[Distal sciatic nerve blocks: randomized comparison of nerve stimulation and ultrasound guided intraepineural block]

Abstract

Background and objectives: The design of this study is related to an important current issue: should local anesthetics be intentionally injected into peripheral nerves? Answering this question is not possible without better knowledge regarding classical methods of nerve localization (e.g. cause of paresthesias and nerve stimulation technique). Have intraneural injections ever been avoided? This prospective, randomized comparison of distal sciatic nerve block with ultrasound guidance tested the hypothesis that intraneural injection of local anesthetics using the nerve stimulation technique is common and associated with a higher success rate.

Material and methods: In this study 250 adult patients were randomly allocated either to the nerve stimulation group (group NS, n = 125) or to the ultrasound guidance group (group US, n = 125). The sciatic nerve was anesthetized with 20 ml prilocaine 1% and 10 ml ropivacaine 0.75%. In the US group the goal was an intraepineural needle position. In the NS group progress of the block was observed by a second physician using ultrasound imaging but blinded for the investigator performing the nerve stimulation. The main outcome variables were time until readiness for surgery (performance time and onset time), success rate and frequency of paresthesias. In the NS group needle positions and corresponding stimulation thresholds were recorded.

Results: In both groups seven patients were excluded from further analysis because of protocol violation. In the NS group (n = 118) the following needle positions were estimated: intraepineural (NS 1, n = 51), extraparaneural (NS 2, n = 33), needle tip dislocation from intraepineural to extraparaneural while injecting local anesthetic (NS 3, n = 19) and other or not determined needle positions (n = 15). Paresthesias indicated an intraneural needle position with an odds ratio of 27.4 (specificity 98.8%, sensitivity 45.9%). The success rate without supplementation was significantly higher in the US group (94.9% vs. 61.9%, p < 0.001) and the time until readiness for surgery was significantly (p < 0.001) shorter for successful blocks: 15.1 min (95% confidence interval CI 13.6-16.5 min) vs. 28 min (95% CI 24.9-31.1 min). In the NS subgroups the results were as follows (95% CI in brackets): NS1 88.2% and 22.7 min (19.5-25.9 min), NS2 24.2% and 43.3 min (35.5-51.1 min) and NS3 36.8% and 35.3 min (22.1-48.4 min).

Conclusions: For distal sciatic nerve blocks using the nerve stimulation technique, intraepineural injection of local anesthetics is common and associated with significant and clinically important higher success rates as well as shorter times until readiness for surgery. In both groups no block-related nerve damage was observed. The results indicate that for some blocks (e.g. sciatic, supraclavicular) perforation of the outer layers of connective tissue was always an important prerequisite for success using classical methods of nerve localization (cause of paresthesias and nerve stimulation technique). Additional nerve stimulation with an ultrasound-guided distal sciatic nerve block cannot make any additional contribution to the safety or success of the block. New insights concerning the architecture of the sciatic nerve are discussed and associated implications for the performance of distal ultrasound-guided sciatic nerve block are addressed.