Ureterorenoscopic Lithotripsy for Pediatric Kidney Stones Using Holmium:Yttrium–Aluminum

Abstract

Introduction:

We aimed to compare the effectiveness of 15- and 30-W holmium:yttrium–aluminum–garnet (Ho:YAG) laser devices used in the treatment of pediatric kidney stones.

Methods:

Eighty-six consecutive pediatric patients who underwent retrograde intrarenal surgery (RIRS) between February 2010 and August 2020 were enrolled in the study. After exclusion criteria were applied, the data of 79 children were evaluated retrospectively. Patients were divided into two groups according to the laser device power of 15 W (Group 15: N = 30) and 30 W (Group 30: N = 49). The groups were compared according to demographic characteristics, stone feature, and clinical efficacy.

Results:

The age, gender, height, weight, and stone characteristics were similar between the groups. The mean operation time was shorter in Group 30. The stone-free rate after the first RIRS session (SF₁) was 66.7% in Group 15 and 83.3% in Group 30. The SF₁ for 20-mm or larger kidney stones was found to be 0% in Group 15 and 62.5% in Group 30. However, there was no statistically significant difference between the two groups in terms of stone-free rate.

Conclusions:

In pediatric kidney stone treatment, 30-W Ho:YAG laser devices should be preferred as they shorten the operation time compared with 15-W devices and provide the final stone-free status with fewer procedures, especially in large kidney stones.

Introduction

Minimally invasive treatment in pediatric stone disease has been increasingly applied in the last three decades.^1,2 Retrograde intrarenal surgery (RIRS) is not the primary treatment option for any renal stones according to European Association of Urology (EAU)/European Society of Pediatric Urology (ESPU) guidelines; however, clinicians have preferred RIRS instead of percutaneous nephrolithotomy (PCNL) for renal stones in recent years because of its minimally invasive nature.^3,4

The optimum laser for lithotripsy during RIRS is currently the holmium:yttrium–aluminum–garnet (Ho:YAG) laser.⁵ With advances in laser technology, the use of high-power laser lithotripters has started. High-power laser provides low pulse energy with longer pulse duration and high frequency according to low-power laser devices that facilitate stone dusting.⁶

We hypothesized that the 30-W Ho:YAG laser lithotripter device would increase the success rate and shorten the operative time in pediatric RIRS compared with 15-W Ho:YAG laser lithotripters. To our knowledge, no studies have compared different laser devices for pediatric RIRS in the literature. The objective of this study was to comparatively evaluate the results of 15- and 30-W Ho:YAG lithotripter devices used to treat pediatric renal stones.

Materials and Methods

This retrospective study was approved by the ethics committee of Bursa Uludag University (No.: 2021-KAEK-26/419).

We included 86 consecutive pediatric patients who underwent flexible ureterorenoscopic laser lithotripsy for kidney stones between February 2010 and August 2020. Exclusion criteria were patients with achondroplasia, severe skeletal dysplasia, kidney failure, atrophic kidney, and nephrocalcinosis. After exclusion criteria were applied, the data of 79 children were evaluated retrospectively. The first consecutive 30 children underwent laser lithotripsy with the 15-W (Group 15/number [N]:30) Ho:YAG surgical laser system (StoneLight™ Laser Therapy System) and the following 49 children underwent RIRS with the 30-W (Group 30: N:49) Ho:YAG surgical laser system (StoneLight Laser Therapy System).

During the preoperative preparation period, urine culture, urinary system ultrasonography (US), and noncontrast abdominopelvic CT were performed in all patients. The stone/kidney index was calculated as the length of the stone (longest axis)/the length of the kidney (longest axis).⁷ The residual stone was evaluated with both US and kidney, ureter, and bladder radiograph for all children. Stone-free status was defined as residual fragment <2 mm. The final stone-free status in children was evaluated 3 months later or after the last additional treatment. Stone freeness achieved after the first session was defined as SF₁, and stone freeness after the last treatment was defined as SF_last. The perioperative complications were classified according to the modified Satava classification.⁸ Postoperative complications were evaluated according to the modified Clavien-Dindo classification (CDC) system.⁹ All operations were performed by the same surgeon (O.K).

