and endourologists’ expertise may have contributed to the controversy. Hollenbeck and associates10 reported that synchronous bilateral ureteroscopy resulted in a stone-free rate similar to that of unilateral ureteroscopy but was associated with a significantly higher risk of procedural complications (odds ratio = 4.0, P = 0.020), in proportion to the number of renal units

Table 2. Perioperative Outcomes Among Bilateral Retrograde Intrarenal Surgery and Unilateral Retrograde Intrarenal Surgery Patients (n = 59 Pairs) Variables Operative time, min, median (Q1–Q3) Postoperative hospitalization, days, median (Q1–Q3) Decrease in hematocrit level, %, median (Q1–Q3) Decline in serum creatinine level, lM, median (Q1–Q3) Stone-free rate per renal unit, n (%) Stone-free rate per patient, n (%) Second-look surgery, n (%)

B-RIRS 100 2 1.3 3 105 50 3

(66–126) (1–3) (-0.5–3.0) (-5–7) (89.0) (84.7) (5.1)

65 1 1.0 2 54 54 1

U-RIRS

P value

(50–80) (1–2) ( - 0.2–3.0) ( - 3–9) (91.5) (91.5) (1.7)

< 0.001 0.011 0.964 0.716 0.598 0.255 0.619

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Table 3. Retrograde Intrarenal Surgery-Associated Complications by Clavien Scale and Category

Clavien scale Any/low grade Grade I Grade II Category Fever Bleeding Sepsis

B-RIRS, n (%)

U-RIRS, n (%)

P value

7 (11.9) 3 (5.1) 4 (6.8)

5 (8.5) 2 (3.4) 3 (5.1)

0.542 1.000 1.000

3 (5.1) 2 (3.4) 2 (3.4)

2 (3.4) 3 (5.1) 0 (0.0)

1.000 1.000 0.496

endourologically treated. Grossi and colleagues9 reported, however, that synchronous bilateral and metachronous unilateral ureteroscopy were comparable with regard to calculus elimination and morbidity. El-Hefnawy and coworkers16 reported that no differences were found in the complication rate and stone-free rate between bilateral ureteroscopy and unilateral ureteroscopy for the management of ureteral stones. Stones with challenging characteristics, including impacted stones, proximal ureteral stones, and large-size stones, were found to be significant risk factors for unsuccessful ureteroscopy.16 A previous study predicting the stone-free rate suggested that age, sex, and BMI are not predictive factors of stone-free status after RIRS, whereas stone burden, stone composition, and renal malformations are independent predictive factors.17 In addition, stone locations in the lower pole and at an infundibulopelvic angle less than 45 degrees are unfavorable factors for stone elimination after RIRS.17 Finally, stone burden and struvite calculi were shown to be independent predictive factors for systemic inflammatory response syndrome after flexible ureteroscopy.18 A large stone burden is associated with a low stone-free rate but a high risk of procedural complications.18,19 Multiple parameters are available for estimating stone burden, including cumulative stone diameter, surface area, and volume, among which cumulative stone diameter is the most widely used in urologic practice because of its easy accessibility. Cumulative stone diameter, however, has a limited predictive accuracy, especially in cases with a large number or volume of stones, because the width and depth of stones are not considered.19 Ito and colleagues19 reported that cumulative stone diameter and stone volume had an equal predictivity of stonefree status for patients with a cumulative stone diameter < 20 mm or 1 to 3 stones. For patients with a cumulative stone diameter ‡ 20 mm or more than 3 stones, however, the cumulative stone diameter has a lower predictive power for stone-free rate compared with stone volume. Thus, stone volume was used to assess the stone burden in our bilateral nephrolithiasis patients with complicating multiple stones. Therapeutic outcomes of B-RIRS as treatment for patients with bilateral nephrolithiasis have been reported at a few endourologic centers.6–8 Huang and associates7 reported a greater than 90% overall stone-free rate and a 16% overall complication rate in 25 patients with a mean stone number of 24, and no observed complications were determined to be severe. Watson and coworkers8 also reported a bilateral stone-free rate of 64% and overall morbidity rate of 9.7% in 84 patients with ureteronephrolithiasis.

