Int Urol Nephrol (2015) 47:573–577 DOI 10.1007/s11255-015-0938-8
UROLOGY - ORIGINAL PAPER
Risk factors for formation of steinstrasse after extracorporeal shock wave lithotripsy for pediatric renal calculi: a multivariate analysis model Ahmed El‑Assmy · Ahmed R. El‑Nahas · Mohammed M. Elsaadany · Samer El‑Halwagy · Khaled Z. Sheir
Received: 14 January 2015 / Accepted: 20 February 2015 / Published online: 4 March 2015 © Springer Science+Business Media Dordrecht 2015
Abstract Objectives To define various stone, renal and therapy factors that could affect steinstrasse (SS) formation after extracorporeal shock wave lithotripsy (SWL) for pediatric kidney stones. Thus, SS could be anticipated and prophylactically avoided Methods From January 1999 through December 2012, 317 children underwent SWL with Dornier Lithotripter S for the treatment of renal stones. Univariate and multivariate statistical analyses of patients, stones and therapy characteristics in relation to the incidence of SS were performed to detect the factors that had a significant impact on SS formation. Results The overall incidence of SS was 8.5 %. The steinstrasse was in the pelvic ureter in 74.1 % of the cases, lumbar ureter in 18.5 % and iliac ureter in 7.4 %. Steinstrasse incidence significantly correlated with stone size, site and age of child. Steinstrasse was more common with increasing stone length and stones located in renal pelvis or upper calyx with the age below 4 years. A statistical model was constructed to estimate the risk of steinstrasse formation accurately. The equation for logistic regression is Z = −4.758 + B for age + B for size stone X length in mm + B for stone site. Conclusions The stone size, site and age are the most important risk factors responsible for SS formation in children. Our regression analysis model can help with prospective identification of children who will be at risk of SS formation. Those children at high risk of SS formation should
A. El‑Assmy (*) · A. R. El‑Nahas · M. M. Elsaadany · S. El‑Halwagy · K. Z. Sheir Urology and Nephrology Center, Urology Department, Mansoura University, Mansoura, Egypt e-mail: [email protected]
be closely monitored or treated by endoscopic maneuvers from the start. Keywords Shock wave lithotripsy · Steinstrasse · Renal calculi · Pediatric · Risk factors
Introduction The incidence of pediatric urolithiasis shows a wide geographic variation. Although the incidence is low in Western countries , pediatric stone disease is endemic in developing nations such as India, Turkey, Pakistan and Far East countries . The introduction of shock wave lithotripsy (SWL) by Chaussy et al.  in the early 1980s revolutionized the armamentarium for the management of upper urinary tract urolithiasis. Successful SWL in children was first published in 1986 . Since then several reports showed safety and stone-free rates comparable to those of adults [5–7]. Steinstrasse (SS) is a well-recognized complication of SWL and defined as the presence of more than one ureteral stone simultaneously [8–10]. In adults, several reports evaluated the risk factors for SS formation and showed that stone size, site and composition, and the power used for disintegration are possible predictors of SS [11–13]. However, in pediatric population only one study evaluated the predictive factors of SS and showed that the stone burden was the only statistically significant factor predicting the formation of SS on logistic regression analysis . That study suffered from some limitations; first it included only those who presented with radiopaque stones; thus, multivariate analysis is specific for predicting the formation of SS after SWL in radiopaque stones. Second it included children with pre-SWL JJ ureteral stenting which could
hinder stone fragment passage, thus affecting the incidence of SS. These previously mentioned limitations triggered us to evaluate the various stone, renal and therapy factors that could affect SS formation after SWL for pediatric urolithiasis. We did not include stone composition, as there is no accurate method to identify it except after treatment and stone retrieval for analysis, and we attempted to predict who would be predisposed to SS formation before treatment. Using our statistical model, SS could be prospectively predicted and prophylactically avoided.
