Discussion
In the literature, the success rate of SWL treatment performed on proximal ureteral stones ranges from 48% to 96%. [7,9,10] In a study conducted by Besien et al, it was stated that the success of SWL was affected by the experience and skill of the urologist. [11] In our study, the stone-free rate after SWL treatment was 92.5%, which is and close to the upper limit given in the literature. We consider that in our study, stone targeting was standardized as it was performed by a single urologist with 30 years of SWL experience.
It is stated that the increase in the UWT in the SWL treatment is one of the factors affecting success. In a study conducted by Sarıca et al., SWL treatment was performed in 111 patients who had impacted proximal ureteral stones, and the success rate was found to be 78.4% and the failure rate as 21.6%. The median transverse stone size was reported as 8.9 mm, and the optimal threshold value for UWT was calculated as 3.55 mm, at which this parameter had 91.7% sensitivity and 77% specificity. It was concluded that UWT > 3.55 mm was an independent parameter affecting treatment failure.[6] In our study, the stone-free rate (92.5%) was higher, which may be correlated with the lower mean transverse stone size in our study (7.5 mm vs 8.9 mm). The UWT threshold value of our study (5.25 mm) was also higher than reported by Sarıca et al. (3.55 mm). In our study, the mean UWT was 4.2 mm, and the selected cross-sectional range was 1.25 mm (a very thin cross-section range). However, Sarica et al. determined the mean UWT as 3.28 mm and did not specify the cross-sectional range. These factors may have led to the difference in the threshold values obtained. In a study by Yoshida et al., it was stated that the increase in UWT also decreased the success of URS.[12]
Extracorporeal shock wave lithotripsy treatment is minimally invasive and can be performed without anesthesia, but it is not as effective as the more invasive URS in the presence of a higher number of stones. Eden et al. found that in distal ureteral stones, the presence of multiple stones increased SWL treatment failure. The authors stated that the stone-free rate of the patients with single stones was 88%, which decreased to 50% in those with multiple stones. [13] Similarly, in our study, the stone-free rate decreased by 50% in the patients with multiple stones, and the presence of multiple ureteral stones located above the iliac crest level further increased the SWL treatment failure rate. Yamashita et al. also stated that the presence of multiple stones was a parameter that increased URS failure.[14]
In a recent study by Guler et al., the diameter of the proximal ureter was found to be an independent parameter affecting the treatment success of proximal ureteral stones with SWL. [15] In our study, the diameter of the ureter in the proximal of the stone was found to increase treatment failure in the univariate analysis, but a similar relationship was not observed in the multivariate analysis. In parallel with the study of Guler et al., the hydronephrosis degree increased failure according to the univariate analysis, but no relation was found in the multivariate analysis.
It is considered that systemic diseases may affect inflammatory reaction caused by the stone on the ureteral mucosa. Hubert et al. found that patients with comorbidities had decreased SWL success. The treatment being successful was 2.4 times more likely in patients with one comorbidity compared to those without any comorbidity. [16] Dalibor et al. stated that there was no relationship between SWL success and comorbidities. [17] In our study, the univariate analysis revealed that among the ureteral stones above the iliac crest, the failure rate increased in the presence of comorbidities while no such association was detected in the multivariate analysis.
In a previous multi-center study conducted in our clinic, we created a nomogram for patients who underwent SWL in the pediatric group and found that a history of stone surgery on the same side affected the success of treatment. [18] However, when we conducted the current study with adult patients, we observed no relationship between treatment failure and having a history of previous stone surgery or ureteral stones above the iliac crest level.
In some patients, placing a ureteral double J catheter before SWL may be considered to improve treatment success and associated complications. Ghonelm et al. stated that there was no significant relationship between urethral stent placement and treatment success in impacted proximal ureteral stones. [19] As in our study, the univariate analysis undertaken by Yamashita et al. showed that patients undergoing urinary drainage had increased treatment failure, but no relation was found in the multivariate analysis.[7]
Ng et al. stated that SSD was a predictor of the success of SWL treatment, but they did not specify the mean SDD of the patients.[20] However, Mains et al. reported the mean SSD as 13 cm and noted that it did not affect SWL success. [21] We determined a similar mean SSD value (12.5 cm) in our study. Furthermore, similar to Mains et al., we did not find a relationship between SSD and SWL success. In many studies, it has been suggested that the size of the ureteral stone affects the success of SWL. [22,23] However, in our study, no significant relationship was found between stone size and treatment success. Contrary to what is stated in the EAU guidelines [1], no relationship was found in SWL success and stone-free status according to the stone density (HU) of patients in both the recent study of Yamashita et al. [7] and our study.
The limitation of our study was its retrospective nature. Since the definition of impacted stones is subjective, and there is no consensus among the clinicians in the literature, we did not classify the stones as impacted or non-impacted in this study. With the growing SWL experience globally and according to our data, the predictive factors used to increase SWL success may change in the coming years.