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Fitzpatrick et al. (2014) recorded ECochG responses at the round window intraoperatively prior to CI insertion in 21 adults and subsequently correlated ECochG magnitude with postoperative CNC word recognition scores. In this study, the metric for ECochG magnitude was termed total response (TR) and defined as the sum of all significant first and second harmonic responses across all frequencies at the highest sound level (90 dB nHL). They reported that TR accounted for 47% of variability in outcomes on the CNC word recognition task making it, at the time, the highest known predictor of CI outcomes even over other predictors such as duration of deafness (<25%; e.g., Rubinstein et al., 1999; Friedland et al., 2003; Plant et al., 2016) and degree of residual hearing (e.g., Plant et al., 2016). Scott et al. (2016) completed intraoperative ECochG with a needle electrode at the round window prior to electrode insertion for 238 CI recipients with postoperative CNC word recognition obtained for 51 adult CI recipients. Similar to Fitzpatrick et al. (2014), they found a significant correlation between TR and CNC word recognition at 6 months post activation (r = 0.43); however, the ECochG CAP only weakly correlated postoperative word recognition (r = 0.20, p < 0.001). Thus, while ECochG appears to be a promising measure for helping explain postoperative outcomes, much additional research is needed to carefully investigate this relationship.

Researchers have also investigated the relationship between intraoperative ECochG and acoustic hearing preservation in the implanted ear. Adunka et al. (2016) recorded ECochG at the round window before and after CI insertion and found no correlation between the ECochG response and postoperative residual hearing as measured by audiometric thresholds—though the results may have been limited by the extracochlear nature of the recording electrode.

ECochG can also be recorded using the CI electrode array which offers advantages given its proximity to the organ of Corti. Koka et al. (2016) measured difference and summation responses from ECochG waveforms postoperatively from patients with residual hearing and compared with behavioral audiometric thresholds. The group found that 87% percent of the variability in postoperative behavioral audiometric thresholds across all frequencies tested could be predicted by difference response thresholds and 82% predicted by summation response thresholds; concluding that ECochG thresholds may be useful to estimate postoperative preserved acoustic hearing in CI patients who cannot participate in behavioral audiometry.

Campbell et al. (2016) recorded ECochG measurements intraoperatively from the CI array in 18 recipients with residual acoustic hearing and (1) explored providing real-time surgical feedback as well as (2) investigated the correlation between ECochG recordings and postoperative acoustic hearing. They found this method to be potentially useful for providing feedback regarding surgical trauma and that patients who had a preserved ECochG at the end of surgery were more likely to have preserved hearing. In fact, postoperative audiometric thresholds for patients with preserved CM were, on average, 15 dB better than individuals without a preserved ECochG. Similar findings were reported by Acharya et al. (2016) for two pediatric patients.

Building on this previous work, in the present study intracochlear ECochG responses were measured for 18 (n = 18) adult Advanced Bionics (AB) CI recipients with preoperative acoustic hearing in the ear to be implanted. ECochG measurements were made both during and after CI insertion, and these measures were compared with pre- and postoperative audiometric thresholds. Sixteen (n = 16) participants also underwent postoperative computerized tomography (CT) scanning to verify scalar placement. The objectives of the current study were (1) to determine the relationship between ECochG, measured from the CI array either during cochlear implantation or after surgery, and postoperative audiometric thresholds, (2) to determine if the CM amplitudes correlated with electrode scalar location/translocation as determined by CT scanning, and (3) to determine if change in CM during electrode insertion is associated with postoperative residual hearing.

Adult patients with residual acoustic hearing (≤80 dB HL at 250 Hz) who were seeking cochlear implantation with an Advanced Bionics (AB) Mid-Scala device between April and December 2016 were prospectively recruited for participation. Exclusion criteria included previous history of middle ear surgery, sudden sensorineural hearing loss, auditory neuropathy spectrum disorder (ANSD), single-sided deafness, and/or abnormal anatomy as detected by CT or MRI scanning. Eighteen (n = 18) subjects met inclusion criteria and were implanted by one of five cochlear implant surgeons using a round window (n = 14) or extended round window approach (n = 4). Patient demographics are shown in Table 1. The methods used in this study were in accordance with the ethical standards of the institutional review board at Vanderbilt University (IRB approval: 151808), and all subjects provided written informed consent before participation.

