Intra- and Postoperative Electrocochleography May Be Predictive of Final Electrode Position and Postoperative Hearing Preservation

Abstract

Introduction: The objectives of the current study were to (1) determine the relationship between electrocochleography (ECochG), measured from the cochlear implant (CI) electrode array during and after implantation, and postoperative audiometric thresholds, (2) determine the relationship between ECochG amplitude and electrode scalar location determined by computerized tomography (CT); and (3) determine whether changes in cochlear microphonic (CM) amplitude during electrode insertion were associated with postoperative hearing. Materials and Methods: Eighteen subjects undergoing CI with an Advanced Bionics Mid-Scala device were prospectively studied. ECochG responses were recorded using the implant coupled to a custom signal recording unit. ECochG amplitude collected intraoperatively concurrent with CI insertion and at activation was compared with audiometric thresholds postoperatively. Sixteen patients also underwent postoperative CT to determine scalar location and the relationship to ECochG measures and residual hearing. Results: Mean low-frequency pure tone average (LFPTA) increased following surgery by an average of 28 dB (range 8-50). Threshold elevation was significantly greater for electrodes with scalar dislocation. No correlation was found between intraoperative ECochG and postoperative behavioral thresholds collapsed across frequency; however, mean differences in thresholds measured by intraoperative ECochG and postoperative audiometry were significantly smaller for electrodes inserted completely within scala tympani (ST) vs. those translocating from ST to scala vestibuli. A significant correlation was observed between postoperative ECochG thresholds and behavioral thresholds obtained at activation. Discussion: Postoperative audiometry currently serves as a marker for intracochlear trauma though thresholds are not obtained until device activation or later. When measured at the same time-point postoperatively, low-frequency ECochG thresholds correlated with behavioral thresholds. Intraoperative ECochG thresholds, however, did not correlate significantly with postoperative behavioral thresholds suggesting that changes in cochlear physiology occur between electrode insertion and activation. ECochG may hold clinical utility providing surgeons with feedback regarding insertion trauma due to scalar translocation, which may be predictive of postoperative hearing preservation. Conclusion: CI insertion trauma is generally not evident until postoperative audiometry when loss of residual hearing is confirmed. ECochG has potential to provide estimates of trauma during insertion as well as reliable information regarding degree of hearing preservation.

Keywords: audiometry; cochlear implant; cochlear microphonic; electrocochleography; hearing loss; hearing preservation; residual hearing.

Figure 1

Pre- and postoperative pure-tone thresholds; each symbol represents an individual patient. Scalar location of electrode, when available, has been denoted (the ^* represents the electrode abutting the basilar membrane). Diagonal lines are used to depict hearing preservation in relation to pure-tone average (PTA) shift bins as follows: PTA shift <15 dB, PTA shift between 15–30 dB, and PTA shift >30 dB.

Figure 2

Scatter plot of low-frequency pure-tone average (PTA) shift depicted according to scalar electrode location. Lower median shift (i.e., better hearing preservation) was noted when comparing electrodes inserted entirely into the scala tympani (ST) to electrodes that translocated into scala vestibuli (SV). Shown are the median and the range of the 25–75th percentile.

Figure 3

The relationship between intraoperative ECochG thresholds, in dB HL, and postoperative behavioral thresholds, also in dB HL, for 125, 250, and 500 Hz are depicted in the entire cohort, and for those cases in which scalar location is known. Bonferroni correction is applied for multiple comparisons, with p < 0.017 indicative of statistical significance. The diagonal and dotted lines represent the ±20 dB difference between ECochG thresholds and behavioral thresholds.

Figure 4

The relationship between postoperative ECochG thresholds and postoperative behavioral thresholds for 125, 250, and 500 Hz frequencies are depicted in the entire cohort, and for those cases in which scalar location is known. Bonferroni correction is applied for multiple comparisons, with p < 0.017 indicative of statistical significance.

Figure 5

Bland-Altman plots depict the average and difference between postoperative behavioral and ECochG thresholds at 125, 250, and 500 Hz. The 95% limits of agreement are shown as two dotted lines. The biases, or average of the differences at each frequency, are reported.

Figure 6

Change in cochlear microphonic (CM) amplitude, in dB re: microVolts, during insertion is shown according to scalar location of the electrode array.