RIRS procedure

Surgical procedures were performed in the lithotomy position, under general anesthesia. While ceftriaxone was preferred as a routine prophylactic antibiotic, children with positive pre-RIRS cultures were treated with appropriate antibiotics. First, a sensor guide (Boston Scientific nitinol guidewire with hydrophilic tip) and safety catheter were inserted into the renal pelvis with a 6.0F semirigid ureterorenoscope (R. Wolf™ Germany). A Cook Flexor^® 9.5F/11.5F ureteral access sheath (UAS) was used for six children. A 7.5F Flex-X2 (Flex-X2; Karl Storz, Tuttlingen, Germany) flexible ureterorenoscope was placed in the kidney through the sensor guide. A 273-nm Ho-YAG laser probe was used. Laser device settings were 0.8–1.5 J pulse energy and 8–15 Hz pulse rate on the 30-W laser system and 0.6–0.8 J pulse energy and 8–12 Hz pulse rate on the 15-W laser system. A suprapubic Angiocath catheter was placed in boys and a 6F Nelaton catheter was placed in girls through the urethra for bladder drainage. Retrograde pyelography (RGP) and fluoroscopy mapping were performed when necessary. At the end of the procedure, a Double-J stent was placed.

Statistical analysis was performed using the SPSS, version 23, software (SPSS, Inc., Chicago, IL). The Shapiro–Wilk test was used to test the normality of variables. The normally distributed variables were compared using Student's t-test and presented as mean ± standard deviation. Variables that were not normally distributed were compared using the Mann–Whitney U test and presented as median (minimum–maximum) and mean ± standard deviation. Nominal data have been presented as numbers and percentages and compared with chi-square and Fisher's exact tests; p < 0.05 was considered significant.

Results

The mean age of children was 7.9 ± 4.81 years. The mean stone size was 12.9 ± 5.70 mm. There was no significant difference between the groups in terms of age, gender, height, weight, stone characteristics (location, side of the kidney stone, and density of the stone), stone–kidney size ratio, and degree of hydronephrosis (Table 1).

Table 1.

Comparison of Patient Demographics and Kidney Stone Features Between Groups

	Group 15 (n = 30)	Group 30 (n = 48)	p
Age, years	7.1 ± 4.28	8.4 ± 5.09	0.243
Gender (female ratio)	14 (46.7%)	23 (47.9%)	1.00
BMI	15.5 (12.5–39.2)	17.2 (11.6–34.9)	0.170
BMI	17.8 ± 5.78	19.1 ± 5.78	0.170
Stone size	13.1 ± 4.70	12.8 ± 6.29	0.834
Stone–kidney ratio	1.68 ± 0.69	1.51 ± 0.83	0.344
Stone density	1048.5 ± 347.31	1078.0 ± 430.08	0.824
Hydronephrosis grade
0–1	16 (53.3%)	25 (52.1%)	1.00
2–4	14 (46.7%)	23 (47.9%)	1.00
Stone side (left)	15 (50.0%)	29 (60.4%)	0.482
Stone size
<1 cm	8 (26.7%)	13 (27.1%)	0.846
1–2 cm	18 (60.0%)	27 (56.3%)
> 2 cm	4 (13.3%)	8 (16.7%)
Multiple stones	15 (50.0%)	21 (44.7%)	0.815
Stone location
Solitary except for lower calix	13 (43.3%)	29 (60.4%)	0.295
Solitary lower calix	7 (23.3%)	8 (16.7%)
Multiple locations	10 (33.3%)	11 (22.9%)
Prior stone treatment	12 (40.0%)	19 (39.6%)	1.000
Preoperative Double-J stent placement	11 (36.7%)	26 (54.2%)	0.164

Normally distributed variables were presented as mean ± standard deviation (SD): Student's t-test, Non-normally distributed variables were presented as median (minimum-maximum): Mann–Whitney U test, Nominal variables, n% percentage in the group: Chi-squared test.

BMI = body mass index.

Preoperative Double-J stent placement (prestenting) was performed in two children for pain during the COVID-19 pandemic and in two children for pyuria after stone movement with the guidewire. In addition, prestenting was performed in 33 children for passive dilatation. There was no significant difference in terms of prior stone treatment and preoperative Double-J stent placement (p = 1.00, p = 0.165, respectively). The UAS was used in five children in Group 15 and one child in Group 30 (p = 0.027).

The mean operation time was significantly shorter in Group 30 (48.8 ± 25.74 minutes) compared with Group 15 (75.8 ± 33.50 minutes) (p = 0.001) (Table 2).

Table 2.