The equipment used for data collection was previously described by Koka et al. (2016). The Bionic Ear Data Collection System (BEDCS) was used to measure ECochG responses. A NI DAQ system (NI DAQ 6216, National Instruments Corporation, 11500 Mopac Expwy, Austin, TX) and an audio amplifier (Sony PHA-2, Sony Corporation, New York, NY) were used to generate the acoustic stimuli, which was presented through an ER-3A (Etymotic Research, Inc. 61 Martin Lane, Elk Grove Village, IL) insert earphone. An ER-7 (Etymotic Research, Inc. 61 Martin Lane, Elk Grove Village, IL) probe microphone was used to calibrate and monitor the stimulus level in the ear canal. The ECochG response was measured using an AB Clinical Programming Interface Platinum Series Sound Processor (PSP) and Universal Headpiece (UHP) with additional magnets for retention and secure connection.

Pure-tone audiometry was assessed prior to implantation and at activation approximately 2–3 weeks after surgery. Audiometric thresholds were completed in a double-walled sound treated booth. Air-conduction thresholds were obtained for all octaves and inter-octave frequencies from 125 to 8,000 Hz using an insert earphone. Bone-conduction thresholds were obtained for octave frequencies from 500 to 4,000 Hz using a bone oscillator placed on the mastoid. Contralateral masking was implemented when appropriate. Low-frequency PTA was calculated using the average of unaided air-conduction thresholds at 125, 250, and 500 Hz.

ECochG potentials were measured from the most apical electrode of the implant array intraoperatively as the surgeon was inserting the CI and postoperatively at each subject's CI activation. Intraoperatively, after the patient was intubated, an ER-3A (Etymotic Research, Inc. 61 Martin Lane, Elk Grove Village, IL) insert earphone and an ER-7 (Etymotic Research, Inc. 61 Martin Lane, Elk Grove Village, IL) probe microphone were placed in the external auditory canal of the surgical ear (See Koka et al., 2016, Figure 1). Since the insert earphone and probe microphone were not sterilized, these pieces were kept out of the sterile field by folding the pinna anteriorly and securing it with a large Tegaderm® transparent adhesive film dressing (3M, 2501 Hudson Rd., Maplewood, MN) taking caution to not compromise the tube delivering sound to the ear. At this point, calibration was completed to ensure that the tube was not crimped or that the insert placement was faulty. The cables/tubes connecting the insert earphone and probe microphone to the measurement equipment were then disconnected, wrapped in a cloth, and placed underneath the surgical table so as to minimize interference with the surgical procedure. The surgical preparation (i.e., sterilization and draping) and surgical procedure (cortical mastoidectomy, facial recess, and round window exposure) then progressed according to normal protocols until just before insertion of the electrode array at which point the cables/tubes were reconnected to the recording equipment and the Universal Headpiece and cable were covered with a sterile ultrasound bag and magnetically coupled to the patient's newly implanted receiver/stimulator. Calibration was repeated, and the ECochG recording was started. The CI electrode was introduced via the round window or extended round window and inserted according to the manufacturer's recommendations (i.e., insertion with the stylet to the first blue marker at which point the pre-curved electrode was advanced off the stylet until the second blue marker was located at the round window). The surgeon reported a full insertion in all cases. While the surgeon was inserting the electrode, the audiologist used markers to identify different key points during the surgery (i.e., round window, first blue marker, second blue marker, complete insertion). For the duration of electrode insertion and ECochG insertion, an acoustic tone burst was delivered via the insert earphone (500-Hz, toneburst, 110 dB SPL or 97 dB HL, alternating polarity, 50-ms duration with 5-ms onset/offset ramp time) while the ECochG response was recorded from the most apical electrode. The neural response imaging (NRI) amplifier in the implant was used for amplification of the response (gain of 1,000). The recordings were done with alternating polarities (2 rarefaction and 2 condensation traces) and averaged in the implant amplifier, then transferred to the processor. Data plotting for the insertion tracks depends on SNR of the signal, which usually averages and plots at a single point until SNR reaches 18 dB, or 8 averages have been performed (internally 16 averages). The SNR benefit can be achieved by 55 ms recordings that can be seen in frequency spectrum with larger acquisition times; the acquisitions were done at 4–6 stimuli per second. In presenting this data, the CM amplitude during the insertion track is normalized with respect to the amplitude obtained at the round window, therefore values are presented as dB. After insertion was complete, the recording electrode was changed to 1, 5, 9, and then 13; additional ECochG measurements were obtained from these electrodes to try and understand electrode location with respect to the 500-Hz stimulus. Subsequently, the stimulus frequency was changed from 125 to 2,000 Hz in octave steps using electrode 1 as the recording electrode to estimate each subject's CM threshold in dB HL at each frequency. Surgery concluded per standard. It is estimated that intraoperative ECochG testing added approximately 5 min of time to each case. It should be noted that for this study, the surgeon was not informed of the ECochG results during the insertion of the electrode.