Methods Patients After approval of the institutional review board, the computerized patients’ records of children who underwent SWL monotherapy for renal stones from January 1999 through December 2012 were retrospectively reviewed. Patients with congenital renal anomalies (such as duplex, ectopic or horseshoe kidneys) and patients who underwent pre-SWL procedures, such as JJ stent or nephrostomy tube, were excluded. The present study included 317 children (191 [60.3 %] boys and 126 [39.7 %] girls) with mean age of 7.16 ± 4 years (range 1–16 years). Pre-SWL radiological evaluation included plain X-ray of kidney, ureter and bladder (KUB) and renal ultrasonography (RUS) for all children. Excretory urography or NCCT was carried out for delineation of stone location and calyceal anatomy. Laboratory investigations included urinalysis and culture if there was pyuria. Children with positive urine cultures were treated with the appropriate antibiotics before SWL until the culture became sterile. Stone length was defined as the largest cross-sectional diameter in a single stone measured on pre-SWL KUB in single stone or the sum of the largest diameters for multiple stones. SWL technique All children underwent SWL using the Dornier Doli S electromagnetic lithotriptor (Dornier MedTech GmbH, Germering, Germany). The stones were localized by fluoroscopy or ultrasonography with real-time monitoring. All children underwent SWL monotherapy in situ with no auxiliary procedures, and all were treated on an outpatient basis. General anesthesia was required for 231 children (72.9 %), and 86 children (27.1 %) received sedoanalgesia with meperidine HCl (1 mg/kg). Therapy was started at a low power of 12 kV; the power was then increased in steps of 200 shocks up to 16 kV. We gave the number of shocks
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required for complete disintegration of the stone, with a maximum of 2000 shocks/session at a rate of 80 shocks/ min. A water cushion was used as a coupling medium, and an urologist experienced in SWL attended the whole procedure. The interval between sessions was 2 weeks. Follow‑up The patients were examined 1 week after each session using KUB and renal ultrasonography to assess fragmentation and the presence of renal obstruction. Repeat sessions were performed if inadequate fragmentation of the stone was observed. Patients were finally evaluated 3 months after the last lithotripsy session using low-dose NCCT. Success was defined as stone-free status at the 3-month follow-up visit after the last lithotripsy session without the need for ancillary procedures. Statistical analysis Data were processed using SPSS-16 for Windows (SPSS, Inc., Chicago, IL). Patient (age, sex and kidney morphology), stone (length, site, opacity, number, nature and side) and therapy (number of shock waves and energy level) characteristics were related to the SS formation using the Chi-square for categorical variables and t tests for continuous ones. A P value of 4 years, younger children have 2.6-fold increase in SS formation. In comparison with stones in the middle calyx, the odds ratio of SS formation had increased 2.9- and 2.6fold for stones in the upper calyx and pelvis, respectively. The equation for logistic regression is Z = constant + B for age + B for size stone X length in mm + B for stone site, where Z is the linear combination of the variables, B is the regression coefficient and the constant of the model is −4.758. The probability of the developing SS is 1/(1 + e−z), where e is the base of the natural logarithm (=2.718). In general, if the estimated probability is 2 cm compared with stones 200 mm2 . In Onal study, the incidence of SS formation was 1.9 % for stones 2 cm . In our study, a direct correlation was found between stone length and subsequent SS development. This relationship to stone size was confirmed statistically in our patients and sustained a statistical significance on univariate and multivariate analyses. The odds ratio for SS formation had increased 1.11-fold for each 1-mm increase in the stone length.
