Optimal stimulus duration for intraoperative facial nerve monitoring

Objectives/Hypothesis: The charge delivered to the facial nerve during intr aoperative facial nerve, monitoring (IOFNM) is the product of the stimulati on amplitude and the duration for which the pulse of charge is applied, In the literature, no standard for pulse duration exists, precluding meaningfu l comparison of IOFNM between studies. The optimal stimulus pulse duration can be derived from facial nerve strength duration curve analysis and calcu lation of chronaxy, Chronaxy is directly related to the time constant, tau, of the neuronal membrane, and is a function of neuronal membrane resistanc e and capacitance. Study Design: A prospective trial of facial nerve stimul ation in both an animal and a human model, Methods: Five rabbits and 17 hum ans underwent intraoperative stimulation of healthy facial nerves, Pulse du rations using pulses of 10, 20, 50, and 100 microseconds were employed, and the corresponding threshold stimulation amplitudes were recorded. From the se data sets, strength duration curves were plotted and chronaxy values cal culated, Results: Average chronaxy values of 18 microseconds in the rabbit and 32 microseconds in the human were found, Given IOFNM system accuracy li mitations, the optimal pulse duration for facial nerve stimulation is 50 mi croseconds. Conclusion: Most commercially available intraoperative monitori ng systems employ a pulse duration default setting of 100 microseconds. Dou bling of the 50-microsecond optimal pulse duration may result in a loss of sensitivity of predictive facial nerve data. Both the stimulation amplitude and the selected pulse duration should be reported by investigators so tha t meaningful comparison of the IOFNM data in the literature can be made.