Postoperative ECochG measurement occurred in the audiology clinic on the same day as the patient's CI activation appointment, typically 2 weeks after surgery. An ER-3A (Etymotic Research, Inc. 61 Martin Lane, Elk Grove Village, IL) insert earphone and an ER-7 (Etymotic Research, Inc. 61 Martin Lane, Elk Grove Village, IL) probe microphone were placed in the external auditory canal of the implanted ear, and the Universal Headpiece was coupled with the patient's receiver stimulator. Calibration was completed to ensure that the tube was not crimped or that the insert placement was faulty. Tone bursts were presented sequentially at 125, 250, 500, 1,000, and 2,000 Hz. The patient's ECochG response was measured from the apical electrode and recorded for each frequency. These frequency scan responses were used to estimate subjects' CM thresholds.

The amplitude of the ECochG response was calculated using fast Fourier transformation (FFT) analysis within the Bionic Ear Data Collection System. A sample rate of 9,280 and a low pass filter of 5 kHz in the NRI amplifier were used to acquire the responses over a 54.5 ms recording duration through back-telemetry.

A subset (n = 16) of patients received postoperative CT scans using a low-dose, flat-panel, volumetric computerized tomography machine (Xoran XCAT, Xoran Technologies; Ann Arbor, MI). Using previously described and validated image-processing algorithms (Noble et al., 2011) scans were analyzed for scalar location of the electrode array (Noble et al., 2011). ST insertions were defined as insertions in which all electrode contacts were located entirely within the ST. Conversely, SV insertions were characterized by electrode arrays that translocated from the ST into the SV, such that at least one electrode contact was located within the SV.

Data were plotted and analyzed using GraphPad Prism 7.0 software (GraphPad Software Inc, 2012). Continuous variables were tested for normal distribution with D'Agostino and Pearson omnibus normality test.

Correlations were performed to examine the relationships between ECochG thresholds and behavioral thresholds at individual frequencies (125, 250, and 500 Hz). Parametric and nonparametric data were examined using a Pearson or Spearman correlation analysis, respectively. Spearman correlation was also used if the sample size of a group was too small to determine distribution of data. Given that correlations were performed at multiple frequencies, the Bonferroni correction was used adjusting the critical p-value. Patients were then categorized by the scalar location of their electrode array (ST and SV), and correlations between ECochG and behavioral thresholds within both these groups were assessed.

The following dependent variables were also assessed: (1) the absolute difference between ECochG thresholds and behavioral thresholds at individual frequencies (125, 250, and 500 Hz), (2) low-frequency PTA shift, (3) rise in CM amplitude from start of insertion to the peak value during insertion, and (4) the drop in CM amplitude from the peak value during insertion to completion of insertion. Patients were again characterized into groups according to scalar location and comparisons of the aforementioned variables were made between ST vs. SV insertions with an independent t-test (normal distribution) or a Mann-Whitney U-test (non-normal distribution). A p < 0.05 was considered indicative of statistical significance, with the exception of data pertaining to absolute differences between ECochG thresholds and behavioral thresholds, as multiple frequencies were analyzed; the Bonferroni correction was used in these analyses.

