Safety,Feasibility,and Efficacy of Bilateral Synchronous Percutaneous Nephrolithotomy for Bilateral Stone Disease: Evidence from a Systematic Review

Abstract

Introduction:

With the evolution of endourologic technology, bilateral synchronous percutaneous nephrolithotomy (BS-PCNL) has emerged as a potentially practical intervention for patients with bilateral lithiasis. Although tradition has favored a staged approach, an increasing number of original studies have reported their experiences with the synchronous approach.

Materials and Methods:

A Cochrane style search was performed after development of a sensitive and predefined search strategy. Primary outcomes measured were initial and final stone-free rate (SFR), drop in hemoglobin, hospital stay, operative time, and complication rates. Additional information was collected on (but not limited to) baseline characteristics, stone complexity, number of tracts made, success rate, and transfusion rate.

Results:

From a total 187 studies, 11 were identified (published between 1997 and 2015), and they were included in this review. In total, 594 patients with a mean age of 46 years and a male:female ratio of 3:1 underwent BS-PCNL procedures, the majority of which was under the prone position. In 87.1% (range: 71.4%–100%) of cases, the synchronous approach was performed as planned. Multiple access tracts were established in an average of 16.7% (4.1%–24%) renal units. Mean initial SFR and final SFR were 72.6% (49%–85%) and 92.4% (87%–96.9%), respectively, with a mean operative time of 171.1 minutes (range: 107.4–269 minutes). Mean hospital stay was 3.9 days (range: 1.25–15 days). Mean complication rate per study was 23.4% (range: 12.1%–54% per study). The majority were Clavien Grade 1 (60.9%), of which fever resolving spontaneously was the most common complication. No deaths were reported in any of the papers.

Conclusion:

BS-PCNL seems to be a good endourologic approach for patients with bilateral stone disease, which can render high SFRs and maintain a noninferior safety profile compared with the staged approach. This technique demands careful patient selection, counseling and should be preferably performed in endourology centers with large case volumes.

Introduction

Percutaneous nephrolithotomy (PCNL) is the benchmark intervention for complex urinary stones and has undergone significant advancements since it was first described in 1976.¹ With the evolvement of technique and technology, in cases of bilateral stone disease, bilateral synchronous percutaneous nephrolithotomy (BS-PCNL) has emerged in recent years as a potential treatment solution in the endourologist's armamentarium.² Given its potential for reducing total anesthetic time and hospital stay and the need to re-operate, it may represent an expeditious choice for stone clearance in both kidneys. However, concerns persist regarding its safety and, as such, standard tradition has favored a staged (also termed asynchronous) approach.

The question remains, therefore, as to whether the asynchronous staged approach is driven by conventional habit, custom, or clinical necessity. However, although there have been an increasing number of original studies on BS-PCNL (advocating its feasibility), a comprehensive evaluation of current evidence remains under-reported. To this effect, our objective was to perform the first systematic review addressing the safety, feasibility, and efficacy of BS-PCNL.

Materials and Methods

In accordance with Cochrane guidelines and Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) checklist,³ a sensitive search strategy was developed to identify original studies published between January 1996 and November 2016 applied to the following bibliographic databases: Medline, Pubmed, Scopus, Web of Science, and Cochrane Library. Individual journals and citation lists were also hand searched. Abstracts form conference proceedings were excluded. The following search terms were employed (but not limited to): “percutaneous nephrolithotomy,” “PNL,” “PCNL,” “synchronous,” “single-session,” “same-session,” and “bilateral.” Medical subject headings (MeSH) included the following terms: [Percutaneous Nephrolithotomy], [Urinary Lithiasis], and [Urologic Surgical Procedures].

Inclusion and exclusion criteria

Only studies pertaining to adult patients reported in the English language were included. Therefore, pediatric and animal studies were excluded accordingly. A sample size cutoff of 10 patients was set to give results from centers with relevant endourologic experience. All types of original study were included from case series to randomized trials. In selected cases, where a number of articles were related to the same study, the most recent publication was selected.

