(169) Electrocochleography in the Facial Nerve Canal

Introduction: Electrocochleography (ECochG) measures the electrophysiological responses of the cochlea. We investigated potentially effective techniques for measuring ECochG in mice so that we can then use this procedure intraoperatively and found that the facial nerve approach was reliable and reproducible.

Methods: From Apr-Sep 2022, we investigated two techniques for measuring ECochG: the tympanic (TM) technique and the facial nerve (FN) technique. This investigation was done in BALB/c mice at the University of Utah. The TM was performed by simply placing a coiled electrode on the TM. The FN approach entailed doing a post-auricular incision and subsequent blunt dissection to expose the cleidomastoideus and CN XI. A retractor then pulled the cleidomastoideus dorsally. Continued blunt dissention directed towards the external auditory meatus revealed the facial nerve. The facial nerve was followed dorsally until it descended into the facial nerve canal. A Teflon-coated silver wire was then inserted 1-2mm into the canal. This approach was adapted from a paper from Li et al (2021). In both approaches, mice were placed in a sound-attenuating booth with the negative electrode in the contralateral ear and ground electrode in the midback, and compound action potential was recorded. Auditory brain response (ABR) was completed before the techniques as a reference.

Results: When comparing ABR to the TM ECochG, the threshold was elevated. We hypothesize that some conductive hearing loss was caused by the placement of the electrode on the TM. Reproducing tympanic ECochG was difficult. The facial nerve approach had a similar threshold to ABR and placing the electrode was reproducible.

Conclusions: The FN approach served as the best choice because of its reproducibility . The placement of the electrode in the tympanic approach resulted in a conductive hearing loss (likely through dampening vibrations of the TM) and was difficult to reproduce. Because the facial nerve approach was more effective, we plan to use this technique as a tool to help elucidate the mechanism of injury in cochlear implantation.