Assessment of Cochlear Function during Cochlear Implantation by Extra- and Intracochlear Electrocochleography

Abstract

Objective: The aims of this study were: (1) To investigate the correlation between electrophysiological changes during cochlear implantation and postoperative hearing loss, and (2) to detect the time points that electrophysiological changes occur during cochlear implantation. Material and Methods: Extra- and intracochlear electrocochleography (ECoG) were used to detect electrophysiological changes during cochlear implantation. Extracochlear ECoG recordings were conducted through a needle electrode placed on the promontory; for intracochlear ECoG recordings, the most apical contact of the cochlear implant (CI) electrode itself was used as the recording electrode. Tone bursts at 250, 500, 750, and 1000 Hz were used as low-frequency acoustic stimuli and clicks as high-frequency acoustic stimuli. Changes of extracochlear ECoG recordings after full insertion of the CI electrode were correlated with pure-tone audiometric findings 4 weeks after surgery. Results: Changes in extracochlear ECoG recordings correlated with postoperative hearing change (r = -0.44, p = 0.055, n = 20). Mean hearing loss in subjects without decrease or loss of extracochlear ECoG signals was 12 dB, compared to a mean hearing loss of 22 dB in subjects with a detectable decrease or a loss of ECoG signals (p = 0.0058, n = 51). In extracochlear ECoG recordings, a mean increase of the ECoG signal of 4.4 dB occurred after opening the cochlea. If a decrease of ECoG signals occurred during insertion of the CI electrode, the decrease was detectable during the second half of the insertion. Conclusion: ECoG recordings allow detection of electrophysiological changes in the cochlea during cochlear implantation. Decrease of extracochlear ECoG recordings during surgery has a significant correlation with hearing loss 4 weeks after surgery. Trauma to cochlear structures seems to occur during the final phase of the CI electrode insertion. Baseline recordings for extracochlear ECoG recordings should be conducted after opening the cochlea. ECoG responses can be recorded from an intracochlear site using the CI electrode as recording electrode. This technique may prove useful for monitoring cochlear trauma intraoperatively in the future.

Keywords: cochlear implant; cochlear implantation; cochlear trauma; electrocochleography; hearing preservation; residual hearing.

Figures

Figure 1

Two examples of typical ECoG responses before insertion of the CI electrode. (A,B) show the time waveform (A) and the corresponding spectrum (B) of an ECoG signal in response to a sinusoidal tone burst with alternating starting phases at 500 Hz, 95 dB nHL (S54). The blue line represents the difference, the red line the average of the responses with alternating polarity. The black rectangle (A) marks the time window, used for the spectral analysis. (C) Displays an ECoG signal in response to an acoustic click stimulus at 95 dB nHL (S43). Only the average of the responses with alternating starting phases is shown. A clear CAP is visible.

Figure 2

Correlation between the change of the low-frequency ECoG response immediately after full insertion of the CI electrode array (Δ Low-frequency ECoG response) and the change of the pure-tone average 4 weeks after surgery (Δ PTA) (Pearson correlation coefficient, r = −0.44, p = 0.055, n = 20).

Figure 3

Two examples of a decrease of ECoG signals after insertion of the CI electrode. (A,B) show the ECoG response (only the difference curve is shown) in response to a sinusoidal tone burst at 250 Hz, 85 dB nHL before and after insertion of the CI electrode. A decrease of the response amplitude after insertion is visible in the time waveform (A) and the corresponding spectrum (B) (S64). In S66 (C), a decrease of the CAP amplitude in response to an acoustic click stimulus at 95 dB nHL was detectable after insertion of the CI electrode.

Figure 4

Correlation of hearing loss 4 weeks after surgery with intraoperative ECoG findings. The mean postsurgical hearing loss was 12 dB (standard error of the mean 1.4 dB, n = 41) in subjects with no detectable decrease of ECoG signals after insertion of the CI electrode and 22 dB (standard error of the mean 4 dB, n = 10) in subjects with decrease of high- or low-frequency ECoG signals (Unpaired t-test, p = 0.0058).

Figure 5

Mean change of the ongoing ECoG signal in extracochlear ECoG recordings during insertion of the CI electrode.

Figure 6

Changes of ECoG signals in intracochlear ECoG recordings during insertion. The most apical contact of the CI electrode itself was used as recording electrode.