An increasing amount of evidence has shown an increasing incidence of pediatric urolithiasis, with a reported incidence of approximately 50 cases in 100,000 children. ^{1 , 2} Although a mere 2%–3% of the pediatric population will develop urinary calculus, pediatric urolithiasis is associated with a considerable morbidity and a high recurrence rate (approximately 67%) ^{3 , 4} ; thus, it is important for this population to receive appropriate treatment to keep them stone-free.

Guidelines by the European Association of Urology have recommended pediatric percutaneous nephrolithotomy (PCNL) as the primary procedure for stones larger than 20 mm and stones larger than 10 mm located at the lower renal pole. ⁵ Despite achieving a high stone-free rate (SFR), ranging from 50%–98.5%, ^{5 – 9} PCNL using a standard sheath of 24-30 Fr for renal access is associated with severe complications, including hemorrhage, blood transfusion, sepsis, damage to renal parenchyma, and a postoperative need for analgesia due to pain. ^{10 , 11}

PCNL in pediatric patients is always a challenge for urologists because of the smaller anatomical structures and fragility of patients compared to adult patients. Various strategies to decrease morbidity post-PCNL have been described, focusing on reducing the tract size to minimize the risk of kidney parenchymal injury, which provides fewer morbidities without changing the therapeutic efficacy. ^{12 , 13} Recently, mini-PCNL, which uses a smaller sheath size of 14-22 Fr has been regarded as a safer and more effective treatment option for pediatric nephrolithiasis, with studies reporting a promising SFR, as well as reduced bleeding and decreased hospital stay, compared to standard PCNL. ^{6 , 12 , 14 – 16}

In Indonesia, pediatric PCNL began in 2005 using the protocol and instruments of standard PCNL. Even though mini-PCNL has been around since 1997, when it was first described by Helal et al., ¹⁷ the equipment was only introduced to our institution, the Cipto Mangunkusumo General Hospital, Indonesia, in 2014. Thus, this study aimed to describe the experience and development of pediatric PCNL in a tertiary referral hospital in Indonesia, comparing the effects of the use of different sizes of instruments (standard PCNL vs. mini PCNL) on the outcomes of SFR, presence of complications, and postoperative use of drainage tubes.

A total of 42 patients under the age of 18 years underwent PCNL, 27 of which underwent mini-PCNL, while the other 15 underwent standard PCNL. The general characteristics of both groups are shown in Table 1. Comparing the two common characteristics, there were no significant differences between the two groups in terms of patients’ sex, as most patients were male in both groups, number of PCNL sessions, BMI, stone burden, and stone number. Meanwhile, there was a significant difference between both groups in age (p < 0.05); the mean age in the mini-PCNL group was 4.61 ± 3.52 years, while that in the standard PCNL group was 8.0 ± 3.57 years. Moreover, most patients underwent only one PCNL session in both groups.

Intraoperative and postoperative outcomes are shown in Table 2. The outcomes were recorded for all surgeries performed on patients who underwent either mini-PCNL or standard PCNL. From the table, it can be observed that most patients were placed in the prone position in both groups. Access to the calyxes was mostly from the lower calyx in both groups, even though there was no significant difference between groups. The length of operation was significantly longer in the mini-PCNL group than in the standard PCNL group (97.55 vs 71.82 min, p = 0.008). Stone clearance was higher in the mini-PCNL group (SFR, 87.9%) than in the standard PCNL group (SFR, 59.1%); these differences were statistically significant. Generally, patients in the mini-PCNL group were mostly discharged totally tubeless, while they were mostly tubeless in the standard PCNL group, which led to a statistically significant difference (p = 0.012) between both groups in terms of the use of postoperative drainage tubes. In terms of complications, there was no significant difference between groups, and most surgeries did not induce any complications.

