Facial nerve stimulation in cochlear implant stimulation: effects of pulse polarity

Introduction 

The facial nerve stimulation (FNS) is an adverse site effect in cochlear implant (CI) stimulation. Approaches to avoid FNS are silencing the affected CI contacts and shifting to triphasic pulses. However, when switching the pulse shape, also the polarity of the dominant phase (i.e. 1<sup>st</sup > in biphasic and 2<sup>nd</sup > in triphasic) is changed. We assessed influences of stimulation polarity on FNS thresholds in an animal model.

Methods 

We used monopolar, pseudo-monophasic (psm) and biphasic CI-stimulation with charge-balanced pulses of alternating polarity. In electrically-evoked compound action potential (eCAP) recordings in 33 ears of 26 guinea pigs, we assessed the FNS thresholds, via custom-made Matlab routines. Additionally, wWe defined the offset to the eCAP-threshold in 254 cases. The 3 dimensional distance of each CI contact to the facial nerve was assessed using micro computer tomography images.

Results 

FNS was detected in 67% (n=192) of biphasic and 51 %, (n=81) of psm stimulations. The CI-contacts with lowest FNS thresholds had shortest distance to the facial nerve. The FNS was on average 6 dB above the eCAP threshold (max: 14 dB). In 83% of pairwise comparisons cathodic stimulation lead to lower FNS threshold than anodic stimulation. Correspondingly, the eCAP-to-FNS offset was significantly larger for anodic than cathodic, biphasic stimulation (median: 7 dB versus 5 dB).

Conclusion 

We conclude that the polarity of the spike-initating phase is a critical factor for FNS thresholds. We suggest that this factor should be tested for its clinical relevance for human CI-users and is taken into account in speech processor programming.

This work was supported by the MHHplus foundation.

Conflict of Interest

The author declares that there is no conflict of interest.

Publication History

Article published online:
24 May 2022