Pathophysiology of Facial Nerve Stimulation and Its Implications for Electrical Stimulation in Cochlear Implants.

Abstract

A small number of cochlear implant (CI) users experience facial nerve stimulation (FNS), which can manifest as facial twitching. In some patients, this can be resolved by adjusting the electrical stimulation parameters. However, for others, facial stimulation can significantly impair CI outcomes or even prevent its use. The exact mechanisms underlying FNS are unclear and may vary among patients. Transimpedance measurements were used to assess lateral and longitudinal spread of current within 15 cochlea of nucleus CI recipients with FNS (13 unilateral recipients and 1 bilateral recipient). We compared the transimpedance measurements with programming parameters from clinical visits and pre- and postoperative temporal bone computed tomography (CT) scans to identify factors that may contribute to FNS in each CI ear. In nine ears, transimpedance curves showed inflection, which suggests a localized current sink within the cochlea. This indicates a low-impedance pathway through which current exits the cochlea and stimulates the labyrinthine segment of the facial nerve canal. Electrodes near this current sink were disabled or underfit to minimize facial stimulation. In the other seven ears, current flow peaked toward the basal end of the cochlea, suggesting that current exits through the round window or other structures near the basal end of the cochlea, stimulating the tympanic segment of the facial nerve. Objective transimpedance measurements can be used to elucidate the mechanisms of FNS and to develop strategies for optimizing electrical stimulation parameters and speech coding to minimize or eliminate FNS in a small subset of CI users.