Effect of local anesthesia on trigeminal somatosensory-evoked magnetic fields

For objective neurophysiological evaluation of the function of the trigeminal system, magnetoencephalography- based TSEF (trigeminal somatosensory-evoked field) assessment would be valuable in providing spatial and temporal profiles of cortical responses. However, this necessitates knowledge of how TSEF varies with trigeminal nerve dysfunctions. We introduced a conduction block of the trigeminal nerve using local anesthesia (lidocaine) to temporally mimic nerve dysfunctions, and monitored TSEF changes. Following an electrical stimulation of the lower lip, a magnetic response with peak latency of approximately 20 ms was identified in all participants. Dipole for the peak was estimated on the post-central gyrus in the participant's own magnetic resonance image. After normalization to Montreal Neurological Institute (MNI) space and inter-participant data integration, the summary equivalent current dipole localization among participants remained in the post-central gyrus, suggesting validity of the use of MNI space. Partial anesthesia of the lower lip led to a loss of the waveform characteristics of TSEF for electrical stimulation to the trigeminal nerve. We verified that the 20-ms latency cortical response of TSEF components localized at the primary sensory cortex can serve as a robust neurofunctional marker of experimental trigeminal nerve dysfunction.