Intracochlear Electrocochleography: Response Patterns During Cochlear Implantation and Hearing Preservation

Abstract

Objectives: Electrocochleography (ECochG) obtained through a cochlear implant (CI) is increasingly being tested as an intraoperative monitor during implantation with the goal of reducing surgical trauma. Reducing trauma should aid in preserving residual hearing and improve speech perception overall. The purpose of this study was to characterize intracochlear ECochG responses throughout insertion in a range of array types and, when applicable, relate these measures to hearing preservation. The ECochG signal in cochlear implant subjects is complex, consisting of hair cell and neural generators with differing distributions depending on the etiology and history of hearing loss. Consequently, a focus was to observe and characterize response changes as an electrode advances.

Design: In 36 human subjects, responses to 90 dB nHL tone bursts were recorded both at the round window (RW) and then through the apical contact of the CI as the array advanced into the cochlea. The specific setup used a sterile clip in the surgical field, attached to the ground of the implant with a software-controlled short to the apical contact. The end of the clip was then connected to standard audiometric recording equipment. The stimuli were 500 Hz tone bursts at 90 dB nHL. Audiometry for cases with intended hearing preservation (12/36 subjects) was correlated with intraoperative recordings.

Results: Successful intracochlear recordings were obtained in 28 subjects. For the eight unsuccessful cases, the clip introduced excessive line noise, which saturated the amplifier. Among the successful subjects, the initial intracochlear response was a median 5.8 dB larger than the response at the RW. Throughout insertion, modiolar arrays showed median response drops after stylet removal while in lateral wall arrays the maximal median response magnitude was typically at the deepest insertion depth. Four main patterns of response magnitude were seen: increases > 5 dB (12/28), steady responses within 5 dB (4/28), drops > 5 dB (from the initial response) at shallow insertion depths (< 15 mm deep, 7/28), or drops > 5 dB occurring at deeper depths (5/28). Hearing preservation, defined as < 80 dB threshold at 250 Hz, was successful in 9/12 subjects. In these subjects, an intracochlear loss of response magnitude afforded a prediction model with poor sensitivity and specificity, which improved when phase, latency, and proportion of neural components was considered. The change in hearing thresholds across cases was significantly correlated with various measures of the absolute magnitudes of response, including RW response, starting response, maximal response, and final responses (p's < 0.05, minimum of 0.0001 for the maximal response, r's > 0.57, maximum of 0.80 for the maximal response).

Conclusions: Monitoring the cochlea with intracochlear ECochG during cochlear implantation is feasible, and patterns of response vary by device type. Changes in magnitude alone did not account for hearing preservation rates, but considerations of phase, latency, and neural contribution can help to interpret the changes seen and improve sensitivity and specificity. The correlation between the absolute magnitude obtained either before or during insertion of the ECochG and the hearing threshold changes suggest that cochlear health, which varies by subject, plays an important role.

Figure 1.

Intraoperative setup for recording intracochlear ECochG with a Cochlear Corporation array. (A) View through surgical microscope with superimposed schematic diagram. The array’s deepest contact (E22) is digitally shorted to the ground rod contact (ECE1) when the processor is connected and software delivered through the telemetry is used. With this connection made, a clip electrode is connected to the ground rod and fed to the BioLogic recording device. (B) Photograph of the intraoperative recording setup, after the array was fully inserted. The ground and clip are floating above the surgical field (isolated electrically from the patient). Inset: Open vs Closed positioning of the clip electrode.

Figure 2.

Distribution of Total Response (ECochG-TR, see methods) from round window electrocochleography just prior to insertion. The distribution of TRs across subjects in this study (red) was not significantly different from the distribution in our larger database (teal). Noise when recording at the intracochlear site precluded recordings in 8 subjects, and while these subjects had smaller RW responses (beige), there wasn’t a clear RW magnitude cutoff which would predict whether intracochlear recordings would be feasible.

Figure 3.

Example waveforms when recording through the intracochlear clip system. (A) A trigger artifact can be seen when using Cochlear Corporation’s software. (B) For smaller responses, the clip system is also sensitive to line noise (typically 60 Hz). (C) In Advanced Bionics arrays, there is no trigger artifact but 60 Hz noise can be seen (though not in this case).

Figure 4.

