Impact of Percutaneous Nephrostomy on the Efficacy of in Situ Shock Wave Lithotripsy for Upper Ureteral Stones Seung Woo Yang#, Ji Yong Lee#, Ju Hyun Shin, Jae Sung Lim, Ki Hak Song* Purpose: To investigate whether a percutaneous nephrostomy (PCN) has any impact on the success rate of shock wave lithotripsy (SWL) and to estimate the probability of stone-free rate in SWL patients with upper ureter stones. Materials and Methods: Overall, 236 patients who underwent SWL for upper ureter stones between 2015 and 2019 were evaluated. Forty-nine patients who underwent PCN during SWL were identified. Medical data of the patients were retrospectively reviewed, and possible prognostic features were evaluated. Results: Out of all patients, 147 patients were selected through propensity score matching. There were no signif- icant differences between the PCN and no PCN groups, except for a lower stone-free rate (55.1% vs. 74.5%, p = .018) and one-session success rate (24.5% vs. 50.0%, p = .003) in the PCN group. In univariate analysis, a younger age, the female sex, a smaller size of the stone, lower mean stone density (MSD), and absence of PCN were posi- tive predictive factors of being stone-free in patients who underwent SWL. In multivariate analysis, a smaller size, lower MSD, and absence of PCN were positive predictive factors of being stone-free in patients who underwent SWL. Conclusion: Stone size, MSD, and PCN were prognostic factors that influence the outcome of SWL. The presence of PCN during SWL is associated with adverse success rates in patients with upper ureter stones. Keywords: percutaneous nephrostomy; shockwave lithotripsy; stents; ureter; urinary calculi INTRODUCTION Urolithiasis is one of the most prevalent problems in patients visiting a department of urology. The commonest risk factors for the progression of urolithi- asis include metabolic syndrome, dehydration, lifestyle changes, and rise in ambient temperatures(1-3). Usually, upper urinary tract stones pass through the urinary tract without a problem; however, in some cases, complica- tions arise, the most frequent being upper urinary tract obstruction and acute renal colic, which often result in excruciating pain(4). If the presence of stones is com- plicated by acute kidney injury with severe obstruction or infection, either percutaneous nephrostomy (PCN) or a double-J ureteral stent should be attempted as an emergency procedure to allow drainage of urine for de- compression before planning further treatment(5). Shock wave lithotripsy (SWL) was introduced in the 1980s for the treatment of urolithiasis and was accepted imme- diately as a first-line option for treatment(6). A number of factors, such as position, size, and hardness of the stone determine the efficacy of treatment with SWL(7). Retreatment, if required, particularly in the case of large volume stone results in additional expenditure in terms Department of Urology, Chungnam National University Hospital, Chungnam National University College of Medicine, Daejeon, Korea. *Correspondence: Department of Urology, Chungnam National University Hospital, Chungnam National University College of Medicine, 282 Monwha-ro, Jung-gu, Daejeon, Korea. Tel: +82-42-280-7777.Fax: +82-42-280-7206. E-MAIL: urosong@cnu.ac.kr # These authors contributed equally to this work and should be considered co-first authors. Received March 2021 & Accepted July 2021 of time and finances(8). Prior evaluation of whether pa- tients with urolithiasis will respond well to SWL, pre- vents unnecessary wastage of resources and treatment by procedures likely to be ineffective by selecting more appropriate treatment methods for the management of urolithiasis (9). Hence, it is advisable to identify in ad- vance patients who can be better served using an al- ternative modality of treatment. Recently, some studies have demonstrated that double-J ureteral stents reduce the success rate of SWL(10-12); however, it remains un- certain and debatable whether PCN affects the success rate of SWL. This study aimed to investigate whether PCN affects the success rate of SWL for the treatment of ureteral stones. Additionally, various patient popu- lations and stone characteristics were evaluated using non-contrast computed tomography (NCCT) for pre- dicting the stone-free rate after SWL to counsel patients on the various treatment options available for upper ureteral stones. MATERIALS AND METHODS Patient population A retrospective analysis of the database of patients of Urology Journal/Vol 19 No. 4/ July-August 2022/ pp. 262-267. [DOI: 10.22037/uj.v18i.6762] ENDOUROLOGY AND STONE DISEASE our department, who received SWL for a single upper ureteral stone from January 2015 to December 2019, was performed. Overall, 236 patients with previously