Data extraction and outcome measures of interest

As for the search process described earlier, two authors extracted data independently (P.J. and B.K.S.). Where additional information was required or confirmation was needed, the relevant study authors were contacted directly. The primary outcome measures of interest were initial and final stone-free rate (SFR), mean drop in hemoglobin, renal function (if reported), and complication rates. The secondary outcomes included hospitalization time, operative time, and the percentage of cases where bilateral procedures achieved in a single session, second-look nephroscopy, and number of tracts created.

Additional information was also collected on patient demographics and stone size/complexity. Complications were graded according to the modified Clavien-Dindo System.⁴ Each study was assigned a level of evidence according to those outlined by the Centre for Evidence-Based Medicine.⁵ The strength of reporting complications was assessed against the Martin criteria.⁶

Results

From a total 187 studies, 11 studies were identified (published between 1997 and 2015), which satisfied our predefined criteria and were included in this review.^{7

–17} Ten of these were cohort studies (one prospective and nine retrospective studies), and one study was a randomized trial.¹⁴

In total, 594 patients with a mean age of 46 years and a male:female ratio of 3:1 underwent BS-PCNL procedures. With the exception of one study,¹⁷ these were all performed in the prone position. In 87.1% (range: 71.4%–100%) of cases, the synchronous approach was performed as planned. However, in the remainder of cases, this method was abandoned and the contralateral side was performed in a staged manner. Second-look nephroscopy was performed in 14.5% (0%–58.8%) of cases. Multiple access tracts were established in an average of 16.7% (4.1%–24%) renal units. Mean initial SFR and final SFR were 72.6% (49%–85%) and 92.4% (87%–96.9%), respectively, with a mean operative time of 171.1 minutes (range: 107.4–269 minutes). For studies where operative time was not clearly defined, the results were not included. The mean hospital stay was 3.9 days (range: 1.25–15 days) (Tables 1 and 2).

Table 1.

Summary of Baseline Characteristics and Complications

Author	Year	Country	Level of evidence, study type	Sample size	Mean age (SD) years	Male/female	BMI (SD) kg/m²	Stone size (SD, range), mm²		Tubeless vs standard (nephrostomy tube inserted)	Multiple access tracts, %	% Transfused		Complications (n, %)
Dushinski and Lingeman⁷	1997	USA	4, Cohort, retrospective	52	49 (NR)	NR	29.9	929 (NR, 26–5478)		All standard	24	4.2		Ureteral perforation (1, 2.1), fragment obstruction (2, 4.2), hydrothorax (5, 10.4), hematuria causing clot retention (2, 4.2), hemorrhage requiring blood transfusion (2, 4.2)
Maheshwari et al.⁸	2000	India	4, Cohort, retrospective	25	34 (NR)	18/7	NR	NR		All standard	20.8	12.5		Fever (4, 16), urine leak (2, 8)
Holman et al.⁹	2002	Yemen	4, Cohort, retrospective	198	36.2 (NR)	163/35	NR	Range 1–5 cm. No further breakdown provided		All standard	NR	6		Fever (8, 4), hemorrhage requiring transfusion (6, 12), urine leak (10, 5)
Shah et al.¹⁰	2005	India	4, Cohort, retrospective	20	48.4 (NR)	13/7	NR	Tubeless	Standard	50% tubeless (Double-J stent only) vs 50% standard	NR	Tubeless	Standard	Tubeless	Standard
								Right: 356.1 mm² (NR, 100–900) Left: 1049.9 mm² (NR, 90–5175)	Left: 1007.5 mm² (NR, 225–2500) Right: 109 mm² (NR, 100–2800)			10	0	Hemorrhage requiring transfusion (1, 10), fever (1, 10), hydrothorax (1, 10)	Fever (1, 10)
Desai et al.¹¹	2007	India	4, Cohort, retrospective	45	44 (NR)	37/8	NR	22.35 (10–54) mm		All standard	22	0		Urinary leak (3, 7), fever (5, 11), urinary tract infection (3, 6)
Ugras et al.¹²	2008	Turkey	3, Cohort, prospective	30	38.8 (1.6)	15/15	NR	Initial side	Second side	All standard	20	10		Hemorrhage requiring blood transfusion (3, 10), urinary tract infection (4, 13.3)
								457 mm² (30, NR)	299 mm² (19, NR)
Bagrodia et al.¹³	2009	USA	4, Cohort, retrospective	17	51 (11)	10/7	38 (ND)	NR		All standard	7	0		Fever (2, 11.8), hydrothorax (2, 11.8)
Wang et al.¹⁴	2011	Taiwan	2, RCT, prospective	50	57.1 (10.61)	37/13	24.94 (2.62)	4624 (1425.1, NR) mm²		All tubeless (Double-J stent only)	Not clear	6		Clavien Grade 1 (10%), Clavien Grade 2 (6%). No further breakdown provided.
Kadlec et al.¹⁵	2012	USA	Cohort, retrospective	47	54 (17)	26/21	32 (11)	2.8 (11, NR) cm		All standard	19.1	0		Clavien Grade 1 (30%), Clavien Grade 2 (20%), Clavien Grade 3 (4%). No further breakdown provided.
Pillai et al.¹⁶	2014	India	4, Cohort, retrospective	85	45.7 (11.6)	65/20	NR	299 (310, 78.5–1962.5) mm²		Totally tubeless in 21.3%, remainder had Double-J stent	4.1	10.5		Hydrothorax (3, 3.5), pneumonia (1, 1.1), urosepsis (1, 1.1), hemorrhage requiring transfusion (9, 10.5), AKI requiring ICU management (1, 1.1)
Proietti et al.¹⁷	2015	Italy	4, Cohort, retrospective	25	51.9 (11.4)	16/9	25.89 (2.1)	3.1 (0.8, NR) cm		76% Bilateral tubeless (Double-J stent) 16%, unilateral tubeless (Double-J stent only)	NR	0		Fever (4, 16), hydrothorax (1, 4)