Eighteen patients met inclusion criteria and were prospectively enrolled (Table 1). The median age at the time of surgery was 67 years (range 23–80); 61% of the patients were male. Round window insertions were performed in the 78% of cases (n = 14), with extended round window insertions used in the remaining 22% (n = 4). Surgeons reported full insertion in all cases. Resistance during insertion was subjectively noted in one case; with electrode repositioning resistance subsided and a full insertion was achieved.

Sixteen patients consented to undergo postoperative CT imaging such that scalar electrode location could be determined. Two patients electively chose not to participate in the postoperative imaging portion of the study, therefore scalar location of these electrode arrays could not be determined. Because all insertions were performed through either round window or extended round window approaches, all electrodes were initially inserted into the ST within the basal turn. In six patients (38%), electrode translocation from the ST into the SV was observed. In one patient, after analysis, the electrode array was pushing against the basilar membrane but did not clearly translocate into the SV; interestingly, this was the case in which resistance was subjectively felt during insertion. Because of the limits of our image processing algorithms, this patient was excluded from subsequent statistical analyses that examined the impact of scalar location on audiologic outcomes.

Preoperatively, all patients had functional residual hearing (≤80 dB HL at 250 Hz) prior to surgery. The mean preoperative low-frequency PTA was 54 dB HL (range 27–75). At activation, the majority of patients (n = 12, 66%) demonstrated measurable unaided air-conduction thresholds at 125, 250, and 500 Hz. One patient had measurable thresholds at 125, and 250 Hz but did not respond to unaided pure-tones at 500 Hz; the remaining 5 patients demonstrated no responses at 125, 250, and 500 Hz.

Eleven patients (61%) maintained thresholds ≤80 dB HL at 250 Hz. Mean low-frequency PTA at activation was 82 dB HL (range 45–105), yielding an average low-frequency PTA shift of 28 dB (range 8–50). As depicted in Figure 1, 5 patients (28%) demonstrated low-frequency PTA shift <15 dB, 5 patients (28%) demonstrated low-frequency PTA shift between 15 and 30 dB, and the remaining 8 patients demonstrated low-frequency PTA shift >30 dB (44%).

The impact of demographic and surgical variables on low-frequency PTA shift was then assessed. No relation between age at surgery and postoperative PTA shift was noted (r = 0.13, p = 0.60). Further, no difference in median PTA shift was observed when round window insertions (23 dB, range 8–50) were compared to extended round window insertions (22 dB, range 12–47, p = 0.81). The median low-frequency PTA shift was significantly lower for electrodes entirely inserted into the ST (16 dB, range 8–25) as compared to electrodes that translocated into the SV (38 dB, range 10–48, p = 0.02; Figure 2).

The relationship between intraoperative ECochG thresholds and postoperative behavioral audiometric thresholds was analyzed. Intraoperative ECochG thresholds were successfully measured in 17 patients (94.4%); connection between the receiver stimulator and external monitoring equipment was lost in one patient. The absolute mean difference between intraoperative ECochG thresholds and postoperative behavioral thresholds for 125, 250, and 500 Hz is shown in Table 2. The absolute difference between intraoperative ECochG thresholds and postoperative audiometric thresholds was significantly lower (i.e., better) for ST insertions compared to SV insertions at 125 and 250 Hz frequencies (p = 0.001 for both analyses). In the overall cohort, no significant correlations between intraoperative ECochG thresholds and postoperative behavioral thresholds were noted at 125 Hz (r = 0.12, p = 0.64), 250 Hz (r = 0.08, p = 0.77), or 500 Hz (rs = 0.46, p = 0.07; Figure 3). The relationship between ECochG and behavioral thresholds at activation is also plotted as a function of scalar location.

