Laryngeal Muscle-Evoked Potential Recording as an Indicator of Vagal Nerve Fiber Activation

Background: Vagus nerve stimulation (VNS) is an adjunctive therapy for drug-resistant epilepsy. Noninvasive evoked potential recordings in laryngeal muscles (LMEPs) innervated by vagal branches may provide a marker to assess effective vagal nerve fiber activation. We investigated VNS-induced LMEPs in patients with epilepsy in acute and chronic settings. Materials and Methods: A total of 17 of 25 patients underwent LMEP recordings at initiation of therapy (acute group); 15 of 25 patients after one year of VNS (chronic group); and 7 of 25 patients were tested at both time points (acute + chronic group). VNS-induced LMEPs were recorded following different pulse widths and output currents using six surface laryngeal EMG electrodes to calculate input/output curves and estimate LMEP latency, threshold current for minimal (I-threshold), half-maximal (I-50), and 95% of maximal (I-95) response induction and amplitude of maximal response (V-max). These were compared with the acute + chronic group and between responders and nonresponders in the acute and chronic group. Results: VNS-induced LMEPs were present in all patients. I-threshold and I-95 values ranged from 0.25 to 1.00 mA and from 0.42 to 1.77 mA, respectively. Estimated mean LMEP latencies were 10 +/- 0.1 milliseconds. No significant differences between responders and nonresponders were observed. In the acute + chronic group, I-threshold values remained stable over time. However, at the individual level in this group, V-max was lower in all patients after one year compared with baseline. Conclusions: Noninvasive VNS-induced LMEP recording is feasible both at initiation of VNS therapy and after one year. Low output currents (0.25-1.00 mA) may be sufficient to activate vagal A alpha-motor fibers. Maximal LMEP amplitudes seemed to decrease after chronic VNS therapy in patients.

Automated summary

This study investigated laryngeal muscle evoked potential (LMEP) responses induced by vagus nerve stimulation (VNS) in patients with epilepsy. The results showed that recording VNS-induced LMEPs was feasible both at the initiation of therapy and after one year. Low output currents may be sufficient to activate vagal A alpha-motor fibers. However, the maximal amplitude of LMEPs seemed to decrease after chronic VNS therapy.