This study compared the SWL outcomes for ureteral stones between a reduced protocol (30 shocks/min with 1,200 shocks/session) and a standard protocol (60 shocks/min with 2,400 shocks/session). Our findings revealed that the success rate of the reduced protocol was comparable to that of the standard protocol. In multivariate analysis, the predisposing factors for SWL success were age and stone size, consistent with previous reports. According to the multivariate analysis and propensity score matching findings, the reduced protocol had no effect on treatment success. The complication rate was minimal and did not differ significantly between the two groups.
Previous studies, including meta-analyses, reported that a slower SWL delivery rate yielded better results. However, the studies only compared rates of 60, 80, 90, and 120 shocks per minute, and further slower rates have rarely been investigated[2–7]. Thus, there is no consensus or recommendation on the optimal rate of shock waves. In vivo, SWL with 30 shocks per minute had a higher success rate than 200 shocks per minute. However, few clinical reports have investigated the success rate of SWL with 30 shocks per minute. Our findings suggest that SWL with an extremely slow delivery rate would provide more effective lithotripsy. Although the effects of lithotripsy should be compared with the same number of shocks, it was not practical to spend twice as much time in clinical practice. Therefore, times for both protocols were matched. Our group recommended the SWL reduced protocol for patients with renal and ureteral stones, comparable to the standard protocol for renal stones.
The mechanism by which shock wave rate affects treatment success is unknown. The cavitation effect may be implicated in the association. The negative pressure generated by the shock waves creates cavitation bubbles, which absorb some of the energy from the shock waves. The subsequent shocks decay until the bubbles disappear, which is thought to reduce effectiveness at higher frequencies. Small fragments generated from the stone surface can act as cavitation nuclei, creating bubbles that attenuate the shock waves. Therefore, shock waves would be more effective if provided soon after the bubbles and fragments disappear. Since the cavitation bubble dissipates within 1–2 seconds, a shock frequency of fewer than 60 shocks per minute may produce better outcomes.
Few clinical studies investigated the efficacy of SWL at a further slower rate, with only one randomized control trial (RCT) comparing the outcomes of a 30 shocks/min versus 60 shocks/min delivery rate. After three months, the stone-free rate in the 30 shocks/min group (96.3%) was significantly higher than that in the 60 shocks/min group (63.8%). The disagreement with our results is primarily because both groups in this RCT received the same number of shocks every session. Furthermore, the RCT included relatively large (almost all stones were larger than 10 mm), radiopaque, high attenuation value (≥ 1,000 Hounsfield unit) upper ureteral stones. Our study included relatively small, low attenuation value stones. Finally, the number of sessions in this RCT was restricted to three. After three SWL sessions, ureteroscopy is an auxiliary treatment; however, SWL may continue at the patient’s discretion or the urologist. Auxiliary ureteroscopy was performed more frequently in the reduced protocol and considered a treatment failure, although overall treatment success rates were comparable between protocols. Even if SWL had continued, it would eventually fail since patients with difficult-to-treat stones should be converted to ureteroscopy. Additionally, patients on the reduced protocol might be more likely to follow our recommendations.
Although each manufacturer makes recommendations on shock wave number and energy, there is no consensus on the optimal number of shock waves. Acons et al. reported that increasing the number of shocks per session improved the stone-free rate. However, more shocks per session may be excessive and ineffective because disintegrating fragments surrounding the stone may impede shockwave transmission. Several factors, such as stone size and consistency, should be considered to determine the optimal shock number and energy. In general, 1,200 shocks per session are deemed insufficient, yet the reduced protocol achieved comparable outcomes to the standard protocol in our study. Since our study included relatively smaller and lower-density stones, half the number of shocks in the reduced protocol may be sufficient. Another possible explanation is that the slower delivery rate was more efficient, resulting in comparable outcomes with fewer shocks. These assumptions are consistent with the previous RCT described above.
Previous reports have demonstrated that tissue damage increases with the frequency and number of shock waves[21, 24, 25]. In the current study, the complication rates were not statistically different. However, the fewer shocks and slower delivery rate may have had protective effects on the tissues. The complications were only Grade 2 fever, which seemed due to a urinary tract infection, necessitating transurethral stenting. Since all these patients had ureteroscopy following infection cure, SWL was considered unsuccessful. However, whether SWL caused infection is debated because ureteral stones were already present before the procedure.
The reduced protocol has additional advantages. First, the reduced shock numbers are cost-effective. The head of lithotripter therapy is expendable and must be replaced every 500,000–600,000 shock waves. The replacement cost is between $10,000 and $20,000; therefore, reducing the shock number from 2,400 to 1,200 would save $25 to $50 every session. Second, a slower delivery rate reportedly reduced pain and analgesic drug use. The major concern with a slower delivery rate is increased treatment duration, but our reduced protocol demonstrated the same success rate with the same treatment as the standard protocol[21, 27]. Thus, our reduced protocol may be useful for pain-sensitive patients, such as children. Unfortunately, however, we could not examine this issue because the pain scale was not recorded.
In the multivariate analysis, age and stone size were independent predictors of treatment success. Interestingly, the stone size affected the success rate within 30 days, which has not previously been reported. In contrast, other variables, such as stone location, attenuation value, or BMI, were not associated with SWL success. Because the target stones in this study were relatively small, some small stones might pass spontaneously without being disintegrated. Thus, age and stone size were independent predictive factors within 30 days. However, this does not explain the success rate within 90 days. Second, the small stone was difficult to target, which might affect the measurement of the stone size and attenuation values. Another possibility is observer bias in measuring attenuation values, particularly for small stones. Finally, the cut-off may be implicated as age, stone size, BMI, and attenuation value were included as continuous variables, and the stone location was classified into upper and lower. A statistical difference may have been found with an appropriate cut-off value.
There are some limitations to this study. First, there was a selection bias because of the retrospective study design. Although there was no statistically significant difference between the two studied groups, unexamined bias might exist. Furthermore, due to a lack of rigorous specifications before initiating treatment, the follow-up schedule or the number of treatments was inconsistent among protocols. Second, it was unclear how much the slow SWL delivery rate exclusively influenced the success rate because both numbers and rates were reduced. Third, ultrasonography and KUB were used to assess treatment success rather than CT scanning, which has the highest accuracy. However, it is impossible in daily clinical practice to screen all patients on CT after each procedure due to radiation exposure’s cost and adverse effects. If the stone is not identified by KUB or ultrasonography and there is no hydronephrosis, there should be no clinical concerns. Actually, we previously reported the usefulness of ultrasonography for the detection of ureteral stone. Finally, only acute complications were evaluated, while the long-term effects remain unclear. Despite several limitations, this study demonstrates the potential benefits of the new protocol with a slower delivery rate and half the number of shocks. Therefore, further prospective randomized trials are warranted to evaluate our reduced protocol and determine optimal SWL.