Postoperative ECochG thresholds were successfully measured in 17 patients (94%) at activation; testing in one patient was limited by time constraints and patient preference. The mean difference between ECochG thresholds and behavioral thresholds at activation is shown in Table 3. At 125 Hz, the difference between postoperative ECochG threshold and pure tone thresholds was significantly lower (i.e., better) for ST insertions compared to SV insertions (p = 0.0007). A significant correlation between ECochG thresholds and behavioral thresholds at activation was observed at 125 Hz (r = 0.83, p < 0.0001), 250 Hz (r = 0.88, p < 0.0001), and 500 Hz (r = 0.88, p < 0.0001; Figure 4). These relationships are also shown according to scalar location. Bland-Altman plots assessing agreement between methods at activation for low-frequencies are shown in Figure 5.

Changes in CM amplitude during electrode insertion were then analyzed. As mentioned previously, intraoperative ECochG could not be performed in one patient; in addition, the insertion scans from four other patients were invalid secondary to monitoring issues. Insertion scans from the remaining 13 patients are depicted in Figure 6 according to scalar electrode location. The mean rise in CM amplitude from start of insertion at the round window to the peak value during insertion, was 22 dB (range 5–40). On average, the CM amplitude dropped 3 dB (range 0–8) from the peak value during insertion to completion of insertion. These objective measures of CM amplitude change were compared between ST and SV insertions; no significant differences were noted (p = 0.35 and p = 0.61; Table 4). Further, low-frequency PTA shift did not correlate significantly with round window to peak amplitude (r = −0.40, p = 0.17) nor drop from peak to completion of insertion (r = 0.26, p = 0.38).

The primary limitation of the current study was the sample size (n = 18) and as a result, generalizations cannot be made at this time. Further, though ECochG including CM peak amplitude with electrode insertion may hold future surgical value regarding insertion trauma, no feedback was provided to the surgeons during the insertions on any of the cases included here. In order to thoroughly investigate the utility of this measure—particularly in helping to avoid scalar dislocation—real-time feedback is likely a necessary component. Finally, all participants in the current study were recipients of a conventional, pre-curved electrode, the AB mid-scala electrode. That is, none of the subjects were implanted with a lateral-wall electrode specifically designed for hearing preservation. Thus, it is possible that ECochG thresholds may not generalize to recipients of a shorter, lateral-wall electrode who may have lower, and potentially better, audiometric thresholds across a broader range of frequencies. Our research team is actively involved in ongoing efforts to investigate the clinical utility of ECochG as both a measure of intracochlear insertion trauma and postoperative audiometric thresholds in larger sample sizes with patients of varying residual hearing in the low-frequency and both pre-curved and lateral-wall electrodes.

More patients are presenting for CI who have measureable and clinically significant preoperative hearing thresholds. However, we are unable to appreciate the effects of CI insertion trauma and resultant postoperative audiometric thresholds until the point of device activation or even later when behavioral hearing thresholds are measured. The current study investigated the relationship between intraoperative and postoperative ECochG measurements and postoperative audiometry in a group of 18 patients with preoperative 250-Hz thresholds up to 80 dB HL who were implanted with an AB mid-scala electrode. Sixteen of the 18 patient consented to postoperative CT imaging allowing for determination of electrode scalar location. From the current dataset, the primary conclusions were as follows:

Scalar translocation from ST to SV was associated with significantly higher shifts in low-frequency PTA when compared to electrodes inserted entirely within ST.

RG is on the audiology advisory board for Advanced Bionics and Cochlear Americas and the clinical advisory board for Frequency Therapeutics. RL is a consultant for Advanced Bionics, Cochlear Americas, and Ototronix. DH is on the surgical advisory boards for Cochlear, MED-EL, AB, Stryker, Anspach, and Oticon Medical. MB is on the surgical advisory board for MED-EL and is a consultant for Oticon Medical. AR is on the surgical advisory boards for Cochlear, MED-EL, AB, Stryker, Olympus, and Grace Medical. GW is on the surgical advisory board for Oticon Medical and is a consultant for AB, Cochlear, and MED-EL. The other authors declare that the research was conducted in the absence of any commercial or financial relationships that directly affected the current research.