AKI = acute kidney injury; BMI = body mass index; ICU = intensive care unit; NR = not reported; SD = standard deviation.

Table 2.

Summary of Baseline Characteristics Continued and Results

Author	Mean operative time (SD, range), minutes		Renal units with complete staghorn calculi, %	Supine vs prone	Synchronous procedure performed as planned, %	Second-look nephroscopy (for residual fragments) during same admission, %	Post-op imaging	Definition SFR, mm	Initial SFR, %		Final SFR, %	Mean hospital stay (SD, range), days		Mean drop in Hb (SD, range), g/L
Dushinski and Lingeman⁷	269		NR	Prone	92.3	73	NCCT, timing not specified	<4	NR		96.9	5.6		26 (NR, 12–66)
Maheshwari et al.⁸	122 (NR, 75–200)		24	Prone	96	8	Plain XR, day 2	NR	79.3		87.5	4 (NR, NR)		11.5 (NR, 0–25)
Holman et al.⁹	45 (NR, 20–100)		13.1	Prone	100	2.7	Plain XR, day 1	<4	NR		96.6	4.3 (NR, 3–8)		17.3 (NR, NR)
Shah et al.¹⁰	180.2		NR	Prone	83.3	0	Renal USS/plain XR, day 1	<5	T	S	NR	Tubeless	Standard	Tubeless	Standard
									85		80	1.68 (0.85, NR)	2.52 (0.72, NR)	17 (16.5, NR)	15.9 (5, NR)
Desai et al.¹¹	107.4		3.3	Prone	87	13	Plain XR day 2	<3	NR		95.5	6.6 (1.94,4–15)		20.2 (11.9, 3–56)
Ugras et al.¹²	Initial side	Second side	NR	Prone	71.4	0	Plain XR, timing not specified	NR	NR		88.35	3.6		20 (2, NR)
	110 (9, NR)	85 (8, NR)
Bagrodia et al.¹³	367 (123)^a		27	Prone	100	58.8	NNCT, day 1	NR	NR		87	3.0		NR
Wang et al.¹⁴	244.92 (68.8, NR)		80	Prone	100	0	NCCT, day 1	NR	72		NR	3.62 (1.03, NR)		19.9 (5.2, NR)
Kadlec et al.¹⁵	131 (62)		NR	Prone	100	0		<4	49		NR	5 (2, 2–15)		NR
Pillai et al.¹⁶	87.6 (35.5)		7	Prone	100	0	Plain XR at 4 weeks	NR	NR		95.2	2.9 (1.18, 1.25–10.5)		11 (9,4–70)
Proietti et al.¹⁷	120 (45.4)		NR	Supine	100	4	NCCT, timing not specified	NR	80		NR	3.6 (1.8, NR)		21 (5, NR)

Included anesthesia time.