Pediatric urolithiasis has a high recurrence rate, and its prevalence is growing globally in all age groups. The use of PCNL in the treatment of pediatric urolithiasis was initially shown by Woodside et al. ¹⁸ in 1985, with an SFR of 100% in a single session using standard adult instruments. However, the use of a larger tract size in standard PCNL is associated with disadvantages, such as increased bleeding that necessitates transfusions, renal damage, or organ damage, including potential pneumothorax. ^{16 , 19} Therefore, the use of mini-PCNL has grown in prominence due to recent efforts to reduce the morbidities associated with using adult instruments, while maintaining high efficacy. The hypothesis for utilizing smaller devices through smaller diameter sheaths is that the narrower the PCNL tract, the less injury is caused to the renal parenchyma; consequently, related morbidity is reduced without affecting therapeutic efficacy. ^{8 , 20 , 21}

In this study, we observed that older patients mostly underwent standard PNCL compared to younger patients who mostly underwent mini-PCNL, and the differences between the two means were statistically significant. A growing body of studies has put forward the perspective that larger sheaths cause more dilatation and, inadvertently, parenchymal and vascular injury in smaller kidneys. ^{20 – 22} Thus, following this theory, it is a logical step to treat younger patients with a smaller sheath size to avoid these damages and the potentially long-term effects of using larger sheaths, which could explain this observation. However, the studies were mostly observational; hence, more scientific support from molecular studies and randomized controlled trials would allow this theory to be accepted universally.

Another significant observation in this study was the intraoperative position of patients, that is, most patients were placed set in a prone position. Prone position is preferred given the familiarity over the supine position and the larger field of operation. ¹⁶ Furthermore, the success rate was high with acceptable complication rates. ²³ However, given the disadvantages of prone position such as possible cervical trauma ²⁴ and ventilatory difficulty due to restricted thorax and abdomen mobility, ²⁵ there has been great interest towards the application of supine position. Initial reports have shown non-inferiority in the supine position. A study by Zhan et al. ²³ has described equal safety and effectiveness in both supine and prone positions. Another study by Gamal et al. ²⁴ evaluated the efficacy and safety of the supine position using mini-PCNL (sheath size, 19 Fr) and reported a high SFR of 92.5%, with two of 27 patients experiencing intraoperative complications. Furthermore, a study by Nerli et al. ²⁵ reported similar results regarding the safety and efficacy of the supine position in pediatric PCNL. As these reports were observational, further subgroup analysis or future comparative studies should be conducted to ascertain their effects on the outcome of surgeries. Thus, this observation highlighted the importance of the intraoperative positions as a factor affecting procedural outcomes.

Thus, despite the current study and the aforementioned studies being observational, this study highlighted the importance of the intraoperative position as a factor affecting procedural outcomes one of the factors affecting the outcomes of the procedures. Further subgroup analysis or future comparative studies could be conducted to ascertain their effects on the outcome of surgeries.

We observed that operational duration was significantly longer in the mini-PCNL group than that in the standard PCNL group (97.55 vs. 71.82, p < 0.05). This result was in accordance with the results of other studies by Unsal et al., ²⁶ Bilen et al., ⁷ Celik et al., ⁶ Ozden et al., ¹⁶ and Mahmood et al. ²⁷ The prolonged duration could be attributed to the limited visual field of smaller endoscopes. The need to extensively shatter the stones into smaller pieces to remove the pieces through the smaller sheath might also prolong operational length.

In this study, the stone clearance rate was significantly higher in the mini-PCNL group than in the standard PCNL group (87.9% vs. 59.1%, p < 0.05) in a single session. Similar SFR results were reported in several other studies that compared stone clearance using equipment of different sizes (i.e., mini-PCNL vs. standard PCNL); the SFR of mini-PCNL ranged from 76%-91.4% to 50%-98% for standard PCNL. ^{6 , 16 , 20 , 26 , 27} Nevertheless, in these studies, they obtained a higher SFR in the standard PCNL group, while we reported a higher SFR in the mini-PCNL group. Be that as it may, a study by Bilen et al. ⁷ reported similar findings; they reported that the SFR in the 20-Fr sheath group and 26-Fr sheath group were 80% and 69.5%, respectively; the differences, however, were not statistically significant. To summarize, an increasing amount of evidence agrees that mini-PCNL is not inferior to standard PCNL in SFR.

