Pediatric urinary stones are rare but have lifelong consequences. Because children have a small body size and delicate tissues, and because the use of general anesthesia is likely, treatment for pediatric stone disease requires thoughtful consideration and individualized therapy [1,2]. Since extracorporeal shock wave lithotripsy (ESWL) for pediatric urinary stones was first introduced by Newman et al. [3] in 1986, numerous reports have demonstrated the efficacy and safety of ESWL in the pediatric population [4-20]. ESWL is now considered a first-line treatment for pediatric stone disease because of its minimal invasiveness and high success rate [7,21].

Recent reductions in the size of endourological instruments, improvements in electronic video imaging systems, and advancements in endourological skill have given clinicians alternative surgical options such as ureteroscopic surgery (URS) or percutaneous nephrolithotomy (PCNL) for first- or second-line treatment of pediatric urolithiasis, including in cases of ESWL failure [2]. Nevertheless, the identification of variables that predict successful outcomes of ESWL in the pediatric population would be useful. Therefore, we aimed to determine the factors affecting the outcome of ESWL for unilateral urinary stones in children by reviewing 17 years of experience at Samsung Medical Center.

The study population was 64 children, 42 boys and 22 girls (mean age, 9.2±5.2 years; range, 0.5 to 15.9 years). Calculi were on the right side in 34 cases (53.1%) and on the left side in 30 cases (46.9%). Of the 64 patients, 5 (7.8%) had urinary stones in the upper or mid calyx, 5 (7.8%) in the lower calyx, 9 (14.1%) in the renal pelvis, 15 (23.4%) in the upper ureter, 17 (26.6%) in the lower ureter, and 13 (20.3%) in multiple locations. Stone size ranged from 4 to 38 mm with a mean of 10.2±5.7 mm. The mean number of ESWL sessions was 1.7±1.7 (Table 1).

Of the 64 patients, 58 (90.6%) were treated by ESWL without other surgical procedures and 54 (84.4%) were successfully treated within three ESWL sessions. Treatment outcomes according to stone location are shown in Table 2. The percentage of treatment success for a single ESWL session was 80.0% when the stone was solitary and located in the upper or mid calyx, 80% if in the lower calyx, 77.8% if in the renal pelvis, 93.3% if in the upper ureter, and 70.6% if in the lower ureter. The percentage of treatment success for a single ESWL session in patients with stones in multiple locations was 30.8%, which was significantly lower than that for patients with a stone in a single location. The success rate within three ESWL sessions was 100% (5 of 5 patients) when the stone was solitary and located in the upper or mid calyx, 100% (5 of 5 patients) if in the lower calyx, 88.9% (8 of 9 patients) if in the renal pelvis, 100% (15 of 15 patients) if in the upper ureter, and 88.2% (15 of 17 patients) if in the lower ureter. To avoid possible injury to developing reproductive systems, 2 of 17 patients (11.8%) with a solitary lower ureter stone were treated with additional URS after a single ESWL session. Stones in multiple locations were treated successfully within three sessions in 46.2% of cases. Four of 13 patients (30.8%) with stones in multiple locations were treated with additional surgical treatments: two with URS, one with PCNL, and one with URS and PCNL. When the shockwave-related parameters and treatment outcomes were analyzed according to the method of anesthesia, the general anesthesia group was younger (p<0.001) and was more likely to have multiple stones (p=0.005). The success rate was not significantly different between the two groups (Table 3). Early hematuria was noted in most of the patients. Transient renal colic, which was managed by antispasmodic and anti-inflammatory treatment, was observed in 3 patients (4.7%). Steinstrasses occurred in 2 of 13 patients (15.4%) with stones in multiple locations and was treated successfully by URS. No other complications occurred in any patients, such as hemorrhage that necessitated transfusion, infection, or injury of other organs.

In the univariate analysis, multiplicity (p<0.001) and large stone size (>10 mm) (p=0.001) significantly decreased the success rate within three ESWL sessions (Table 4). In the multivariate analysis, multiplicity (odd ratio [OR], 0.080; 95% confidence interval [CI], 0.012 to 0.534; p=0.009) and large stone size (OR, 0.112; 95% CI, 0.018 to 0.707; p=0.020) also decreased the success rate within three ESWL sessions (Table 5).

Since the first report of success of ESWL in a pediatric population by Newman et al. [3], numerous reports have shown the safety and efficacy of ESWL for treating pediatric urinary stones [4-20]. The joint European Association of Urology/American Urological Association Nephrolithiasis Guideline Panel's 2007 Guideline for the Management of Ureteral Calculi states, "Treatment choices should be based on the child's size and urinary tract anatomy. The small size of the pediatric ureter and urethra favors the less invasive approach of ESWL" [22]. Therefore, ESWL remains a first-line treatment option for most pediatric cases of urinary stone disease.

In addition to its noninvasive nature, ESWL has other advantages in the treatment of pediatric urinary stones. For younger patients, stones seem to be more susceptible to shockwaves because of the short indwelling time. The pediatric ureter is more elastic, more distensible, and shorter, which facilitates the passage of stone fragments and compensates for the narrower lumen. The small body volume of children allows the shockwaves to be transmitted with minimal energy loss. However, the application of many ESWL sessions is a burden to pediatric patients because of the likely use of general anesthesia during the procedure and the increased susceptibility of children to radiation exposure.

