Retrograde Intrarenal Surgery Is a Safe Procedure in Severe Obese Patients: Is It Reality or Prediction? A Propensity Score-Matching Analysis from RIRSearch Study Group

Abstract

Objective:

The aim of the study was to assess whether severely obese patients have an increased risk of complications during and after retrograde intrarenal surgery (RIRS).

Materials and Methods:

The data of 639 consecutive patients undergoing RIRS for the treatment of upper tract urinary stones were analyzed retrospectively. The patients were divided into two groups according to their body mass index numbers (Group 1, <35; Group 2, ≥35). The patients' demographics, stone characteristics, operative outcomes, and complication rates were compared between the groups. The primary objective was to examine whether the intraoperative and postoperative complication rates were higher in patients with a body mass index of ≥35 kg/m².

Results:

After matching of confounding factors, Group 1 comprised 135 patients, and Group 2 comprised 47 patients. The baseline characteristics were similar between the groups. There were no significant differences between groups for intraoperative complication rates (11.8% and 12.8%, respectively; p = 0.97). There was statistically significant difference in favor of Group 2 for postoperative complication rates (12.6% and 29.7%; respectively, p < 0.01), overall complication rates (22.9% and 38.2%; respectively, p = 0.02), mean operation time (56.15 vs 66.45 minutes; respectively, p = 0.01), and length of stay (1.4 vs 2.1 days; p = 0.03). Stone-free rates (75.5% vs 85.1%; respectively, p = 0.17) did not differ between groups.

Conclusions:

RIRS is an efficient and feasible treatment option for upper urinary tract stones in severely obese patients. However, higher possibility of postoperative, especially infectious, complication rates should be considered in these patients.

Introduction

Obesity is a global health problem affecting people of all ages. Obesity increases the risk of renal stone formation, as it is closely associated with a low urine volume, increased urinary osmolality, acidic pH, and insulin resistance.^1
–3 The treatment of renal stones in obese patients is a challenging task. Minimally invasive treatments, such as extracorporeal shockwave lithotripsy (SWL), percutaneous nephrolithotomy (PCNL), and retrograde intrarenal surgery (RIRS), can be performed on selected patients. Longer skin-to-stone distances, difficulties related to the prone position, and technical limitations complicate the targeting of the appropriate calix or stone, making SWL and PCNL challenging in obese patients.^4,5

Indeed, SWL and PCNL are associated with increased anesthesia-related risks and lower success rates in these patients.⁶ The European Association of Urology guidelines suggest that RIRS might be more suitable for obese renal stone patients because it can be performed in the supine position, and the absence of specific contraindications to RIRS makes it feasible even for patients receiving anticoagulant therapy.⁷

According to the World Health Organization (WHO), severely obese patients (body mass index [BMI] of ≥35) have a considerably higher risk of life-threatening comorbidities, such as cardiovascular events and diabetes mellitus.⁸ Therefore, we hypothesized that severely obese patients may be more susceptible to intra- and/or postoperative complications related to RIRS. Most studies evaluating the impact of BMI on the outcomes of RIRS have focused primarily on surgical success rates.⁹ Data on the relationship between severe obesity and RIRS safety are limited. To fill this gap, this study aimed to determine whether severely obese patients have an increased risk of complications during and after RIRS.

Materials and Methods

This was a multicenter retrospective study performing a propensity score-matching analysis. The study protocol was approved by the local Ethics Committee of the Faculty of Medicine of Bulent Ecevit University, Zonguldak/Turkey (approval no.: 20.10.2021/2021-20). The data of 1119 consecutive patients undergoing RIRS for the treatment of upper urinary system stone disease at seven referral centers from February 2016 to May 2021 were reviewed. All operations were performed by surgeons with a minimum of 5 years of experience in RIRS. The patients' demographic characteristics, stone-related parameters (stone volume, location, density, and number), and operative outcomes were retrospectively evaluated. The prospectively built RIRSearch database was used for this retrospective analysis.

