S157. Normative data of facial nerve F waves in Chinese

Introduction With the short length of the facial nerve, its F waves tend to appear before the M waves finish, limiting its clinical application. To circumvent this difficulty, we have sought to investigate the methodology on how to better elicit the facial nerve F-wave and to establish its reference values, which provides an electrophysiological basis for facial nerve assessment in clinical practice. Methods We studied 41 healthy volunteers (19 men) aged 19 to 68 years old (mean 44.3 years). Head circumference ranged from 51.0 to 58.5 cm (mean circumference, 55.0 cm). F waves were elicited by 1 Hz supramaximal stimuli of 0.2 ms duration, using AgCl surface electrodes placed on the marginal mandibular branch of the facial nerve. The cathode was positioned as distally as possible, with the anode placed 2 cm more proximally along the presumed course of the nerve. The recording electrode was put on the depressor anguli oris muscle, with a reference electrode located on the tip of the chin. The distance between stimulation and recording sites was usually kept about 2 cm. We used a gain of 200 μv/div, sweep speed at 5 ms/div and the filter width of 30 Hz-10 kHz to evaluate minimum, maximum, and mean F-wave latency, F-wave chronodispersion, F-wave conduction time (FWCT), peripheral conduction time (PCT), F ratio (FWCT/distal motor latency) and F-wave persistence. Results We successfully obtained bilaterally reliable F waves, which were clearly distinguished from the M waves. There was no significant difference in the values of F waves between men and women, or between the two sides. No correlations with age, sex, or head circumference could be detected. The measured values included minimum (8.97 ± 0.53 ms; range, 7.81–10.2 ms), maximum (9.74 ± 0.54 ms; range, 8.39–10.4 ms), and mean F-wave latency (9.39 ± 0.51 ms; range, 8.14–10.5 ms), broadly scattered F-wave chronodispersion (0.80 ± 0.31 ms; range, 0–1.77 ms), F-wave conduction time (2.92 ± 0.29 ms; range, 2.17–3.53 ms), peripheral conduction time (5.47 ± 0.31 ms; range, 4.78–6.19 ms), F ratio (1.17 ± 0.24; range, 0.72–1.90) and F-wave persistence (100% ± 0%; range, 90–100%). Conclusion We hereby established a method to elicit facial nerve F-wave by stimulating the marginal mandibular branch, and recording from the depressor anguli oris muscle and defined a range of reference values in a Chinese healthy population. These findings provide an objective basis for an evaluation of the entire length of the facial nerve.