A plausible reason for the higher SFR in the mini-PCNL group could be the smaller stone burden in the mini-PCNL group than in the standard PCNL group, although the distribution was not significantly different. Indeed, an association between stone burden and SFR has been reported. ^{28 , 29} A study by Hussain et al. ³⁰ reported a trend where single treatment success decreased as stone size increased.

Another possible reason for the higher SFR in the mini-PCNL group was the lower BMI, which was observed in this study; the mean BMI in the mini-PCNL group was lower than that in the standard PCNL group (17.0 vs. 17.6, p > 0.05). Although no prior studies have compared the outcomes of mini-PCNL vs. standard PCNL in obese vs. non-obese pediatric settings, the Clinical Research Office of the Endourological Society (CROES) study by Fuller et al., ³¹ where standard PCNL was performed on 4 different groups based on BMI, demonstrated a lower SFR in the morbidly obese group than in the obese group (65.6% vs. 78.9%). Other similar studies also reported similar results, where SFR after standard PCNL seemed to be lower in patients with a higher BMI. ^{32 , 33} However, the effect of BMI on PCNL outcomes remains debatable, as several studies by Ferreira et al., ³⁴ Alyami et al., ³⁵ and Akbulut et al. ³⁶ showed that PCNL outcomes were independent of BMI. Thus, a large population-based study or randomized controlled trial is needed to ascertain the effect of BMI on PCNL outcomes, especially in the more recent mini-PCNL group.

The standard procedure for drainage after pediatric PCNL includes inserting nephrostomy tubes and ureteric stents. Given the use of standard postoperative tube procedures, alternative procedures, such as tubeless and totally tubeless PCNL, began to gain traction. Instead of utilizing both ureteric stents and nephrostomy tubes, tubeless PCNL only uses ureteric stents, while totally tubeless PCNL forgoes both nephrostomy tubes and ureteric stents. These procedures have been associated with a shorter hospital stay and reduced need for analgesics. ^{37 , 38} Ultimately, the use of tubed or tubeless PCNL depends on the current complications, bleeding, and residual fragments. In this study, most patients were discharged totally tubeless after mini-PCNL, and the proportion was significantly different from that of standard PCNL. In contrast, with standard PCNL, most patients underwent a tubeless procedure. These results demonstrate the shifting trend of tubeless procedures after PCNL. Furthermore, the use of a totally tubeless procedure would imply that mini-PCNL induced minimal postoperative complications and thus continued to show the safety of mini-PCNL.

Similar complication rates were observed after mini-PCNL and standard PCNL (15.2% vs. 13.6%), even though the differences were not significant. Major complications, such as pneumothorax and other organ injuries, were not recorded; however, lacerations of kidney parts, such as the infundibulum and inferior calyces, were observed. Signs of infection, such as pus, were documented in one patient who underwent mini-PCNL. While other common complications such as hemorrhage, fever, and the need for transfusion were not detailed in this study, preliminary data showed a similar safety profile between mini-PCNL and standard PCNL, which is in accordance with other studies that reported a similar overall complication rate between mini-PCNL and standard PCNL. ^{6 , 13 , 27}

This study has some limitations. As this was an observational study, the results might suffer from selection and information biases and be undermined by confounding factors. This was true for age, which is one of the general characteristics of the populations and should be similar between the two populations. Data were also retrieved from a single institution with a limited sample size. Hence, further comparative research, particularly randomized controlled trials, are needed to corroborate the conclusions of this study.

The management of pediatric urolithiasis has evolved over time. Mini-PCNL, which is a miniaturization of conventional PCNL, is the fundamental result of this evolution. In this study, we demonstrated that mini-PCNL is non-inferior, if not superior, to standard PCNL in safety and effectiveness in treating pediatric renal calculi. Mini-PCNL provided a high SFR and tubeless follow-up.