Patients younger than 18 years of age, patients with incomplete data, renal anatomical abnormalities, solitary kidney, and previous ipsilateral PCNL procedure, and patients undergoing bilateral RIRS were excluded from the study. After exclusions, a total of 639 patients undergoing RIRS for renal and/or upper ureteral stones were included before propensity score matching. BMI was used to assess patients' adiposity status. The patients were categorized into normal weight (<25 kg/m²), overweight (25–29 kg/m²), Class 1 obesity (30–34 kg/m²), Class 2 (severe) obesity (35–39 kg/m²), and Class III (very severe) obesity (≥40 kg/m²). Using a cutoff value of ≥35 kg/m², patients with a BMI of <35 kg/m² were assigned to Group 1 (control group), and patients with a BMI of ≥35 kg/m² were assigned to Group 2 (case group).

Propensity score matching was then performed to reduce the risk of selection bias and control for confounding factors in our analyses. The variables used for propensity score matching were age, gender, stone location, volume, presence of multiple stones, and ureteral access sheath use. A total of 47 patients were included in Group 2 and were matched in a one-to-three manner to 135 Group 1 patients using a fuzzy matching algorithm. Thus, a total of 182 patients were included in the analysis.

The preoperative evaluation of patients consisted of a detailed anamnesis, physical examination, routine laboratory studies, including serum creatinine levels, urinalysis, and urine culture, and radiological evaluation using noncontrast computed tomography (NCCT). If the urine culture was positive, appropriate antibiotics were prescribed according to the patient's sensitivity profile. The operation was performed only after the patient's urine was confirmed to be sterile. The stone volume was calculated based on the NCCT images using the following formula: length × width × height × π × 0.167 (in cubic millimeters). In cases with multiple stones, the stone size was calculated as the sum of all the stones' volumes. The stone locations were classified based on the NCCT images as follows: renal pelvis, upper, middle, and lower calix, and upper ureter for stones located above the upper border of the sacroiliac joint.

The patients' demographic characteristics, stone-related parameters, and operative outcomes were compared between the two groups before and after propensity score matching. The primary objective was to examine whether the intraoperative and postoperative complication rates were higher in patients with a BMI of ≥35 kg/m². A modified Satava classification system (MSCS) was used to classify intraoperative complications, and the modified Clavien classification system (MCCS) was used for postoperative complications. Postoperative complications were further subdivided into two groups: Grade 1 and 2 complications were defined as “minor,” and Grade 3, 4, and 5 complications were classified as “major.”

Stone-free status was defined as no evidence of a residual stone (or stones) <3 mm on first-month postoperative NCCT. In all cases, laser settings were adjusted as 0.5 J - 20 Hz to crumble stones finely (dusting) or 1 J - 12 Hz for the stone fragmentation. If the stones were completely fragmented with the dusting method, the extraction was not applied. Otherwise, the fragmented stones were extracted by the nitinol basket catheter (Dakota®; Boston Scientific, Marlborough, MA). The surgical technique was described in our previous study.¹⁰

Statistical analysis

Categorical variables were expressed as frequencies and percentages. Continuous variables were expressed as means ± standard deviations. The Shapiro–Wilk test was used to assess the normality assumption for continuous variables. Differences in proportions between the groups were compared using the chi-square test or Fisher's exact test as appropriate. Student's t-test was used to evaluate differences between normally distributed continuous variables, and the Mann–Whitney U-test was used to assess differences between non-normally distributed variables. Values of p < 0.05 were considered statistically significant. All statistical analyses were performed using SPSS (SPSS, Inc., Chicago, IL).