Our results showed that B-RIRS and U-RIRS had similar stone-free rates per renal unit and per patient. Stone size larger than 20 mm was reported to be associated with a higher risk of residual stones after B-RIRS. Alkan and coworkers6 reported that synchronous B-RIRS is associated with a stonefree rate per patient of 100% for stones < 25 mm and 80% for those ‡ 25 mm. Huang and colleagues7 reported similar results for stone-free rates per renal unit: 100% for stones £ 20 mm and 85.7% for stones > 20 mm. Ureteroscopy is considered to be generally safe for the treatment of patients with renal and/or ureteral calculosis. The Clinical Research Office of the Endourological Society Ureteroscopy Global Study prospectively reviewed 11,885 cases of ureteroscopy and reported an intraoperative complication rate of 6.3% and postoperative complication rate of 3.5%.20 Our results showed that the overall complication rate of B-RIRS was generally equivalent to that of the unilateral approach (11.9% vs 8.5%, P = 0.542), and all complications were minor in severity, necessitating no specific intervention. These findings are similar to the results of previous studies.3,6–8 No clinically significant bleeding occurred in either group in the present study, and hematuria was the most frequently reported postoperative complication. As reported by Oguz and associates,3 among patients who underwent RIRS, 9.5% experienced mild late-onset hematuria and 2.1% had severe bleeding necessitating termination of RIRS. Hemorrhagic events in RIRS are normally self-limiting and rarely (0.2%) necessitate blood transfusion.20 Postoperative fever or urinary sepsis may result from preexisting urinary tract infection and prolonged operative time. It was reported that use of a ureteral access sheath could minimize the occurrence of septic events by halving the pelvic irrigation pressure.21 A major safety concern regarding the use of a ureteral access sheath, however, is iatrogenic ureteropelvic injury,22 but no relevant procedural complications were observed in our patients. Because of the bilateral ureteral access and stent placement, the B-RIRS group had a significantly longer operative time compared with the U-RIRS group. Longer postoperative hospital stay associated with B-RIRS might result from a limited access to community-based postoperative follow-up care in the Chinese healthcare system; therefore, the patients undergoing B-RIRS were hospitalized longer after operation compared with those with U-RIRS. For a patient with bilateral renal stones, a same-session B-RIRS is advantageous in saving overall operative time compared with staged U-RIRS in addition to avoidance of repeated anesthesia and reduction in overall hospital stay and medical costs.10 High-risk patients, however, should be carefully evaluated for potential complications, especially in those with diabetes, the elderly ( > 70 years), and those with struvite calculus or preexisting renal insufficiency.18 Our study does have some limitations. First, this casecontrol study had a small sample size and enrolled retrospectively reviewed, nonrandomized patients. The two groups, however, were well-balanced in terms of baseline patient characteristics. Second, the study results were subject to patient selection bias, especially for stone-free rate because patients were assigned to the B-RIRS or U-RIRS group based on the presence of bilateral or unilateral nephrolithiasis. The two techniques were associated with similar stone-free rates, however, both per renal unit and per patient. Third, longerterm follow-up is normally needed for evaluation of stone-

BILATERAL VS UNILATERAL RIRS FOR RENAL STONES

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eliminating efficacy for a given medical, surgical, or interventional technique for management of urolithiasis. 13. Conclusion

Our paired-matched study demonstrated that in comparison with U-RIRS, single-stage B-RIRS was equally effective for eliminating renal stones without compromising the procedural safety. Long-term, randomized, controlled studies are needed to confirm the effectiveness and safety of synchronous B-RIRS for the management of bilateral nephrolithiasis.

14. 15.

Acknowledgments

The study received funding from Foundation of Shanghai Changhai Hospital ‘‘1255’’ Discipline Construction Projects (CH125520302) and Medical Foundation of Shanghai Science and Technology Committee (11411950102). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Disclosure Statement

16.

17. 18.

