Study: Second procedures more likely after shock wave lithotripsy

October 1, 2008

Stone patients undergoing shock wave lithotripsy are significantly more likely to undergo a subsequent surgical procedure than are patients undergoing ureteroscopy with or without laser lithotripsy.

The analysis used data from a nationwide database of privately insured beneficiaries enrolled in over 30 health plans. It included 17,669 subjects who underwent a fragmentation procedure in 2004 and had at least 1 year of continuous follow-up.

SWL accounted for 56% of initial procedures, URS with lithotripsy laser and URS alone each accounted for about 20%, and about 3% of patients underwent percutaneous nephrolithotomy.

In a Cox proportional hazards model, where patients who underwent URS plus lithotripsy represented the reference group, the risk of a second treatment was almost 2.4 times greater in patients who had SWL as the first procedure and about 1.2-fold higher in those undergoing percutaneous nephrolithotomy initially.

"We know from clinical trials that SWL is a good treatment for certain stones, but our study in a community-based cohort shows it has a high second procedure rate," said first author Charles D. Scales, Jr, MD, a urology resident at Duke University Medical Center, Durham, NC, working with Glenn M. Preminger, MD, and colleagues.

"These results need to be interpreted with caution, given the limitations of our claims-based analysis. However, we used multivariate analysis techniques to control for potential confounders and we believe our findings are robust. Now, keeping in mind a goal of optimizing quality of care for patients, we need to understand the causes of our findings."

The patients included in the study had an average age of 51 years, and 61% were men. Patients at high risk for recurrent stones, ie, individuals with a stone claim within 6 months prior to the index encounter, cystinuria, or uric acid nephrolithiasis, were excluded from the study.

"Our retrospective study could not account for whether the second treatment was for bilateral stones, and it also could not control for stone size. We believe the number of patients with a bilateral stone is small, since we excluded those with high-risk disease. However, we would recommend that future investigations be conducted in independent, real-world data sets and aim to correlate SWL and URS use with clinical data that would also include stone size, stone location, and shock rate," Dr. Scales said.

The study was undertaken recognizing that there may be discrepancies between efficacy results demonstrated for a technology in clinical trials, which are often conducted in tertiary care settings, and effectiveness in daily clinical practice.

"Effectiveness is defined as outcomes in the real world, and results achieved with lithotripsy in a community-based cohort have not been previously documented," Dr. Scales said.

Looking at practice patterns

The study also aimed to characterize practice patterns. Dr. Scales noted that the utilization rates for the different technologies in the present study are consistent with previously reported data from the Urologic Diseases in America project. However, he acknowledged that patient preference can be a strong driver in treatment selection and is a confounder that could not be teased out in the analysis due to its claims-based nature.

The study also showed that for each of the initial procedures, the same procedure was most commonly performed as the second procedure. For example, among patients who were initially treated with SWL, almost three-fourths of the second stone procedures were also SWL.

The analyses also showed that a stent was inserted in about 20% of patients undergoing initial SWL and in about 60% of patients treated initially with URS. Stent placement was a marker for second procedures.

"We would speculate that this finding may be related to a complicated initial procedure or a large stone burden, but we cannot directly test this hypothesis in our data," Dr. Scales said.