Results

After propensity score matching, there were no statistically significant differences between the two groups in terms of age, gender, preoperative urinary tract infection, hydronephrosis, or stone characteristics (volume, side, location, density, or number), as given in Table 1. The two groups were also similar in terms of preoperative Double-J stenting and SWL history. The operative outcomes are summarized in Table 2. The fluoroscopy times, ureteral access sheath use, and preoperative and postoperative serum creatinine levels did not differ significantly between the two groups. Conversely, the mean operation time was significantly longer in Group 1 than in Group 2. By contrast, the mean length of hospital stay was significantly greater in Group 2 than in Group 1. Stone-free status was achieved in 102 (75.5%) patients in Group 1 and 40 (85.1%) patients in Group 2 (p = 0.17).

Table 1.

Patients' Demographics and Stone Characteristics

Variable	Before propensity score matching			After propensity score matching		p
Variable	Group 1	Group 2	p	Group 1	Group 2	p
Number of patients	592	47		135	47
Age (years), mean ± SD	47.55 ± 14	48.81 ± 12	0.52	47.77 ± 13	48.81 ± 12	0.66
Gender, n (%)
Male	357 (60.3)	20 (42.6)	0.01^*	77 (57)	20 (42.5)	0.08
Female	235 (39.7)	27 (57.4)	0.01^*	58 (42.9)	27 (57.4)	0.08
BMI, mean ± SD	26.7 ± 3.3	38 ± 3.1	<0.01^*	28 ± 2.9	38 ± 3.1	<0.01^*
Age-adjusted CCI, n (%)
0–1	436 (73.7)	17 (36.1)	<0.01^*	105 (77.7)	17 (36.1)	<0.01^*
≥2	156 (26.3)	30 (63.8)	<0.01^*	20 (14.8)	30 (63.8)	<0.01^*
Hydronephrosis, n (%)	251 (42.4)	16 (34)	0.26	50 (37)	16 (34)	0.71
Pyuria, n (%)	324 (54.7)	23 (48.9)	0.23	70 (51.8)	23 (48.9)	0.26
Positive urine culture, n (%)	32 (5.4)	3 (6.3)		7 (5.1)	3 (6.3)	0.24
Side, n (%)
Right	303 (51.1)	26 (55.3)	0.68	66 (48.8)	26 (55.3)	0.21
Left	289 (48.9)	21 (44.6)	0.68	69 (51.1)	21 (44.6)	0.21
Number of stones, n (%)
One	390 (65.9)	33 (70.2)	0.75	97 (71.9)	33 (70.2)	0.08
More than one	202 (39.2)	14 (29.8)	0.75	38 (28.1)	14 (29.8)	0.08
Stone size (mm³), mean ± SD	1100.81 ± 2401.63	1439.60 ± 2037.10	0.04^*	1222.17 ± 1901.00	1439.60 ± 2037.10	0.31
Stone attenuation (HU), mean ± SD	959.63 ± 275.29	893.43 ± 264.51	0.12	929.18 ± 251.02	893.43 ± 264.51	0.40
Stone location, n (%)
Upper pole	23 (3.9)	4 (8.5)	0.72	6 (4.4)	4 (8.5)	0.87
Middle pole	39 (6.6)	3 (6.4)		7 (5.2)	3 (6.4)
Lower pole	105 (17.7)	8 (17)		31 (23)	8 (17)
Renal pelvis	117 (19.8)	9 (19.1)		25 (18.5)	9 (19.1)
Upper ureter	145 (24.5)	9 (19.1)		28 (20.7)	9 (19.1)
Multiple locations	163 (27.5)	14 (29.8)		38 (28.1)	14 (29.8)
Anticoagulant use, n (%)	60 (10.1)	7 (14.8)	0.30	14 (10.3)	7 (14.8)	0.40
SWL history, n (%)	184 (31.1)	18 (38.2)	0.31	48 (35.5)	18 (38.2)	0.73
Prestenting, n (%)	233 (39.4)	15 (31.9)	0.31	50 (37)	15 (31.9)	0.52

Statistically significant difference.

BMI = body mass index; CCI = Charlson comorbidity index; n = patient number; SD = standard deviation; SWL = extracorporeal shockwave lithotripsy.

