Hypothesis: Triphasic pulse stimulation of the auditory nerve can prevent unintended facial nerve stimulation (FNS) due to a different electromyographic (EMG) input-output function compared with biphasic pulses.
Background: FNS is sometimes observed in cochlear implant (CI) users as an unpleasant side effect of electrical stimulation using biphasic pulse patterns (BPP). Clinical remedies to alleviate FNS are 1) to extend stimulus phase duration or 2) to completely deactivate the electrode. In some cases, these options do not provide sufficient FNS reduction or are detrimental to subject performance. Stimulation using triphasic pulse patterns (TPP) has been shown to prevent FNS more effectively, yet the underlying mechanism remains unclear.
Methods: EMG potentials of muscles innervated by the facial nerve (orbicularis oculi and oris muscles) were recorded to quantitatively compare the effect of BPP and TPP stimulation on FNS. Recordings were conducted in five subjects during CI surgery. In two exemplary cases, different leading phase polarities in alternating and non-alternating order were tested.
Results: Compared with our previous study in awake patients using surface electrodes (Bahmer and Baumann, 2016), intraoperative recordings using subdermal electrodes showed lower noise content and allowed higher sampling resolution. While inter-subject variation remained high, intra-subject results for different electrode positions were comparable: FNS was strongly reduced for cathodic-first TPP stimulation. In contrast, exemplary cases showed little reduction for anodic-first TPP as well as for alternating stimulation.
Conclusion: FNS in CI users can be reduced using TPP stimulation, but the ameliorative effect appears to be dependent on the leading stimulus polarity.