BACKGROUND AND PURPOSE: Obesity has been identified as a limitation of extracorporeal shockwave lithotripsy (SWL). The obesity metrics of body mass index (BMI) and skin-to-stone distance (SSD) have been evaluated as predictors of SWL success. While SSD has demonstrated a strong correlation with success, BMI has not. Bioimpedance analysis (BIA) is an accurate way of determining body adiposity. We evaluated fat mass percentage (FMP) as measured by BIA as a predictor of SWL success.
PATIENTS AND METHODS: We prospectively collected body composition data using the Imp-DF50 Body Impedance Analyzer on consecutive patients undergoing SWL. All generated variables, including FMP, along with demographics, BMI, stone size, and stone composition, were analyzed. Patients were evaluated for success, defined as no evidence of stones on radiography of the kidneys, ureters, and bladder at follow-up.
RESULTS: Fifty-two consecutive patients were enrolled in the study, of which 37 had the necessary metrics to be included in the analysis. Twenty-three (62.2%) patients were stone free while 14 (37.8%) were found to have residual stone at follow-up. There was no difference in sex, stone laterality, mean age, and stone size between the groups. For the success and failure groups, the mean BMI was 25.8 kg/m(2) and 29.8 kg/m(2) (P=0.0091), and mean FMP 24.6% and 32.2% (P=0.0034). On mirrored multivariable analysis, both BMI (OR=0.735, P=0.026) and FMP (OR=0.806, P=0.010) were associated with success. Patients with a FMP ≥35% had a reduced success rate compared with those with a FMP <35% (14% vs 73%, respectively, P=0.0028).
CONCLUSIONS: Both BMI and FMP both appear to be independent predictors of success. Based on these findings, a large study examining the relationship between BMI, FMP, SSD, and SWL success is warranted. A preoperative FMP ≥35% is associated with a 14% success rate, and alternative treatment strategies for urolithiasis should be considered.
J Endourol. 2011 Sep;25(9):1503-6. doi: 10.1089/end.2010.0687. Epub 2011 Aug 4
PMID: 21815805 [PubMed - in process]
The outcome of shockwave lithotripsy is determined by two factors: disintegration of stone material and clearance of stone fragments. When lithotripsy effects are studied it thus is important to analyse the results from both perspectives. In this study the authors did choose to measure the success of ESWL as »no evidence of stone fragments« remaining in the kidneys.
Although in obese patients with a high BMI it is likely that the reduced shockwave power results in sub-optimal stone disintegration and accordingly residual stones and fragments, it is also likely that these patients have residuals because of reduced mobility. The bio-impedance which is the focus of this article is a factor that further will influence stone fragmentation. It is indeed of interest to learn more about how the tissue properties influence the shockwave power and the approach presented in this article seems promising.
In the larger study that is planned it would be desirable if a distinction was made between the effects in terms of disintegration and that of stone clearance.