NCCT = non contrast computed tomography; SFR = stone-free rate; XR = x-ray.

Complications

The mean complication rate per study was 23.4% (range: 12.1%–54%/study). The majority were Clavien Grade 1 (60.9%), of which fever resolving spontaneously within 48 hours was the most common complication. Clavien Grade II and III represented 20.3% and 17.8% of adverse events, respectively. Only a single Clavien Grade 4 complication was reported where the patient had acute kidney injury postprocedure requiring Intensive Care management.¹⁶ The mean drop in Hb was 14.9 g/L (0–70 g/L), with 5.4% of patients requiring blood transfusion. There were no deaths reported in any of the studies (Tables 1 and 3).

Table 3.

Nature of (Early) Complications, Frequency, and Clavien Grade

Nature of complication	Frequency (n)	Clavien Grade
Fever	25	I
Hematuria causing clot retention	2	I
Urine leak resolving spontaneously	15	I
Urinary tract infection	7	II
Pneumonia	1	II
Hemorrhage requiring blood transfusion	9	II
Urosepsis	1	III
Hydrothorax	12	IIIa
Fragment obstruction	2	IIIb
Ureteral perforation	1	IIIb
AKI requiring ICU management	1	IVa
Death	0	V

The mean Martin score was 7.45/10, indicating that the overall quality in complication reporting was good (Table 4). Key areas that commonly scored poorly were a lack of inclusion of risk factors in analyses and use of severity grading systems. More recent studies have adopted the Clavien system and improved this process accordingly.

Table 4.

Summary of Martin Criteria

Martin criteria	Dushinski and Lingeman	Maheshwari et al.	Holman et al.	Shah et al.	Desai et al.	Ugras et al.	Bagrodia et al.	Wang et al.	Kadlec et al.	Pillai et al.	Proietti et al.
Method of accruing data defined	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y
Duration of follow-up indicated	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y
Outpatient information included	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y
Definition of complications defined	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y
Mortality rate and causes of death listed	N	N	P	N	N	N	N	N	P	N	N
Morbidity rate and total complications indicated	N	N	Y	Y	Y	Y	Y	Y	Y	Y	Y
Procedure-specific complications included	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y
Severity grade utilized	N	N	N	N	N	N	N	Y	Y	Y	Y
Length-of-stay data	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y
Risk factors included in the analyses	N	N	N	N	N	N	N	N	Y	N	N
Total (n/10)	7/10	7/10	7/10	7/10	7/10	7/10	7/10	8/10	9/10	8/10	8/10

N = no; P = partial; Y = yes.

Discussion

Findings of our study

This is the first systematic review to report on the outcomes of BS-PCNL for bilateral urolithiasis in adult patients. The findings reveal that this same session approach can deliver high SFRs while maintaining an acceptable safety profile. Given the large numbers of nonindexed and therefore high-risk patients who typically undergo PCNL, careful patient selection is paramount to surgical success.

Safety of BS-PCNL

Silverstein and colleagues reported an overall complication rate of 28% and 19% for synchronous and staged PCNL, respectively.² Their findings correlate with data from other studies as well as with those in this review, which included both treatment arms. However, these figures represent calculations based on the total number of patients. Holman and colleagues argue that studies reporting synchronous bilateral procedures should calculate complication rates according to the number of renal units operated on rather than the number of patients.⁹ Otherwise, the frequency of complications for such simultaneous procedures can be potentially misleading. If this method is adopted, the total number of complications for BS-PCNL is less than that for staged PCNL. Multiple studies have supported the theory that high case volumes favorably impact PCNL outcomes, including complications.^18,19 In this review, it was institutions performing more than 40 cases that yielded SFRs more than 90%.^7,9,11,16 Moreover, the study performing BS-PCNL on the largest sample size recorded the lowest complication rate.⁹

Tubeless or standard PCNL

The vast majority of institutions selected to place nephrostomies as part of their standard practice policy. Although at first glance, efficacy and safety outcomes for tubeless BS-PCNL appear comparable to the standard method, insufficient evidence is currently available to be able to draw any reliable conclusions regarding this issue. For the time being, therefore, the authors are of the opinion that standard placement of nephrostomy tubes should continue for BS-PCNL and especially for centers lacking high-volume experience.

