In the October 2006 issue of Anesthesiology, Bigeleisen reported that intraneural injections do not invariably lead to neurologic injury.1After premedicating patients, Bigeleisen used a 22-gauge short bevel needle and a 10-MHz linear ultrasound transducer to anesthetize brachial plexus at the axilla by guiding the needle to elicit paresthesia or piercing the fascia around the individual nerves by the sensation of a pop. In all, 72 of total of 104 nerves received an intraneural injection, without neurologic consequences.

Dr. Bigeleisen deserves accolade for taking on a responsibility of formally documenting what we long suspected: Intraneural injections indeed do not inevitably result in neurologic injury. In clinical practice of peripheral nerve blockade (PNB), injection of local anesthetic is typically followed by a latency of 10–20 min for the blockade to develop. In contrast, injections of the same local anesthetic for the same PNBs occasionally result in nearly instantaneous, dense, and unusually long-lasting nerve blockade. It is almost certain that such blocks are the result of intraneural injections and the consequent intimate exposure of neural tissue to high concentrations and volumes of local anesthetics.1,2However, the potentially hazardous clinical implications of Bigeleisen’s data deserve careful considerations.

First, intraneural injections can be extrafascicular or intrafascicular. The intraneural–extra  fascicular injections are characterized by a diffuse spread of the injectate within the epineurium with escape of the fluid into the extraneural space. Such injections indeed do not necessarily result in nerve injury.1–6In contrast, intra  fascicular injections almost invariably lead to some degree of neurologic impairment,4and possibly a substantial proximal spread of the injectate toward the neuraxis.7,8As Dr. Bigeleisen correctly points out, neurologic injury after PNBs is uncommon; therefore, his study is underpowered to draw any meaningful conclusions on the safety of intraneural injections.

Second, our ability to monitor and avoid intrafascicular injection during PNBs has been limited. Real-time monitoring of needle placement by ultrasound guidance is useful, but of inadequate resolution to avoid intrafascicular injection.6Recent data in animal models suggest that nerve stimulation with currents of less than 0.2 mA (0.1 ms) may be associated with intraneural injection9; however, nerve stimulation is inconsistent and unreliable after injection of even miniscule volumes of local anesthetic.

Third, we do not agree with Dr. Bigeleisen in that an initial injection of a small volume (2–3 ml) of local anesthetic is a satisfactory precautionary measure to avoid an intrafascicular injection. Fascicles are small structures, and injury occurs even with minute volumes of local anesthetic (≤ 0.5 ml).2,4,7,10Injections into fascicles are characterized by high opening injection pressure (≥ 20 psi), followed by a rapid decrease of injection pressure to normal as the perineurium ruptures and local anesthetic leaks out perineurally.2,7,10Therefore, intraneural injection of even small volumes of local anesthetic are hazardous without monitoring the injection pressure.11 

Fourth, Bigeleisen1reports that paresthesia results in intraneural injections in 96% of attempts; however, intraneural injection may not always result in paresthesia. These findings speak once more against the use of paresthesia-guided techniques for block placement.

In conclusion, we are in agreement with the accompanying editorial12; needles should not  be routinely inserted intraneurally in the absence of reliable monitoring to guard against an intrafascicular needle placement. We believe that for success and safety of PNBs, a combination of real-time ultrasound needle guidance along with in-line injection pressure monitoring11and avoidance of injection with stimulation of less than 0.2 mA9may prove to be the ultimate monitoring during PNBs. However, more clinical data are needed before any such monitoring can be suggested as a routine practice.

*University Hospital of Parma, Parma, Italy.

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