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.