Recommendations for synchronous procedure

Although no standardized protocol exists for performing BS-PCNL, the authors have proposed an ideal set-up for a potential patient and criteria for deferring contralateral surgery (Table 5), based on the findings of this article and the experiences of other endourologists, which have been shared in the literature. The first important consideration is case selection. The decision to proceed with a planned synchronous approach should be tailored to the particular patient and made by a mutual agreement between the surgeon and anesthetist after careful patient counselling. The patient should be fully aware that a decision may be made to defer the contralateral procedure to a later date.

Table 5.

Shows the Potential Patient for Initial Bilateral Synchronous Percutaneous Nephrolithotomy and Criteria for Deferring Contralateral Surgery

Potential patient for initial BS-PCNL	Criteria for deferring contralateral surgery
• Tailored approach to patient (informed consent obtained), not suitable for all patients	• Shared decision with anesthetist and any anesthetic concerns taken into account
• Mutual agreement between patient, surgeon, and anesthetist	• Operative time >180 minutes
• Case selection paramount, discussion in an MDT set-up first	• >2 tracts formed on initial side
• Patient with favorable caliceal anatomy with anticipated single tract only	• Severe hemorrhage or Hb drop >30 g/L or level <11 g/L after first PCNL
• Patient with good performance status, normal renal function, and limited comorbidities	• Arterial oxygen saturation <95%, pH <7.35
• Avoiding tubeless procedure if in center with limited experience	• Serum sodium <128 mg/mL
• Patient fully aware that contralateral surgery may need to be postponed	• Systolic pressure <100 mm Hg
	• Multiple comorbidities or renal dysfunction
	• Any other unforeseen complication

BS-PCNL = bilateral synchronous percutaneous nephrolithotomy; MDT = multidisciplinary.

Centers should start with cases of bilateral simple calculi, favorable caliceal anatomy, and where fewer punctures and tracts are likely to be required.¹¹ With increased experience and proficiency, case selection can be extended to include patients with a lower performance status, chronic kidney disease, and complex stones such as staghorn calculi. As for staged procedures, the first renal unit to be operated on should be the one with more complex stone burden or a greater degree of obstruction.² If the procedure goes smoothly as per plan, the team can embark on the contralateral stone. Until sufficient experience has been gained, the authors advocate that a tubeless procedure should be avoided.

Second, operating teams attempting the simultaneous method should have a shared and established plan for when a simultaneous approach should be abandoned in favor of postponed surgery to the contralateral renal unit. It is arguably the most pivotal point in the operation and again, the decision should be shared with the anesthetist. There are several factors that are important in this decision-making process. Setting a time restriction (180 minutes) for the initial side has been adopted by a number of centers. Not only has prolonged operative time been cited as a risk factor for hemoglobin decrease but also this strategy helps decrease other time-related complications, for example, hypothermia and DVT formation.¹⁹

Given the recognized relationship between the formation of multiple tracts and hemoglobin loss, establishing a predefined cutoff for the maximum number of tracts allowed on the initial side (such as 2) is a sensible consideration.¹² Performing blood gas analysis after the completion of the initial side allows for Hb to be measured in addition to pH and arterial oxygenation, which can highlight patients suffering pulmonary complications. Hb drop >30 g/L or level <11 g/L should prompt the contralateral procedure to be cancelled. Caution in proceeding should be taken if any of the following are incurred during the surgery on the initial side: arterial oxygen saturation <95%, pH <7.35, serum sodium <128 mg/mL, or systolic pressure <100 mm Hg. Ugras and colleagues estimated that as many as 30% of synchronous procedures may have to be abandoned for any of the reasons outlined earlier.¹²

A general rule may, therefore, be that if surgery on the initial side is successful and accomplished quickly, a decision can then be made to proceed to the contralateral PCNL procedure.