Table 2.

Operative Outcomes

Variable	Group 1	Group 2	p
Operation time (minutes), mean ± SD	66.45 ± 29.6	56.15 ± 21.5	0.01^*
Fluoroscopy screening time (seconds), mean ± SD	32.03 ± 81	23.09 ± 48.2	0.29
Ureteral access sheath use, n (%)	99 (73.3)	33 (70.2)	0.68
Postoperative creatinine (mg/dL), mean ± SD	1.11 ± 0.3	1.03 ± 0.3	0.81
Intraoperative complications, n (%)	16 (11.8)	6 (12.8)	0.97
Postoperative complications, n (%)	17 (12.6)	14 (29.7)	<0.01^*
Minor	15 (11.1)	8 (17)	0.31
Major	2 (1.4)	6 (12.7)	<0.01^*
Total complications, n (%)	31 (22.9)	18 (38.2)	0.02^*
Length of hospital stay (days), mean ± SD	1.4 ± 0.6	2.1 ± 0.9	0.03^*
Stone-free rate, n (%)	102 (75.5)	40 (85.1)	0.17
Auxiliary procedure, n (%)	22 (16.2)	5 (10.6)	0.34

Statistically significant difference.

Intraoperative complications were seen in 16 (11.8%) patients in Group 1 and 6 (12.7%) patients in Group 2. In Group 1, 7 (5.1%) patients had Grade 1 complications, 3 (2.2%) had Grade 2 complications, and 6 (4.4%) had Grade 2b complications. Failure to reach the stone was the most common intraoperative complication in Group 1. In Group 2, 4 (8.5%) patients had Grade 1 complications and 2 (4.2%) had Grade 2b complications. The most common intraoperative complications in Group 2 were mild hematuria and failure to reach the stone. The intraoperative complication rates did not differ significantly between the two groups (11.8% and 12.7%, respectively; p = 0.97). The intraoperative complications classified according to the MSCS are given in Table 3.

Table 3.

Classification of Intraoperative Complications According to the Modified Satava Classification System

Grade	Complication	Group 1, n (%)	Group 2, n (%)	p
1	Mild hematuria	2	2
	Minimal mucosal injury of ureter (mucosal tears)	2	1
	Inability to reach stone necessitating observation	3	1
2a	Mucosal injury necessitating stent insertion	2
2a	Ureteral perforation managed by placing ureteral stent	1
2b	Inability to access ureter or reach stone necessitating secondary retrograde intrarenal surgery	3	2
	Secondary ureteroscopy owing to ureteral stone fragments	1
	Severe bleeding necessitating termination of the procedure and secondary retrograde intrarenal surgery	2
Total		16 (11.8)	6 (12.7)	0.9

The postoperative complications according to the MCCS are given in Table 4. Complications were noted in 17 (12.5%) patients in Group 1 and 14 (29.7%) patients in Group 2. In Group 1, most complications were minor (MCCS Grade 2 or lower), and the rate of major complications was 1.4%. In Group 2, Grades 1, 2, 3b, and 4b complications were observed in 10.6%, 6.4%, 6.4%, and 8.5% of the patients, respectively. There were no Grade 5 complications in either group. The most common postoperative complications were fever necessitating antibiotherapy (n = 9, 6.6%) in Group 1 and urosepsis (n = 4, 8.5%) in Group 2. The rate of postoperative complications was significantly higher in Group 2 than in Group 1. Likewise, Group 2 had a significantly higher rate of major complications than Group 1. The overall (intraoperative and postoperative) complication rate was also significantly higher in Group 2 than in Group 1.

Table 4.

Classification of Postoperative Complications According to the Modified Clavien Classification System

Grade	Complication	Group 1, n (%)	Group 2, n (%)	p
1	Mild hematuria	4	3
1	Fever treated with antipyretics	1	2
2	Renal colic	1	2
2	Fever necessitating antibiotherapy	9	1
3b	Steinstrasse	1	2
4b	Urosepsis	1	4
Total		17 (12.5)	14 (29.7)	0.003^*

Statistically significant difference.

