Different injection speeds produce distinct temporal dynamics of optic nerve sheath diameter increase during caudal block in children: a prospective randomized trial

Background Caudal block increases optic nerve sheath diameter (ONSD), but its temporal dynamics vary considerably across centers. Injection speed is one of the unstandardized factors in caudal block, and whether it influences ONSD dynamics remains unclear. Purpose We therefore evaluated the effect of injection speed on ONSD changes during caudal block in children. Design A prospective, randomized, observer-blinded trial. Methods This study enrolled children aged 1–3 years undergoing perineal surgery. Participants received a caudal block with 0.2% ropivacaine (0.5 mL/kg) at either 1 mL/s (high-speed) or 0.1 mL/s (low-speed). ONSD was measured at baseline, immediately after injection, and at 10, 20 and 40 min time points. The primary outcome was ONSD change over time. Linear mixed-effects models were used for analysis. Results Eighty children were analyzed. At T2, ONSD was significantly higher in the high-speed group (4.73 ± 0.43 mm) than in the low-speed group (4.52 ± 0.35 mm, P = 0.027); at T3, ONSD was significantly higher in the low-speed group (4.73 ± 0.57 mm) than in the high-speed group (4.49 ± 0.42 mm, P = 0.010). Both groups showed significant ONSD increases from T1 onward ( P < 0.05), without return to baseline by T4. A significant time-by-group interaction ( P < 0.001) indicated different temporal patterns. The maximum ONSD value did not differ between groups (4.8 mm, p = 0.537). No significant differences were observed in mean arterial pressure, heart rate, or end-tidal carbon dioxide at any time point ( P > 0.05). No adverse events occurred. Conclusion At an injection volume of 0.5 mL/kg, both speeds were well tolerated. Faster injection advances the observed ONSD peak; slower injection produces a more gradual rise and is therefore recommended for routine practice. Clinical trial registration https://www.chictr.org.cn , identifier ChiCTR2200057459.