Comparison of Treatment Outcomes of Groups

	Group 15 (n = 30)	Group 30 (n = 48)	p
Usage of ureteral access sheaths	5 (16.7%)	1 (2.1%)	0.029
Mean operation time (minutes)	75.8 ± 33.50	48.8 ± 25.74	0.001
SF₁ (after the first session)	20 (66.7%)	40 (83.3%)	0.104
SF₁ according to location
Solitary location	10/13 (76.9%)	27/29 (93.1%)	0.162
Lower calix	7/7 (100%)	7/8 (87.5%)	1.00
Multiple locations	3/10 (30.0%)	6/11 (54.5%)	0.387
SF₁ according to stone size
Stones <10 mm	7/8 (87.5%)	13/13 (100%)	0.381
10 mm ≥ stones <20 mm	13/18 (72.2%)	22/27 (81.5%)	0.489
Stones ≥20 mm	0/4 (0%)	5/8 (62.5%)	0.081
Retreatment	8 (26.7%)	9 (18.8%)	0.416
Additional treatment
SWL	2 (6.7%)	2 (4.2%)	0.636
Median procedure number	1 (1–3)	1 (1–3)	0.258
Median procedure number	1.40 ± 0.68	1.25 ± 0.57	0.258
SF_last	29 (96.7%)	48 (100%)	0.385
LoHS (day)	1 (1–4)	1 (1–7)	0.065
LoHS (day)	1.1 ± 0.57	1.5 ± 1.22	0.065
Satava
1	0	1 (2.1%) (fornix rupture)	0.340
3	1 (3.3%) (abdominal distention)	0	0.340
CDC grade 1 (fever)	3 (10%)	3 (6.3%)	0.670

CDC = Clavien-Dindo classification; LoHS = length of hospital stay; SF = stone-free; SF₁ = stone-free rate after the first RIRS session; SF_last = stone freeness after the last treatment; SWL = extracorporeal shockwave lithotripsy.

The stone-free rate after the first RIRS session (SF₁) was 66.7% in Group 15 and 83.3% in Group 30 (p = 0.104) (Table 2). In addition, in the subgroup analysis, SF₁ was similar between groups according to stone location and size (<10, 10–20, and ≥20 mm) (Table 2). However, SF₁ for 20-mm or larger kidney stones was found to be 0% in Group 15 and 62.5% in Group 30 (p = 0.081) (Table 2).

The retreatment rate was 26.7% in Group 15 and 18.8% in Group 30 (p = 0.416). Two children received SWL after the RIRS procedure in both groups as an additional treatment (p = 0.636) (Table 2). The final stone-free rate after retreatment and additional treatment was 96.7% in Group 15 with a mean of 1.40 procedures/child and 100% in Group 30 with a mean of 1.25 procedures/child (p = 0.385) (Table 2). Stone-free status was achieved in all of the children except one in Group 15 after additional treatments.

In Group 15, abdominal distension was observed in a 2-year-old girl, when the operative time was 90 minutes. Fluid was removed with a minilaparotomy incision perioperatively (Satava grade 3). The peritoneal catheter was placed at the end of the procedure. The intraperitoneal catheter and urethral catheter were removed on the first and third postoperative days, respectively. In Group 30, at the end of the RIRS, fornix rupture was observed in a 7-year-old boy when the operative time was 20 minutes. The urethral catheter was removed on the second postoperative day. The perioperative complication rate was similar between the groups (p = 0.0.340). Three children in both groups had fever (CDC stage 1) on the day of surgery and were managed with a conservative approach without any additional treatment (0.670).

The median length of hospital stay was similar in both groups (p = 0.065). The median follow-up time is 27 months (9–132 months). No ureteral stricture was observed in both groups during the follow-up period.

Discussion

This study compared the efficacy of two different power settings of (15 and 30 W) Ho:YAG surgical laser devices in pediatric RIRS for kidney stones.

Total laser application time and laser energy level were reported to be correlated with the stone size and density.¹⁰ In the current study, stone size, density, and location were similar between groups, making it safe to compare the different laser devices for pediatric RIRS.

We preferred the dusting mode as it does not need a basket catheter for stone removal and we did not use the UAS. A laser setting with high frequency and low pulse energy is used for stone dusting, and high-power laser systems provide these parameters.^6,11 High-power laser systems allow rapid stone dusting compared with low-power laser systems. In the present study, the operation time was longer in Group 15 than in Group 30. This result may be explained by the mentioned mechanisms. This result was consistent with the pediatric ureteral laser lithotripsy study.¹²

Children had a 66.7% SF rate in Group 15, while this ratio was 81.6% in Group 30 after the first session (p = 0.104). Although there is a clinically important rational difference, the lack of statistical difference may be related to the number of children in the present study. In addition, the mean number of procedures was 1.4 in Group 15 and 1.25 in Group 30. The SF status was achieved with fewer mean procedures/child numbers in Group 30 than in Group 15 for medium stones (10–20 mm). In a recent adult RIRS study, the 30-W laser device had a higher SF rate compared with the 20-W device for 1–2-cm kidney stones.¹³ Rapid and easy stone dusting with high-power laser systems may explain the higher success rate in Group 30. However, in our study, SF₁ was found to be insignificantly higher in Group 30 compared with Group 15 for medium stones. The difference between our study and the adult study might be due to variations in the sample.