Discussion

In this multicenter retrospective study, we examined whether there is a link between severe obesity and an increased risk of RIRS complications. We found that the stone-free and intraoperative complication rates of severely obese patients were comparable with those of normal/overweight and moderately obese patients. These results suggest that RIRS is a safe and feasible option for the treatment of upper urinary tract stones in severely obese patients. However, care should be taken while monitoring these patients postoperatively, as our findings indicate that severe obesity is associated with higher postoperative complication rates.

BMI provides the most useful and practical population-level measure of obesity. However, it does not correspond to the same degree of adiposity or associated health risks in different individuals or populations owing to wide variations in body fat distribution, such as muscle and fat weight.¹¹ According to the WHO, the overall health risks are considerably higher for individuals with a BMI of ≥35 kg/m² than in those with a BMI of <35 kg/m².⁸ In fact, severe obesity was defined as a BMI of ≥35 kg/m² in many reports related to bariatric procedure in published literature.¹² Therefore, we used the WHO classification system of obesity and defined severe obesity as a BMI of ≥35 kg/m².

Several studies have documented the safety of RIRS with low intraoperative complication rates.^13,14 However, few have evaluated its safety for obese patients. A single-center retrospective study reported no intraoperative complications in morbidly obese patients and concluded that RIRS was an efficient and safe option for the treatment of upper urinary tract stones in these patients.¹⁵ These findings are in line with Caskurlu and colleagues, who found no association between BMI and the incidence of intraoperative RIRS complications.¹⁶ In our study, after controlling for confounding factors, we found that a higher BMI had no significant effect on intraoperative RIRS complications. Failure to reach the stone was the most common intraoperative complication in both groups.

The observed intraoperative complication rates in severely obese patients (12.5%) are also comparable with nonobesity-specific rates (7%–30%) reported in the literature.^14,17 Moreover, the mean operation time was significantly shorter in severely obese patients, further confirming that RIRS is a feasible procedure for these patients. In the published literature, longer operation time was often associated with the higher likelihood of postoperative, especially infectious, complications after RIRS.¹⁸ In addition, despite shorter operation time, a higher rate of postoperative complications suggests that physicians should be more careful during preoperative preparations and postoperative care of these patients.

Although RIRS is considered a first-line treatment for obese patients, urologists may face challenges postoperatively because these patients usually have several comorbidities, such as diabetes and heart diseases. This subject is debatable because of a lack of studies directly evaluating postoperative RIRS complications in these patients. Several studies reported similar complication rates in patients with different BMI numbers. However, these were formed retrospectively and unmatched groups in these studies led to increase the influence of selection bias between the groups.^16,19,20 Moreover, there are conflicting reports on the relationship between a high BMI and postoperative complication rates.

A multicenter retrospective study found that, although obese and morbidly obese patients had significantly more comorbidities than other patients undergoing RIRS, a higher BMI did not increase the incidence of postoperative complications.²⁰ In contrast, a high-volume multicenter prospective study on the safety and efficacy of ureteroscopic lithotripsy found that a higher BMI predicted a higher incidence of complications after either semirigid or flexible ureteroscopy.²¹ Similarly, in our study, the postoperative complication rate was significantly higher in severely obese patients. The incidence of major complications was also significantly higher. The postoperative complication rate in these patients (29.7%) is slightly higher than the overall, nonobesity-specific rates (8.4%–26.1%) reported in the literature.^14,22