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Table 3 Risk factors for SS maintained in stepwise logistic regression Variables
Regression coefficient (B)
Relative risk (95 % CI) (lower, upper)
Stone length (mm) Age (years) >4 (reference) ≤4 Stone site Upper calyx Middle calyx (reference) Lower calyx Pelvis
1.090 0 −17.861 0.960
2.976 0 0.00 2.613
0.243–36.367 1 0.000–0.000 0.329–20.730
In our study, the stone nature did not have a statistically significant impact on SS formation. This could be explained by the fact that the nature of stone being de novo or recurrent is of minor significance as long as the renal anatomy is not distorted. The imaging of the 20 recurrent stones revealed no anatomical alteration or distortion in 17 children while the remaining three cases showed mild pyelonephritic changes. In our study, stone site had a statistically significant impact on steinstrasse formation and the commonest site associated with SS was renal pelvis followed by upper calyx. The incidence rates of a steinstrasse in stones located in upper calices, middle calices, lower calices and renal pelvis were 9.5, 3.6, 0 and 12.2 %, respectively. Our finding is similar to that reported by Soyupek et al. who found also the highest incidence of SS with stones located in the renal pelvis, and the incidence rates of a SS in stones located in upper calices, middle calices, lower calices and renal pelvis were 6.12, 10.52, 6.36 and 19.32 %, respectively . Also Madbouly et al. highlighted the importance of stone site on the incidence of stone formation and reported that the incidence of steinstrasse was 2.7 times less for lumbar ureteral stones compared to renal stones. However, they found no difference in SS formation between renal stones and those at the pelviureteral junction . On the other hand, the stone location was not statistically significant in pediatric group published by Onal et al. . The shock wave energy level was previously reported as a predictive factor for the development of SS . This is because high-energy shock wave applications produce a coarse stone disintegration with the formation of large fragments in comparison with fine disintegration using repeated low-energy shock wave applications . Madbouly et al.  reported that SS formation was three and two times less using energy levels of 14–17 and 18–22 kV, respectively, compared with using > 22 kV for stone disintegration. In our study, the energy level was not found to be a significant factor, in contrast to the cited reports. This
might have originated from our SWL treatment was usually started at a low power of 12 kV; the power was then increased in steps of 200 shocks up to 16 kV. Our results are in harmony with that reported by Onal et al. in pediatric group who found that the energy level was not found to be a significant factor for SS formation . A new finding in our study was the significant influence of age on the formation of SS. The incidence of SS was 14.7 % in children ≤4 years compared to only 5.6 % for older children. In comparison with children >4 years, younger children and infants have more than double the chance to develop SS. Our finding contradicts the finding of Onal et al.  who did not find an influence of age on SS formation among children. However, our finding could be explained by the fact that children ≤4 years are less actively walking than older ones which may not help stone passage. It is well documented that walking and exercise improve passage of calculi . However, the correlation between age of child and incidence of SS should be interpreted with caution, and further prospective studies are warranted to evaluate this issue. In our study, eight (29.7 %) cases with uncomplicated SS passed the stones spontaneously by conservative management while four (14.8 %) were successfully managed by repeat SWL monotherapy. We used URS in 10 children (37 %) after failure of SWL repeat treatment, and URS was successful in all cases. In the study by Onal and associates , conservative management was effective in 15.4 %, repeat SWL monotherapy was successful in 65.4 %, and URS was utilized in 15.4 % of cases. The present study had several limitations. First, data were collected retrospectively, so it may carry all the bias of retrospective studies. Second, our series had small number of children with SS; however, SS in children is not a frequent event. In addition, there is an absence of full stone characteristics in NCCT, namely the accurate stone volume and stone density, as pre-SWL NCCT was not available in all cases. Lastly, the correlation of SS to stone composition
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was not tested; being a retrospective study, the data of stone composition were not available and stone fragments were not available for chemical analysis. Finally, by using our model if the estimated risk is high, close follow-up monitoring is mandatory, or after counseling, the parents of children with urolithiasis other line of treatment such as endoscopic intervention may be chosen. Also we recommend that another prospective study should be done including a larger number of cases and all risk factors and stone characteristics should be statistically analyzed to gain more solid and reliable data.
Conclusions The stone size, site and child age are the most important risk factors affecting SS in children. Steinstrasse formation occurs more with increasing stone length, stones located in renal pelvis and upper calyx, and young children ≤4 years. Our regression analysis model can predict the probability of SS formation and help with prospective identification of children who will be at risk of SS formation. Conflict of interest There is no conflict of interest to declare.
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