Until now, studies on ESWL for pediatric urinary stone disease have reported a wide variation in success rate resulting from variation in the methods of different studies, the age of included patients, the machines used, the definition of success, and stone characteristics including size, location, and multiplicity (Table 6) [4-20]. Thus, care must be taken in interpreting the success rate of different series, because 1) some reports on ESWL monotherapy report success rates resulting from a single session and others report results from several sessions, and 2) some series define success as only a stone-free state whereas others include CIRFs. In our study, we analyzed results by several definitions, namely, stone-free rate at 3 months after ESWL, success within a single session, and success within three sessions. Most previous studies defined success as a stone-free rate at 3 months after ESWL, reporting success rates of 71% to 97% [7,10,13,14,16,17,19,20]. In our study, the stone-free rate at 3 months after ESWL was 90.6%.

We believe that our analysis of treatment success within one or three ESWL sessions is an innovative approach. In clinical practice, an important issue during ESWL treatment for pediatric urinary stone disease is the number of ESWL sessions required for success. Because ESWL in pediatric patients frequently requires general anesthesia, the number of ESWL sessions for treatment success is particularly important in children. When a clinician encounters a pediatric patient with stone disease, several factors affect success, such as stone location, size, and multiplicity. We surmised that three sessions of ESWL would be a generally acceptable number for patients and their parents; thus, we analyzed factors affecting treatment success within three ESWL sessions. The overall success rates were 70.3% for a single session and 84.4% for three ESWL sessions. When urinary stones were not found in multiple locations, the success rate was 80.4% for a single ESWL session and 94.1% for three sessions. Few data have been published on ESWL success within one or three sessions. The definition of CIRFs also differs among studies. Some studies suggest that no urinary stones in children are insignificant; others define CIRFs as 3 or 4 mm [8,9,15]. Although children's ureters are more elastic, more distensible, and shorter than the ureters of adults, thus permitting easier passage of stone fragments, the narrow lumen of the pediatric ureter interferes with the expulsion of stone fragments. Therefore, we defined CIRFs as 3 mm.

Our analysis revealed that factors lowering the treatment success rate were stone multiplicity and large size (>10 mm), as expected from the results of previous reports [15,23]. When patients with multiple stones were excluded, three patients did not achieve success within three ESWL sessions. Of these three, one had a 22-mm staghorn stone in the renal pelvis that completely fragmented without complication in 5 ESWL sessions. The remaining two patients had lower ureteral stones that were not fragmented in a single ESWL session. We did not attempt further ESWL because of difficulties with localization over the sacroiliac joint and to avoid possible injury to the developing reproductive systems. These patients received URS.

An interesting finding of this study was that the success rate did not differ by stone location. When stones are in the lower pole calyx, the success rate of ESWL depends not only on successful fragmentation but also on stone fragment clearance. ESWL treatment for lower pole calyceal stones is controversial. Several studies have demonstrated a significantly lower success rate after ESWL for lower pole stones than for other kidney locations [24,25]. However, a recent study by Goktas et al. [26] reported that ESWL was highly successful for lower calyceal stones in pediatric patients compared with adults. The study suggested that ESWL can be a first-line treatment for managing lower calyceal stones in children. The authors hypothesized that the high success rate of ESWL treatment of pediatric lower calyceal stone resulted from 1) less shockwave energy loss during travel through the small body volume of a child, 2) the shorter ureter length in children, and 3) the more elastic and distensible ureters in children. In our study, the success rate for stones in the lower pole calyx was not significantly different from that of stones in other kidney locations: 80% for a single ESWL session and 100% for three sessions. Concurring with previous reports, we recommend ESWL as a first-line treatment, even for stones located in the lower pole calyx. However, our results were from a retrospective analysis with a small number of patients (n=5); thus, definite conclusions cannot be drawn about the success of ESWL for lower calyceal stones.

In our study, 2 of 64 patients (3.1%) experienced steinstrasse, which was treated by URS without sequelae. Both patients initially had multiple urinary stones. Of the possible complications after ESWL for pediatric stone disease, steinstrasses seem to be most challenging and mainly results from large fragments or multiplicity [9,27]. However, with advances in medical technology, the miniaturization of ureteroscopes has made treatment of steinstrasses easier, resulting in fewer complications. Therefore, we recommend ESWL as a first-line treatment even for large or multiple stones, although this results in a risk of steinstrasse.

A potential limitation of this study is the retrospective cohort design. However, during the study period, our interpretation of stone characteristics including stone location and size did not change. Neither did the ESWL protocol for stone treatment. To the best of our knowledge, this study includes the largest number of pediatric stone patients in Korea. We think that the results could be a useful reference for pediatric stone treatment. If a child has a large urinary stone (>10 mm) or multiplicity, the parents should be informed of the probability of needing several ESWL sessions for complete stone fragmentation.

In our 17-year single-institution experience, most pediatric urinary stone patients (58 of 64 [90.6%]) were successfully treated by ESWL alone without additional procedures. The factors decreasing the success rate within three ESWL sessions were multiplicity or large stone size (>10 mm). No significant difference in success rate was observed by stone location for patients without multiplicity.