Trigeminal Somatosensory Evoked Potentials During Neurosurgical Procedures Using Electrical Stimulation of the Lips Under General Anesthesia

Background Intraoperative neurophysiology monitoring is rapidly evolving with the advent of newer modalities. Long latency sensory evoked potentials from the trigeminal nerve distribution have rarely been demonstrated during neurosurgical procedures. Trigeminal sensory evoked potential (TSEP) can be used to prevent nerve injury during surgical procedures, such as those for trigeminal neuralgia and tumors involving the trigeminal nerve and pathway. Methodology We attempted to record TSEP from 12 subjects who underwent various neurosurgical procedures with low doses of inhalational anesthetic agents. We stimulated the upper and lower lip and recorded from C6 and Fz locations. We used 14-17 mA current stimuli with a pulse width of 50-150 microseconds and a stimulation rate of 2.1 Hz. Results We could obtain a clear, reproducible TSEP response in two out of 12 subjects. We observed a TSEP waveform with negative peaks at 13 and 27 milliseconds and a positive wave at around 19 milliseconds. Conclusions The TSEP produced by the electrical stimulation of the upper and lower lip can be detected from the scalp C5, C6, and Fz area even during neurosurgical procedures, even if inhalational anesthesia was used at induction, but only in a small proportion of cases. It appeared to reflect the activity of trigeminal cortical response. Avoiding the notch filter and turning off the inhalational agents are essential for a good response.