Ocena wpływu zmiany kształtu impulsu stymulującego na stymulację nerwu twarzowego u dzieci korzystających z implantu ślimakowego Assessment of the effectiveness of changing the shape of the stimulus pulse for facial nerve stimulation in cochlear implanted children

Background: Unwanted stimulation of the facial nerve, which occurs occasionally in cochlear implant users, leads to involuntary contraction of the facial muscles as a result of the flow of current used to induce auditory sensations. The occurrence of facial nerve excitation often leads to reduction of stimulation levels, which finally causes too quiet hearing through the implant. In order to reduce the undesirable non-acoustic sensations, Med-El proposed stimulating pulses shape modification. Material and methods: The study involved 6 children aged 6 to 15, users of the Med-El cochlear implant system. The study involved patients who showed facial nerve stimulation during stimulation by the cochlear implant, and able to subjectively determine the impression of loudness using a categorical scale. Loudness scaling was performed by conducting stimulation through a single electrode. The test was performed for each active electrode using both standard two-phase stimulation and three-phase stimulation. Moreover, for each stimulated electrode and for each type of stimulation, the threshold for the occurrence of facial nerve stimulation and the subjective loudness for the obtained threshold values were determined. Based on the loudness scaling, two programs were suggested for each patient, one using biphasic stimulation and one using triphasic stimulation. The values of the stimulating charge for the individual electrodes of the implant were set below the threshold of the facial nerve stimulation, and if there was no stimulation of the facial nerve, they were set at the level of comfortable hearing. Then, using speech as a test signal, the loudness sensation under listening conditions by the cochlear implant system was compared for both programs that differed in the method and value of the charge used for stimulation. Results: The facial nerve irritation thresholds for the three-phase stimulation were 10.6 qu higher than the facial nerve irritation thresholds for the two-phase stimulation. The mean value of the MCL levels obtained for the standard biphasic stimulation was 32 qu, comparing to 42.1 qu for the three-phase stimulation. Conclusions: Unwanted stimulation of the facial nerve can be reduced by changing the stimulus shape from biphasic to triphasic while maintaining an appropriate loudness sensation of sounds heard through the cochlear implant system.