untreated stones were registered. The inclusion criteria for the current study were a solitary stone measuring 0.5 – 2 cm in diameter, radiopaque, and located within the upper ureter on NCCT. Patients with the following state were excluded: uncontrolled bleeding disorders, uncorrected obstruction inferior to the stone, had a gen- itourinary tract abnormality, younger than 15 years, double-J ureteral stents inserted state. The medical data of these patients were reviewed to evaluate their suita- bility as prognostic features. The factors evaluated were age, sex, diabetes mellitus (DM), hypertension (HTN), stone laterality, stone length (X, Y, and Z axes), stone volume, mean stone density (MSD), stone heteroge- neity index (SHI), skin to stone distance (SSD), psoas muscle cross-sectional area, colic pain and presence of PCN before SWL. PCN was performed as an emergen- cy procedure in complicated upper ureter stone with severe obstruction and infection. An 8-French tube was introduced into the obstructed renal pelvis through a PCN puncture. All data analyses were performed ac- cording to the relevant regulations and guidelines de- scribed in the Declaration of Helsinki; the study was approved by Chungnam National University Hospital institutional ethics committee (Approval No. CNUH 201807047004). Stone characteristics The characteristics of the stones were interpreted using NCCT, and maximum stone length was measured on axial and coronal view. The stone volume was calculat- ed using the ellipsoid formula (π/6 × length × width × height). The MSD was obtained by measuring the mean Hounsfield units (HU) of the defined regions of a circle with a diameter smaller than that of the stone without including the adjacent tissue. The SHI was obtained as the standard deviation of HU. The SSD was obtained by measuring distance from the center of the stone to the skin at 90o in the horizontal axis. Successful SWL outcome was categorized as stone-free and one-session success. Stone-free was defined as an asymptomatic state with residual stone debris of less than 3 mm in the largest diameter or absence of observed stones for four weeks after the first SWL. One-session success was de- fined as stone-free state after only once SWL. SWL protocol The same electromagnetic lithotripter was used for treating all patients under fluoroscopic guidance on an outpatient basis. The lithotripter was an electromagnet- ic lithotripter made by the DirexGroup (Integra SL, In- itia Ltd., Israel). The number of shock waves delivered per session was 2500 to 2800 at a low frequency (sixty times per minute). The voltage of the shock wave start- ed from 10.0 kV and was increased stepwise to a max- imum of 18.0 kV in order to reduce the risk of damage to adjacent organs. Additional SWL was carried out at one-week intervals if any evidence of residual stones remained. All patients underwent SWL under opened PCN state. No antibiotics and diuretics were adminis- tered during SWL. All patients were instructed to drink sufficient water and continue daily activities with prop- er exercise. Statistical analyses with propensity-score matching The method of propensity-score matching was per- formed to further clarify patients’ characteristics after total group analysis. Combined continuous and cate- gorical factors were evaluated to produce a propensity score for each individual in the surveyed population. Propensity scores were then calculated using a multi- variate logistic regression model with a binomial meth- od based on factors that demonstrated significant dif- ferences between the PCN and no PCN groups in the total groups. Propensity-score analysis with 1:2 match- ing was performed with the nearest neighbor matching method. Statistical comparisons of continuous variables were expressed as mean ± standard deviation and per- formed using the Student’s two-sample t-test. Statistical comparisons of categorical variables were performed using the Pearson’s chi-square test. Univariate logistic regression was used to identify factors having an effect on stone-free and one-session success. Significant fac- tors in the univariate logistic regression were further analyzed by multivariate logistic regression. P-values ≤ 0.05 were considered statistically significant. All sta- tistical analyses used the IBM SPSS Statistics version 23.0 (IBM Corp., Armonk, NY, USA). Impact of percutaneous nephrostomy on SWL-Yang et al. Table 1. Demographic data and SWL success rate comparisons between PCN and no PCN groups Variable PCN group (n=49) No PCN group (n=187) p-value Age, mean ± SD 65.31 ± 12.46 58.08 ± 14.26 .001 Sex, numbers of female, % 24, 49.0 81, 43.3 .478 Diabetes mellitus, % 21, 42.9 40, 21.4 .002 Hypertension, % 33, 67.3 63, 33.7 <.001 Stone laterality, numbers