Another important finding of our study was that infective complications were more common in severely obese patients. The most common complication was urosepsis necessitating intensive care unit management. Some explanations for this finding may be proposed. In our study, we matched the groups in terms of stone volume and gender, which have been reported to be independent risk factors of infectious complications after RIRS.²³ Although the effect of ureteral access sheath use on the complication rates is debatable in the current literature, a recent study published by our group showed that there was a significant association with the ureteral access sheath use and a lower likelihood of complication rates during and after RIRS.²⁴ Therefore, we also matched groups in terms of ureteral access sheath use. In addition, in that study, it was found that there was no significant difference in terms of complication rates between the different calibrations of ureteral access sheaths. On the contrary, a multicenter prospective study reported that concomitant diseases and related disorders often found in severely obese patients, such as coronary heart disease, and alteration of lipid metabolism, were preoperative predictors of infective complications after RIRS.²⁵ We defined patients' comorbidity levels based on the age-adjusted Charlson comorbidity index.²⁶ In univariate analyses, severely obese patients had significantly higher Charlson comorbidity index scores, suggesting the presence of multiple concomitant diseases and related disorders. This might be a reason for the higher incidence of infectious events in these patients.

Obesity may not only increase the risk of infections but also worsen the outcomes of infectious events. Several mechanisms have been reported to determine obesity-related factors affecting the risk and course of infectious complications. One of them is obesity-related immune system dysregulation, such as impaired chemotaxis and dysregulated cytokine production. Another is related to respiratory dysfunction, such as decreased pulmonary volumes and ventilation–perfusion mismatch. A third one is associated with obesity-related comorbidities, such as diabetes mellitus and atherosclerosis.²⁷

In light of these factors, our results suggest that RIRS may be more complicated in severely obese patients. In our opinion, the same conclusion cannot be made for PCNL and SWL as it was shown that the high intrarenal pressure during the RIRS promotes local pathogens and toxins into blood circulation. In fact, patients with positive urine culture are more likely to suffer sepsis after RIRS than PCNL.²⁸ Therefore, possible detrimental outcomes should be kept in mind when planning RIRS for these patients.

Our analysis revealed no significant association between BMI and stone-free or re-treatment rates. This is in line with the results of several previous studies. For instance, a single-center retrospective study comparing the effectiveness of RIRS between obese and nonobese patients found that BMI was not associated with the stone-free rate.¹⁹ In our study, we found no correlation between the severity of obesity and stone-free rates. In fact, the stone-free rate in severely obese patients was slightly higher than in normal weight, overweight, and obese patients, although the difference was not statistically significant. These findings suggest that RIRS can be performed on severely obese patients.

A main limitation of our study is its retrospective design, which entails possible selection bias. However, we used the RIRSearch study group database, which was carefully built prospectively. Moreover, we used a one-to-three matching analysis and included previously reported confounding factors as covariate-matching factors. Another limitation is that we were unable to measure renal pelvic pressure, which has been shown to be closely associated with higher operative RIRS complication rates.²⁹ On the contrary, all procedures were performed using a surgical technique and equipment standardized since 2015. A third limitation is that, although the CCI is widely used to classify various comorbid conditions and underlying diseases such as diabetes and heart disease in published literature, patients' comorbidities were not taken into account separately. If the patients' comorbidities were enrolled one by one, there would be more concise results. Finally, although this was a one to three matching study, the small number of patients in Group 2 may have resulted in insufficient statistical power to draw more definite conclusions from the results.

Conclusion

RIRS can be safely and efficiently performed on severely obese patients, with surgical success and intraoperative complication rates comparable with those of other patients. However, severely obese patients can have significantly higher postoperative complication rates. Therefore, urologists should always consider the possibility of postoperative complications, especially infectious complications, in these patients.

Footnotes

Authors' Contributions

C.B., C.M.Y., and E.B.S. contributed to the concept and design of the study. C.B., H.M.A., O.O., Ö.Ç., E.B.S., A.B.B., H.Ç. involved in data acquisition. D.S. and O.Ö. performed the statistical analysis. The first version of the article was written by C.B., C.M.Y., E.B.S., and C.B. wrote the main article. All authors read and approved the final version of the study and article.

Acknowledgment

The authors thank all those who contributed to this study.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

No funding was received for this article.

Abbreviations Used

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