Comparison of intracochlear response magnitude to those at the RW. Intracochlear responses could be (A) larger than the RW, (B) of similar magnitude to the RW, or (C) smaller than the RW. A ‘response’ was defined as sum of the FFT peaks to the first and second harmonics. (D)Across all 36 subjects, 28 intracochlear responses were above the noise floor among these, the median intracochlear response was 5.8 dB larger than the RW, and correlated positively (r² = 0.51). Labels within (D) refer to the subjects illustrated in (A) to (C), while the ‘x’ symbols refer to cases with intracochlear noise which precluded subsequent analysis.

Figure 5.

Intracochlear Response Track. (A) Response waveforms to a 90 dB, 500 Hz tone in one subject were assessed at 5 stages during CI insertion. (B) Plotting the response magnitude of the ongoing response as a function of insertion depth reveals a “Response Track”.

Figure 6.

Response Tracks vary by device type. (A) In the Cochlear Corporation CI512 array, responses in dB scale (top row) typically demonstrate early growth, and response magnitude when normalized in uV (bottom row) demonstrates the median response was greatest at an insertion depth of 14.2 mm (white arrow). (B) In the Cochlear Corporation 422/522 arrays, responses in dB could dip (n=3, black arrow) but most growths were steady and in uV/uV scale (bottom row) the maximal median response was achieved at the deepest insertion depth (white arrow). (C) in Advanced Bionics MidScala devices, sample size was limited but responses were steady and in one case dropped with depth.

Figure 7.

Response magnitude could drop because of differences in phase relationship of generators rather than trauma. (A) A Response Track for one subject shows a large drop (10 dB) which recovered to within 3 dB by the end of insertion. (B) Average cycles from the ongoing response (left) at the starting point demonstrates a large response at the fundamental frequency evident in the FFT (right). (C) Mid-insertion, the phase inverts. (D) By the end of insertion, the phase again reverts and the response contains more distortions (arrow), indicating the array is likely recording from a different population of generators than those at the first intracochlear location. (E) This phase change is due to a latency shift beyond a cycle.

Figure 8.

Response Tracks by pattern. Categories were (A) Overall growth of 5 dB by the end of insertion, (B), a response which remained +/− 5 dB throughout insertion, (C) an early drop of >5 dB during insertion, and (D) a late drop in response during insertion. The top row demonstrates change in response (dB) whereas the bottom row shows dB re 1 uV. Cases in blue are cases where hearing was preserved, red demonstrate hearing was lost, and black are for subjects where preservation was not a goal. As is evident, there isn’t a clear pattern category which contains the hearing preserved vs lost subjects, implying trauma can occur with or without a characteristic response pattern. Note: in the top row of panel A there are two red tracks which mostly overlap, but are more distinct in absolute scale on the bottom row.

Figure 9.

Response drops in two subjects without a concurrent change in phase. The first subject demonstrates (A) a 10 dB drop between (B) and (C). While the phase of the best-fit sine doesn’t change between (B) and (C), the proportion of neural activity is seen by both the distortions in the average cycle (arrows) and the changing proportion of ANN/CM (see methods for this calculation). In a second subject, a response drop of 4.3 dB (D) shows no change in phase or proportion of ANN/CM.

Figure 10.

Contingency tables for using three approaches of ECochG to identify trauma leading to hearing loss. In the first model (A), a 5 dB cutoff is used to connote trauma, which was associated with poor sensitivity and specificity. (B) The best magnitude cutoff we found, 2 dB, properly identified all cases of hearing loss but the specificity was poor. (C) Using magnitude drops and analysis of phase and ANN/CM, the sensitivity remained high and the specificity was improved. PV, predictive value.

Figure 11.

Response Tracks for Hearing Preservation cases and Relationship between hearing loss at 500 Hz and intraoperative track magnitudes. (A) The absolute track values demonstrate those who lost hearing (red) had smaller responses. Specifically, the starting magnitude (B), final magnitude (C), and maximal magnitude (D) all correlated significantly with amount of threshold gain. (E) The change in response does not in and of itself help predict which cases will have preserved hearing versus hearing loss. Changes in response magnitude including the overall growth (B) and the largest drop (C) were not correlated with hearing loss.

Figure 12.

Relationship between Round Window (RW) response magnitudes prior to CI insertion and subsequent hearing loss at 500 Hz. (A) The magnitude of the response to a 90 dB, 500 Hz tone correlated with the degree of hearing loss. (B) Across a broad range of stimulus frequencies (see ECochG-TR in methods), the total response (TR) also correlates with degree of hearing loss.