on right side, % 31, 63.3 86, 46.0 .031 Stone length (mm, X-axis), mean ± SD 7.01 ± 1.56 6.41 ± 1.49 .015 Stone length (mm, Y-axis), mean ± SD 7.55 ± 1.21 7.22 ± 1.75 .123 Stone length (mm, Z-axis), mean ± SD 10.29 ± 2.95 9.08 ± 2.60 .005 Stone volume (mm3), mean ± SD 308.15 ± 167.62 245.62 ± 173.10 .024 Skin to stone distance (mm), mean ± SD 109.38 ± 22.33 107.90 ± 15.67 .593 Mean stone density, mean ± SD 707.72 ± 276.50 790.53 ± 262.81 .053 Stone heterogeneity index, mean ± SD 140.81 ± 88.13 188.47 ± 79.29 <.001 Psoas muscle cross-sectional area (mm2), mean ± SD 965.98 ± 336.79 1096.98 ± 362.06 .023 Colic pain, % 41, 83.7 148, 79.1 .480 Stone-free, % 27, 55.1 145, 77.5 .002 One-session success, % 12, 24.5 97, 51.9 .001 Abbreviations: SD = standard deviation; PCN = percutaneous nephrostomy. Values are presented as mean ± standard deviation or number (%). Endourology and Stones diseases 263 RESULTS Characteristics of patients and stones are presented in Tables 1 and 2. Table 1 shows the baseline character- istics of all 236 patients who received SWL for upper ureteral stone. Out of these patients, 20.8% (n = 49) had PCN insertion during SWL for upper ureteral stone. Comparisons of the PCN and no PCN groups based on characteristics of patients and stone revealed that age, DM, HTN, stone laterality, size, MSD, SHI, and psoas muscle cross-sectional area were significantly different between the groups. There were no significant differ- ences between the groups in sex, SSD, MSD, and colic pain. Stone-free was significantly less in the PCN group (55.1% vs. 77.5%), as was one-session success (24.5% vs. 51.9%) (Table 1). After 1:2 propensity-score matching with the nearest neighbor algorithm, stone-free was significantly less in the PCN group (55.1% vs. 74.5%), as was one-session success (24.5% vs. 50.0%) (Table 2). Univariate logistic regression model proved that the fol- lowing were significantly related factors of stone-free after SWL for upper ureteral stone: a younger age, the female sex, a smaller stone volume, lower MSD, and no inserted PCN. Multivariate logistic analysis proved that a smaller stone volume, lower MSD, and no inserted PCN were significantly independent predictive factors of stone-free after SWL for upper ureteral stones (Ta- ble 3). Univariate logistic regression model proved that the fol- lowing were significantly related factors of one-session success after SWL for upper ureteral stone: the female sex, a smaller stone volume, shorter SSD, lower MSD, and no inserted PCN. Multivariate logistic analysis proved that a smaller stone volume, lower MSD, and no inserted PCN were significantly independent predictive factors of one-session success after SWL for upper ure- teral stones (Table 4). DISCUSSION Since its introduction in the early 1980s, SWL has been recognized as the preferred treatment for ureteral stones because it is non-invasive, has few contraindications, and demonstrates good clinical results(13). Over the last 40 years, many scholars have attempted to find out the factors that determined high success and low compli- cation rates in SWL treatment of ureteral stones. In the case of SWL, factors that have been reported to affect success and complication rates contain stone char- acteristics (size, location, composition, and density), patients’ characteristics (age, sex, and obesity), SWL frequency range, lithotripter type, and any hemorrhag- ic tendency in the patients(14-16). Additionally, pre-SWL PCN has been proposed as an important method of re- solving upper urinary tract obstruction. It affords satis- factory drainage, is technically simple, and is associated with fewer complications. Persistent obstruction may result in subsequent decline of renal function. Elevated pressure above the ureter stone also increases the tension in the wall of the ureter at the stone location and, therefore, the friction between the mucosa and stone of the ureter. Moreover, high fric- tion at the stone location injures the mucosal layer of the ureter, leading to bleeding and inflammation around the stone. This leads to swelling that narrows the lumen of the ureter and hinders spontaneous discharge of the stone. On the other hand, severe obstruction with hy- dronephrosis can be linked to the impaction of a ureteral stone on the ureteral mucosa(17). Chronically impacted ureteral stones can cause edema of the ureteral wall and are often associated with ureteral polyps or strictures(18). These changes can also adversely affect the discharge of the ureteral stone. We consider whether perform- ing PCN to resolve the obstruction could improve the movement of the ureteral stone and increase the success rate of SWL. However, the findings did not support our expectations. Our results indicated that PCN