No competing financial interests exist. References

1. Akman T, Binbay M, Ozgor F, et al. Comparison of percutaneous nephrolithotomy and retrograde flexible nephrolithotripsy for the management of 2–4 cm stones: A matched-pair analysis. BJU Int 2012;109:1384–1389. 2. Herrera-Gonzalez G, Netsch C, Oberhagemann K, et al. Effectiveness of single flexible ureteroscopy for multiple renal calculi. J Endourol 2011;25:431–435. 3. Oguz U, Resorlu B, Ozyuvali E, et al. Categorizing intraoperative complications of retrograde intrarenal surgery. Urol Int 2014;92:164–168. 4. Turna B, Stein RJ, Smaldone MC, et al. Safety and efficacy of flexible ureterorenoscopy and holmium:YAG lithotripsy for intrarenal stones in anticoagulated cases. J Urol 2008; 179:1415–1419. 5. Giusti G, Proietti S, Cindolo L, et al. Is retrograde intrarenal surgery a viable treatment option for renal stones in patients with solitary kidney? World J Urol 2015;33:309–314. 6. Alkan E, Avci E, Ozkanli AO, et al. Same-session bilateral retrograde intrarenal surgery for upper urinary system stones: Safety and efficacy. J Endourol 2014;28:757–762. 7. Huang Z, Fu F, Zhong Z, et al. Flexible ureteroscopy and laser lithotripsy for bilateral multiple intrarenal stones: Is this a valuable choice? Urology 2012;80:800–804. 8. Watson JM, Chang C, Pattaras JG, Ogan K. Same session bilateral ureteroscopy is safe and efficacious. J Urol 2011; 185:170–174. 9. Grossi FS, Barnaba D, Raguso M, et al. Bilateral same session ureteroscopy: Safety and efficacy. Arcunilateral retrograde intrarenal surgery h Ital Urol Androl 2007;79:20–22. 10. Hollenbeck BK, Schuster TG, Faerber GJ, Wolf JS Jr. Safety and efficacy of same-session bilateral ureteroscopy. J Endourol 2003;17:881–885. 11. Kadlec AO, Greco KA, Fridirici ZC, et al. Comparison of complication rates for unilateral and bilateral percutaneous nephrolithotomy (PCNL) using a modified Clavien grading system. BJU Int 2013;111:E243–E248. 12. Dindo D, Demartines N, Clavien PA. Classification of surgical complications: A new proposal with evaluation in

19.

20.

21.

22.

a cohort of 6336 patients and results of a survey. Ann Surg 2004;240:205–213. Bangalore S, Cohen DJ, Kleiman NS, et al. Bleeding risk comparing targeted low-dose heparin with bivalirudin in patients undergoing percutaneous coronary intervention: Results from a propensity score-matched analysis of the Evaluation of Drug-Eluting Stents and Ischemic Events (EVENT) registry. Circ Cardiovasc Interv 2011;4:463–473. Riley JM, Stearman L, Troxel S. Retrograde ureteroscopy for renal stones larger than 2.5 cm. J Endourol 2009;23:1395–1398. Cocuzza M, Colombo JR, Jr., Ganpule A, et al. Combined retrograde flexible ureteroscopic lithotripsy with holmium YAG laser for renal calculi associated with ipsilateral ureteral stones. J Endourol 2009;23:253–257. El-Hefnawy AS, El-Nahas AR, El-Tabey NA, et al. Bilateral same-session ureteroscopy for treatment of ureteral calculi: Critical analysis of risk factors. Scand J Urol Nephrol 2011;45:97–101. Resorlu B, Unsal A, Gulec H, Oztuna D. A new scoring system for predicting stone-free rate after retrograde intrarenal surgery: The ‘‘resorlu-unsal stone score’’. Urology 2012;80:512–518. Zhong W, Leto G, Wang L, Zeng G. Systemic inflammatory response syndrome after flexible ureteroscopic lithotripsy: A study of risk factors. J Endourol 2015;29:25–28. Ito H, Kawahara T, Terao H, et al. Utility and limitation of cumulative stone diameter in predicting urinary stone burden at flexible ureteroscopy with holmium laser lithotripsy: A single-center experience. PLoS One 2013;8:e65060. de la Rosette J, Denstedt J, Geavlete P, et al. The clinical research office of the endourological society ureteroscopy global study: Indications, complications, and outcomes in 11,885 patients. J Endourol 2014;28:131–139. Auge BK, Pietrow PK, Lallas CD, et al. Ureteral access sheath provides protection against elevated renal pressures during routine flexible ureteroscopic stone manipulation. J Endourol 2004;18:33–36. Traxer O, Thomas A. Prospective evaluation and classification of ureteral wall injuries resulting from insertion of a ureteral access sheath during retrograde intrarenal surgery. J Urol 2013;189:580–584.

Address correspondence to: Xiaofeng Gao, MD, PhD Yinghao Sun, MD, PhD Department of Urology Changhai Hospital Second Military Medical University 168 Changhai Road Shanghai 200433 PR China E-mail: [email protected]; [email protected]

Abbreviations Used ASA ¼ American Society of Anesthesiologists BMI ¼ body mass index B-RIRS ¼ bilateral retrograde intrarenal surgery CT ¼ computed tomography KUB ¼ kidneys, ureters, and bladder RIRS ¼ retrograde intrarenal surgery U-RIRS ¼ unilateral retrograde intrarenal surgery