Unmasking Local Anesthetic Injection Injury in the Peripheral Nerve

Purpose: Local anesthetics (LA) are frequently used for many operative procedures. A well-known complication of LA blocks is peripheral nerve injury, whether from the needle or toxicity of the medication used. The purpose of this study was to illustrate the extent of damage that results from inadvertent intraneural injection of various anesthetics into the peripheral nerve.

Methods and Materials: Two studies were conducted: Group A used a small volume of injectate (8µL), while Group B, used a larger volume (50µL). In Group A, 80 Lewis rats received an injection of saline (negative control), phenol (positive control), or one of three LAs (Lidocaine, Ropivacaine, Bupivacaine) via an extrafascicular or intrafascicular route into the tibial nerve (n=8 per subgroup). In Group B, 16 Lewis rats received an intrafascicular injection of saline (control) or one of the three LAs in the sciatic nerve (n=4 per subgroup). At the 2 week endpoint, nerves from both groups nerve were harvested for histomorphometric analysis. Additionally, animals in Group A underwent functional testing by muscle force analysis prior to nerve harvest, and Group B nerves were further analyzed by electron microscopy.

Results: Small volume LA injections in Group A resulted in no injury and no difference in functionality testing. In contrast, animals receiving larger volume injections in Group B showed increased severity of injury as compared to control. In particular there was a significant loss of large diameter fibers as indicated by decreased counts (p<0.001 for all LAs) and area (p<0.001 for all LAs) of remaining fibers in severely-injured versus non-injured areas of the nerve. No difference in the extent of injury was found between the three different LAs. In all groups, fascicular transection injury from the needle was observed. Electron microscopy confirmed neural injury.

Conclusions: Frequently used LAs at traditional concentrations can injure the peripheral nerve as evidenced by our data. There is a correlation with volume and intrafascicular injection. The nature of injury can vary between 1^st and 6^th degree as was seen in the study. Varying fascicular topography may result in a combined motor and sensory deficit, or motor or sensory alone. Surgical repair may be obligatory dependent on the severity of the injury.