insertions could adversely affect stone-free and one-session suc- cess during SWL. In our study, PCN was the only sig- nificantly different factor between both groups after 1:2 propensity-score matching. Propensity-score matching was used to reduce the impact of treatment-selection bias for estimating causal treatment effects using obser- vational data(19). The main rationale for performing decompression stent- ings (PCN or double-J ureteral stent) was to prevent complications related to upper urinary tract obstruction as stone debris passes through the ureter during SWL. Complete elimination of stones is the supreme goal; Table 2. Demographic data and SWL success rate comparisons between PCN and no PCN groups for propensity-score matching Variable PCN group (n=49) No PCN group(n=98) p-value Age, mean ± SD 65.31 ± 12.46 62.70 ± 13.34 .257 Sex, numbers of female, % 24, 49.0 50, 51.0 .816 Diabetes mellitus, % 21, 42.9 31, 31.6 .180 Hypertension, % 33, 67.3 57, 58.2 .281 Stone laterality, numbers on right side, % 31, 63.3 57, 58.2 .552 Stone length (mm, X-axis), mean ± SD 7.01 ± 1.56 6.54 ± 1.63 .104 Stone length (mm, Y-axis), mean ± SD 7.55 ± 1.21 7.32 ± 1.76 .355 Stone length (mm, Z-axis), mean ± SD 10.29 ± 2.95 9.50 ± 2.69 .105 Stone volume (mm3), mean ± SD 308.15 ± 167.62 264.30 ± 173.43 .146 Skin to stone distance (mm), mean ± SD 109.38 ± 22.33 106.34 ± 17.32 .366 Mean stone density, mean ± SD 707.72 ± 276.50 755.05 ± 266.15 .317 Stone heterogeneity index, mean ± SD 140.81 ± 88.13 157.97 ± 72.69 .212 Psoas muscle cross-sectional area (mm2), mean ± SD 965.98 ± 336.79 997.98 ± 368.55 .611 Colic pain, % 41, 83.7 79, 80.6 .651 Stone-free, % 27, 55.1 73, 74.5 .018 One-session success, % 12, 24.5 49, 50.0 .003 Abbreviations: SD = standard deviation; PCN = percutaneous nephrostomy. Values are presented as mean ± standard deviation or number (%). Impact of percutaneous nephrostomy on SWL-Yang et al. Vol 19 No 4 July-August 2022 264 however, reducing complications during SWL is anoth- er important goal in the management of urolithiasis. In most studies, decompression stents have been shown to be effective in preventing complications. Regrettably, few reports show that decompression stents increase stone-free rates after SWL. Previous studies demon- strated that double-J ureteral stents do not improve the success rate of SWL(20-22). Middela et al., reported that the presence of PCN was not a significant factor in the success rate of SWL(23). Joshi et al., demonstrated that the outcome of SWL in no decompression stent group was better than that in PCN group and double-J ureteral stent group(24). Although these measures may provide more information to urologists before treatment, the precise meaning of these data in the management of stone remains controversial. Our assumption was that when the friction between stone and mucosa of ureter was higher than the driving force, the stone will not be dislodged. If PCN was pres- ent, it could decompress the dilated renal pelvis and the ureter above the stones and can drain the urine through it. Consequently, it reduces the pressure above the ure- teral stone and the friction around the stone, but it also reduces the driving force associated with urine flow, which negatively impacts the success rate of SWL. Urinary tract obstruction resulting from ureteral stones is a common cause of urinary tract infection (UTI)(25). In patients with acute UTI, the infection should be treat- ed first with appropriate antibiotics before commenc- ing treatment for removal of the stone. In some cases, PCN insertion is inevitable. Until stones are removed, urinary diversions can be performed empirically to pre- vent deterioration of renal function or aggravation of the UTI. PCN is a well confirmed procedure that allow for temporary or permanent urinary diversion from the renal pelvis in urinary tract obstruction. PCN is main- ly performed for patients with severe renal colic, acute kidney injury, and urosepsis caused by urinary tract ob- struction(26). We had several cases of SWL in patients who under- went PCN and observed that PCN could interfere with stone debris migration. As mentioned above, PCN can decrease intrarenal pressures if intrarenal urine and flu- id are continuously draining through a PCN, and this may make it difficult for stones to migrate toward the distal ureter and bladder. The results of the current study suggest that physicians may consider stone vol- ume and MSD as factors that influence SWL outcomes when deciding whether to perform PCN prior to SWL. In patients with large upper ureteral stones and a high MSD, the decision to perform PCN should be based on the degree of renal function decline and complications Table 3. Univariate and multivariate logistic regression models