According to EAU/ESPU guidelines, PCNL is the first-choice treatment for renal stones larger than 2 cm.³ To the best of our knowledge, there is no study regarding this issue for the pediatric age group. Two adult studies investigated the efficiency of RIRS for renal stones 2–3 cm in diameter, and the initial SF rates were reported as 47% and 71.4% at a 2-mm threshold for SF status.^14,15 In this study, all children with large stones (20–30 mm) had residual stones in Group 15, whereas five of eight children having large stones had residual stones in Group 30 after the first RIRS session. In terms of 30-W laser devices, the SF rate after the first RIRS session (62.5%) is similar to the adult studies. These results indicated that low-power laser systems are not effective for the treatment of large stones in children; however, the suitability of higher power laser devices for large stones (20–30 mm in dimension) should be investigated in further studies.

The success rate was high for solitary stones of the lower calix for both groups in this study; however, RIRS is not recommended as the primary treatment for lower calix stones in the EAU/ESPU guidelines.¹⁶ Halinski et al. reported that RIRS and micro-PCNL had a similar success rate for pediatric lower calix stones, nearly 85% in both groups.¹⁷ This study supports our results. The deflection was reported to be between 4.4% and 10.21% for the 273-μm laser fiber; however, 1.9F diameter basket catheters do not affect the maximum deflection angle.^18,19 We used a basket catheter to reposition the lower calix stone when deflection was decreased with a laser fiber. This could explain the high success rate.

Ureteral stent placement before RIRS (prestenting) was not done routinely in this study as it is not recommended in the EAU/ESPU guidelines.³ However, in case of the slightest difficulty in insertion of semirigid URS or flexible ureterorenoscopy (FURS), the Double-J stent was placed before the RIRS procedure. The prestenting rate in this study was 47.4%, which is consistent with other studies.^20,21 Prestenting was not routinely preferred to avoid the negative effects of anesthesia in children.²²

In this study, the UAS was used only in six children. We used a 9.5F/11.5 UAS (Flexor Ureteral Access Sheath; Cook Medical) to prevent ureteral complications as it was shown to be safe in a recent study,²⁰ but 9.5/11.5F UASs are insufficient for urinary fluid drainage. It was shown that FURS with a 7.5F outer diameter combined with a UAS with an inner diameter of 9.5F could not decrease the intrapelvic pressure.²³ In addition, in a swine animal model study, it was reported that ureteral blood flow dropped below 50% of the baseline when using the 12/14F UAS and there was no clear information about the risk of long-term ureteral stenosis in children.²⁴ We do not prefer to use the UAS in pediatric RIRS for these reasons. In our study, ureteral stricture was not detected in any child at a median 27-month follow-up.

Abdominal distension occurred due to fluid leakage in a 2-year-old girl at the end of the procedure in Group 15. Young age, no UAS use, no prestenting, and prolonged operative time could be reasons for this complication. Using a 15-W Ho:YAG surgical laser system prolonged the operative time, so using a high-energy laser system and prestenting should be considered, especially for young children. Fornix rupture was detected in one patient in Group 30; this child had a small renal stone and a short operation time. Fornix rupture occurred during RGP; therefore, this complication might not be related to the use of UASs or the power of laser devices. Afterward, RGP was performed with the natural flow and no fornix rupture occurred.

There are some limitations to this study. First, this study had a monocentric retrospective nature. However, this is the first study that compares the efficiency and safety of laser devices with different power settings in pediatric renal stone treatment. The second limitation of this study is that a 15-W Ho:YAG laser device was used in the first 30 children and a 30-W Ho:YAG laser device was used in the last 48 children. The groups were treated by the same surgeon who had gained experience with adults before taking the pediatric cases. It was shown in a recent study that results of the first pediatric RIRS procedures after adult experience were comparable with adults' results.²⁵ Therefore, we believe that the experience of the surgeon with adults before taking pediatric cases minimizes the effect of the learning curve on results and operative times. Another limitation was that the stone fragment analysis was done only on 20 children. However, the mean stone density was similar between the groups and showed the homogeneity of groups concerning stone hardness.

Conclusions

Ureterorenoscopic laser lithotripsy with whole laser devices is an effective treatment for pediatric renal stones; however, the 15-W Ho:YAG laser device is not suitable for large stones.

Our hypothesis was partially supported with a similar success rate and shorter operative time in Group 30 compared with Group 15. Thirty-watt Ho:YAG laser devices should be preferred over 15-W devices due to their shorter operative time.

Ethics Approval

This study was approved by the Ethics Committee of Bursa Uludag University (No.: 2021-KAEK-26/419).

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Funding Information

No funding was received for this article.

Abbreviations Used

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Ureterorenoscopic Lithotripsy for Pediatric Kidney Stones Using Holmium:Yttrium–Aluminum–Garnet Laser Devices: 15 W vs 30 W