Cochlear Microphonic Changes During Cochlear Implantation are Consistent with Hair Cell Survival and Electrode Contact on the Basilar Membrane

Background: Sudden decreases in cochlear microphonic amplitude during cochlear implantation are associated with poor preservation of residual hearing post-operatively. Here we describe changes in the amplitude, latency, and peak frequency response of the microphonic during these events consistent with the electrode array contacting the basilar membrane.

Objective: Correlate specific properties of ECochG intra-operatively with loss of residual and post-operative outcomes.

Methods: Intra-operative electrocochleography was recorded from the apical electrode during implantation in 88 recipients of Cochlear’s slim-straight lateral wall electrode (CI522). Patients had audiometric thresholds of between 20- and 90-dB HL at 0.5-kHz. Acoustic responses were recorded using a 0.5-kHz stimulus tone presented with alternating polarity at 100- or 110-dB HL. The cochlear frequency place (CFP) was derived from CT imaging. Audiometric outcomes were collected pre-operatively and 3-months post-operatively.

Results: During implantation, 59 patients showed a drop of cochlear microphonic amplitude with a loss of hearing. Of these, the peak FFT amplitude shifted (up or down) in 34, and in 17/34 the latency of the microphonic decreased. For electrodes at the 1-, 2-, and 4-kHz CFP, the latency shift between adjacent electrodes (when stimulated at the characteristic frequency) was negatively correlated with hearing loss.

Conclusions: We argue that: (1) the changes in latency and FFT-derived frequency of the cochlear microphonic could have resulted from damping of the basilar membrane, caused by contact with the electrode array, (2) a shift in latency between adjacent electrodes, measured post-operatively, is a biomarker for local outer hair cell survival.