Effect of coupling fluid tonicity on pain and fragmentation outcomes in ESWL: a prospective comparative study

To investigate the effects of coupling fluid tonicity on pain, stone fragmentation success and complications in extracorporeal shock wave lithotripsy (ESWL). This prospective, controlled clinical trial included 294 patients with renal pelvis stones (5-20 mm) allocated to three groups: hypotonic degassed water (n = 97), isotonic 0.9% NaCl (n = 99) and hypertonic 3% NaCl (n = 98). Primary endpoints were post-ESWL Visual Analogue Scale (VAS) pain scores and stone fragmentation success (< 4 mm residual fragments) on follow-up non-contrast computed tomography at 2-4 weeks. Baseline demographic and stone characteristics were comparable across groups. VAS pain scores differed significantly among the three groups (p = 0.005), with the hypertonic group showing the lowest median score (4.0 vs. 5.0 in both other groups). Post-hoc analysis confirmed that this reduction was statistically significant only between the isotonic and hypertonic groups (Bonferroni-corrected p = 0.004), whereas the difference versus hypotonic water did not remain significant after correction. Multivariable regression confirmed that the pain reduction was independent of the recorded energy level (p < 0.001). Stone fragmentation success rates were similar (56.7%, 59.6% and 57.1%; p = 0.907). The incidence of skin petechiae and/or ecchymosis decreased numerically with increasing tonicity (6.2% to 1.0%) but did not reach statistical significance (p = 0.161). Hypertonic 3% NaCl coupling fluid may significantly reduce ESWL-related pain, particularly compared with isotonic fluid, with no statistically significant difference in clinical stone fragmentation success observed among the groups. Validation through larger, multicentre randomised trials is recommended.