The evolution of synchronous endourologic procedures

The modern era of endourology has witnessed the evolution of synchronous procedures. In addition to BS-PCNL in adults, this practical approach has also been performed in pediatric populations. In 2004, Salah and colleagues published the first series (13 children, mean age 8 years), reporting the technique using an adult 26F nephroscope.²⁰ A 100% final SFR was rendered with no serious complications in their series. A subtle but important point raised by the authors is the advantage of reduced psychological stress incurred to the patient when performing PCNL for both sides during the same session. Interestingly, this consideration is mentioned only sparingly in adult studies. Bilateral same-session ureteroscopy has also been reported as an effective surgery, with a final SFR of ∼90%.²¹ It has also been associated with a slightly greater overall complication rate than expected. Another emerging solution for bilateral stone disease is synchronous PCNL and ureteroscopy.²²

Cost effectiveness

The annual spending on urolithiasis in the United States is estimated to exceed $2 billion U.S. dollars.²³ In an era of mounting economic constraints and limited availability of funding, the ability, therefore, to ameliorate the cost profile of a surgical intervention is hugely desirable. The modification of a surgeon's practice to a simultaneous approach potentially allows for such an improvement. Two of the studies in this article provided cost-effectiveness analysis.^13,14 Bagrodia and colleagues reported direct cost savings of $4374 (30%) when a simultaneous surgery was adopted.¹³ Higher use of disposable instruments, duplicated preoperative investigations, for example, blood tests and imaging, as well as greater cumulative hospital stay contribute to this added cost burden. This financial incentive for health institutions (third party payer) is not offset by a slightly higher complication rate for simultaneous compared with staged PCNL. Healthcare policies typically reimburse the second distinct procedure (i.e., PCNL of the contralateral renal unit) at 50% of the allowable. This correlates closely to an estimated pecuniary loss of $1069. This correlates with the 24.6% reduction in physician re-imbursement reported by Wang and colleagues.¹⁴

Limitations/implications for future research

With the exception of one randomized trial,¹⁴ all the studies included in this review were observational cohort studies. Given that the adoption of new techniques necessitates high-quality evidence to achieve disseminated and standardized practice, further randomized trials and prospective large-volume series gathered from a multi-centre setting are, therefore, required to gather such an evidence. Notable variation existed in both the imaging modality (and timing) to assess postprocedure stone-free status and also how SFR was defined. One study described the latter as residual fragments <5 mm,¹⁰ whereas others defined it as <4 mm^7,9,15 or <3 mm.¹¹ Moreover, six studies provided no definition for their SFR. In light of this variation, due caution is, therefore, recommended when interpreting SFRs both in this study and elsewhere in the literature. A proposed remedy for this is the “Stone Free Rate” classification, which aids clarification of post-treatment stone size.²⁴ Similarly, insufficient information was provided across the studies on additional stone characteristics such as complexity, location, and composition.

For this article, the heterogeneity of results reported by the individual studies was such that only a pooled analysis of mean values was possible. Full application of meta-analytic methods could, therefore, not be integrated to calculate treatment effect sizes. Improved reporting on location as well as number of supra-costal punctures performed, for example, would enable a more accurate interpretation of complications and risk stratification accordingly. Another simple and reproducible method, which has the potential to augment such studies, is the Guys Stone Score, which permits the complexity of PCNL to be graded (I–IV). This validated tool also enables an accurate prediction of SFR.²⁵ Only 5 out of 11 studies gave information on BMI. Future studies should include this baseline characteristic as well as other parameters such as renal function, comorbidities such as diabetes, ischemic heart disease, and hypertension. Similarly, the experience of a small-volume center might be different and urologists should consider this before embarking on bilateral PCNL procedures.

Conclusion

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Abbreviations Used

References

Patel

, Nakada

. The modern history and evolution of percutaneous nephrolithotomy. J Endourol, 2015; 29:153–157.

Silverstein

, Terranova

, Auge

, et al. Bilateral renal calculi: Assessment of staged v synchronous percutaneous nephrolithotomy. J Endourol, 2004; 18:145–151.

Moher

, Shamseer

, Clarke

, et al. Preferred reporting items for systematic review and meta-analysis protocols (PRISMA-P) 2015 statement. Syst Rev, 2015; 4:1.

Dindo

, Demartines

, Clavien

. Classification of surgical complications: A new proposal with evaluation in a cohort of 6336 patients and results of a survey. Ann Surg, 2004; 240:205–213.

OCEBM Levels of Evidence Working Group.. The Oxford levels of evidence 1. Oxford Centre for Evidence-Based Medicine. 2011. Available from: www.cebm.net/index aspx?o = 1025 Accessed December 25, 2016 .