for predictive factors of stone-free following shock wave lithotripsy Parameter Univariate Multivariate Odds ratio 95% CI p-value Odds ratio 95% CI p-value Age 0.971 0.943-0.999 .046 0.965 0.926-1.006 .093 Sex (female) 2.051 1.010-4.164 .047 1.960 0.728-5.227 .183 Diabetes mellitus 1.253 0.600-2.614 .548 Hypertension 0.971 0.476-1.979 .935 Stone laterality (right side) 1.704 0.844-3.440 .137 Stone length (mm, X-axis) 0.504 0.379-0.671 <.001 Stone length (mm, Y-axis) 0.546 0.418-0.713 <.001 Stone length (mm, Z-axis) 0.590 0.489-0.713 <.001 Stone volume (mm3) 0.991 0.989-0.994 <.001 0.992 0.988-0.995 <.001 Skin to stone distance (mm) 0.989 0.970-1.008 .244 Mean stone density 0.996 0.994-0.997 <.001 0.996 0.994-0.998 <.001 Stone heterogeneity index 1.003 0.998-1.007 .254 Psoas muscle cross-sectional area (mm2) 1.000 0.999-1.001 .887 Colic pain 1.943 0.826-4.569 .128 Percutaneous nephrostomy 0.420 0.204-0.866 .019 0.292 0.104-0.815 .019 Abbreviations: CI = confidence interval. Parameter Univariate Multivariate Odds ratio 95% CI p-value Odds ratio 95% CI p-value Age 1.002 0.977-1.027 .886 Sex (female) 2.042 1.047-3.982 .036 1.212 0.421-3.487 .721 Diabetes mellitus 1.344 0.678-2.662 .397 Hypertension 1.371 0.695-2.707 .363 Stone laterality (right side) 0.838 0.430-1.635 .605 Stone length (mm, X-axis) 0.402 0.287-0.562 <.001 Stone length (mm, Y-axis) 0.471 0.351-0.633 <.001 Stone length (mm, Z-axis) 0.557 0.452-0.686 <.001 Stone volume (mm3) 0.988 0.984-0.992 <.001 0.990 0.985-0.994 <.001 Skin to stone distance (mm) 0.978 0.959-0.997 .025 0.979 0.952-1.008 .150 Mean stone density 0.995 0.994-0.997 <.001 0.995 0.992-0.997 <.001 Stone heterogeneity index 0.998 0.994-1.003 .443 Psoas muscle cross-sectional area (mm2) 0.999 0.998-1.000 .154 Colic pain 1.529 0.636-3.679 .128 Percutaneous nephrostomy 0.324 0.151-0.695 .004 0.168 0.054-0.523 .002 Abbreviations: CI = confidence interval. Table 4. Univariate and multivariate logistic regression models for predictive factors of one-session success following shock wave lithotripsy Impact of percutaneous nephrostomy on SWL-Yang et al. Endourology and Stones diseases 265 caused by upper urinary tract obstruction. Identifying factors that predict SWL outcomes would help simpli- fy the care of patients with stones. Patients identified to be at high risk of treatment failure may be offered alternative procedures, such as flexible ureteroscopy to manage their urolithiasis. Physicians can determine which types of patients are most likely to benefit from SWL. In an era of limited medical insurance reimburse- ment, cutting down on medical costs is essential. Thus, we do not recommend routine pre-SWL PCN insertion; PCN insertion should be offered only when there are special indications, such as complicated upper urinary tract obstruction. The limitation of this study is that it is a retrospective study conducted at a single institution; therefore, our re- sults were derived from a relatively small sample pop- ulation. A small control group might arouse selection bias in propensity-score matching studies. Also, poten- tial limitation to propensity-score matching studies was that unrecognized risk factors might affect outcomes. Despite this limitation, our study confirmed the effect of PCN on the outcomes of SWL and analysis of stone- free following SWL according to meaningful stone characteristics (stone volume and MSD) in relation to the presence of a PCN. In the future, a well-designed prospective study with a large sample population will be required to prove our observations on the adverse effect of PCN during SWL. CONCLUSIONS The presence of PCN during SWL was an adverse pre- dictive factor of stone-free and one-session success in patients with upper ureteral stones. We believe the PCN insertion status can be used to predict the treatment out- comes of SWL, and this may be helpful in selecting the optimal treatment option for patients with upper ure- teral stones. PCN should still be used in patients with obstruction, those at risk of sepsis, and in those with unbearable pain or decreased renal function. However, if the underlying problem resolves and the general con- dition improves, it is suggested that performing SWL after the removal or clamping of the PCN can lead to positive therapeutic effects. ACKNOWLEDGEMENTS This research was supported by the Chungnam National University Hospital Research Fund. This work was supported by research fund of Chungnam National University. CONFLICT OF INTEREST There are no conflicts of interest to declare from all au- thors. REFERENCES 1. Wong Y, Cook P, Roderick P, Somani BK. Metabolic Syndrome and Kidney Stone Disease: A Systematic Review of Literature. J Endourol. 2016;30:246-53. 2. 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