Intraneural Ultrasound-guided Sciatic Nerve Block: Minimum Effective Volume and Electrophysiologic Effects

Posted on by Dr Clemens
Authors: Gianluca Cappelleri, M.D. et al
Anesthesiology 8 2018, Vol.129, 241-248.
What We Already Know about This Topic:

  • Reducing the volume of local anesthetic administered for neural blockade may increase safety

  • Local anesthetic administration can result in nerve damage

What This Article Tells Us That Is New:

  • The low volume intraneural injection of ropivacaine 1% provided complete sensory-motor nerve block

  • Reductions in action potential amplitudes lasting at least 6 months from the time of nerve block suggest that additional safety studies will be required

Background: Both extra- and intraneural sciatic injection resulted in significant axonal nerve damage. This study aimed to establish the minimum effective volume of intraneural ropivacaine 1% for complete sensory-motor sciatic nerve block in 90% of patients, and related electrophysiologic variations.

Methods: Forty-seven consecutive American Society of Anesthesiologists physical status I-II patients received an ultrasound-guided popliteal intraneural nerve block following the up-and-down biased coin design. The starting volume was 15 ml. Baseline, 5-week, and 6-month electrophysiologic tests were performed. Amplitude, latency, and velocity were evaluated. A follow-up telephone call at 6 months was also performed.

Results: The minimum effective volume of ropivacaine 1% in 90% of patients for complete sensory-motor sciatic nerve block resulted in 6.6 ml (95% CI, 6.4 to 6.7) with an onset time of 19 ± 12 min. Success rate was 98%. Baseline amplitude of action potential (mV) at ankle, fibula, malleolus, and popliteus were 8.4 ± 2.3, 7.1 ± 2.0, 15.4 ± 6.5, and 11.7 ± 5.1 respectively. They were significantly reduced at the fifth week (4.3 ± 2.1, 3.5 ± 1.8, 6.9 ± 3.7, and 5.2 ± 3.0) and at the sixth month (5.9 ± 2.3, 5.1 ± 2.1, 10.3 ± 4.0, and 7.5 ± 2.7) (P < 0.001 in all cases). Latency and velocity did not change from the baseline. No patient reported neurologic symptoms at 6-month follow-up.

Conclusions: The intraneural ultrasound-guided popliteal local anesthetic injection significantly reduces the local anesthetic dose to achieve an effective sensory-motor block, decreasing the risk of systemic toxicity. Persistent electrophysiologic changes suggest possible axonal damage that will require further investigation.

The best place to perform local anesthetic injection during an ultrasound-guided sciatic nerve block is an unresolved issue. In the sciatic nerve, ultrasound allows for distinguishing the connective layer surrounding the nerve tissue (paraneural sheath), and differentiating an external circumferential injection from a subparaneural injection under the sheath and out of the epineurium. The local anesthetic injection in the connective tissue, which is inside the epineurium and among the nerve fascicles, is usually defined as intraneural. The intraneural injection provides a faster onset with a much greater success rate compared to the subparaneural ones. Unfortunately, the safety of this technique still remains questionable, and not extensively investigated. In a previous trial, we demonstrated that the electrophysiologic impairment of the intraneural local anesthetic injection was similar to those that resulted from the extraneural local anesthetic injection. In fact, both techniques resulted in a similar, significant reduction of amplitude of the action potential 5 weeks after surgery compared to the baseline. Considering the higher local anesthetic dose investigated (15 ml ropivacaine, 1%) in previous study, our goals are now to extend the electrophysiologic findings to assess if the worrisome reduction in amplitude of the action potential may be minimized by limiting the dose of anesthetic, and also evaluating if recovery is complete in a longer timeframe of 5 weeks.
Therefore, the purposes of this prospective, up-and-down study were: (1) to assess the minimum effective volume of ropivacaine 1% to achieve a complete sensory-motor sciatic nerve block in 90% of patients, using an ultrasound-guided intraneural popliteal approach; and (2) to evaluate any variations of the electrophysiologic test both at 5 weeks and 6 months after surgery, considering the administered dose.

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