Shock Wave Lithotripsy is an Efficacious Treatment Modality for Obese Patients with Upper Ureteric Calculi: logistic regression and matched-pair analyses from a dedicated centre comparing treatment outcomes by skin-stone distance.

To assess the association of skin-to-stone distance (SSD) and stone-free rates following shock-wave lithotripsy (SWL) using two statistical methods: logistic regression and a matched-pair analysis approach. Patients & Methods Patients with a solitary radio-opaque upper ureteric calculus diagnosed on non-contrast computed tomography were included. Patients were treated with a Sonolith-I-Sys Lithotripter (focal depth 17cm). Stone treatment success was defined as stone free (fragments ≤3mm) at 3 months. Failure was persistent fragments beyond 3 months or requirement for intervention. Logistic regression was used to determine association of patient and stone factors with treatment failure. Matched-pair analysis was undertaken between individuals from the top and bottom SSD tertiles (SSD ≤12cm and SSD ≥14cm). Matching criteria consisted of age, sex, maximum stone diameter (+/-2mm) and stone density (+/-250 HU). Results From a database of 2849 patients who underwent SWL, 397 patients were identified who had treatment of a single upper ureteric stone. Age (OR 1.03, 95% CI1.01-1.04, p=0.007), SSD (OR 1.16, 95%CI 1.03-1.32, p=0.02), stone side (OR 1.65, p=0.05), stone diameter (OR 1.09, 95%CI 1.00-1.19, p=0.05), and multiple sessions (OR4.65, 95%CI 2.61-8.29, p<0.001) were significantly associated with treatment failure by logistic regression univariable analysis. Multiple sessions was the only factor significantly associated with treatment failure on multivariable analysis (OR 4.03, 95%CI 2.18-7.42, p<0.001). From a cohort of 141 patients with SSD ≥ 14cm and 174 patients with a SSD ≤ 12cm, 66 matches were identified (132 patients). 49 patients (74.2%) with SSD ≥14cm were deemed stone free at follow up versus 51 patients (77.3%) with SSD ≤12cm (p=0.85). Conclusion This study demonstrates by two statistical methods that SWL can provide efficacious treatment of upper ureteric stones in obese patients, and that the upper threshold of SSD for ESWL with Sonolith I-SYS could be revised to allow these patients the benefits of SWL.