Martin II

, Brennan

, Jaques

. Quality of complication reporting in the surgical literature. Ann Surg, 2002; 235:803–813.

Dushinski

, Lingeman

. Simultaneous bilateral percutaneous nephrolithotomy. J Urol, 1997; 158:2065–2068.

Maheshwari

, Andankar

, Hegde

, Bansal

. Bilateral single-session percutaneous nephrolithotomy: A feasible and safe treatment. J Endourol, 2000; 14:285–287.

Holman

, Munim Khan

, Pasztor

, Toth

. Simultaneous bilateral compared with unilateral percutaneous nephrolithotomy. BJU Int, 2002; 89:334–338.

10.

Shah

, Kausik

, Hegde

, Shah

, Bansal

. Safety and efficacy of bilateral simultaneous tubeless percutaneous nephrolithotomy. Urology, 2005; 66:500–504.

11.

Desai

, Grover

, Manohar

, Ganpule

. Simultaneous bilateral percutaneous nephrolithotomy: A single-center experience. J Endourol, 2007; 21:508–514.

12.

Ugras

, Gedik

, Gunes

, Yanik

, Soylu

, Baydinc

. Some criteria to attempt second side safely in planned bilateral simultaneous percutaneous nephrolithotomy. Urology, 2008; 72:996–1000.

13.

Bagrodia

, Raman

, Bensalah

, Pearle

, Lotan

. Synchronous bilateral percutaneous nephrostolithotomy: Analysis of clinical outcomes, cost and surgeon reimbursement. J Urol, 2009; 181:149–153.

14.

Wang

, Chang

, Huang

. Simultaneous bilateral tubeless percutaneous nephrolithotomy of staghorn stones: A prospective randomized controlled study. Urol Res, 2011; 39:289–294.

15.

Kadlec

, Greco

, Fridirici

, Hart

, Vellos

, Turk

. Comparison of complication rates for unilateral and bilateral percutaneous nephrolithotomy (PCNL) using a modified Clavien grading system. BJU Int, 2013; 111:E243–E248.

16.

Pillai

, Mishra

, Sharma

, Venkatesh

, Chawla

, Hegde

, Thomas

. Tubeless simultaneous bilateral percutaneous nephrolithotomy: Safety, feasibility and efficacy in an Indian setting. Int J Urol, 2014; 21:497–502.

17.

Proietti

, Sortino

, Giannantoni

, et al. Single-session supine bilateral percutaneous nephrolithotomy. Urology, 2015; 85:304–310.

18.

Opondo

, Tefekli

, Esen

, et al. Impact of case volumes on the outcomes of percutaneous nephrolithotomy. Eur Urol, 2012; 62:1181–1187.

19.

Kadlec

, Ellimoottil

, Guo

, Trinh

, Sun

, Turk

. Contemporary volume–outcome relationships for percutaneous nephrolithotomy: Results from the nationwide inpatient sample. J Endourol, 2013; 27:1107–1113.

20.

Salah

, Tállai

, Holman

, Khan

, Tóth

. Simultaneous bilateral percutaneous nephrolithotomy in children. BJU Int, 2005; 95:137–139.

21.

Rai

, Ishii

, Jones

, et al. Bilateral simultaneous ureteroscopy for bilateral stone disease: A systematic review. Can J Urol, 2016; 23:8220–8226.

22.

Mason

, Koi

, Hafron

, Milhoua

, Hoenig

. Safety and efficacy of synchronous percutaneous nephrostolithotomy and contralateral ureterorenoscopy for bilateral calculi. J Endourol, 2008; 22:889–894.

23.

Pearle

, Calhoun

, Curhan

; Urologic Diseases of America Project. Urologic diseases in America project: Urolithiasis. J Urol, 2005; 173:848–857.

24.

Somani

, Desai

, Traxer

, Lahme

. Stone-free rate (SFR): A new proposal for defining levels of SFR. Urolithiasis, 2014; 42:95.

25.

Thomas

, Smith

, Hegarty

, Glass

. The Guy's stone score—Grading the complexity of percutaneous nephrolithotomy procedures. Urology, 2011; 78:277–281.