Cochlear Implant Electrode Effect on Sound Energy Transfer Within the Cochlea During Acoustic Stimulation

Abstract

Hypothesis: Cochlear implants (CIs) designed for hearing preservation will not alter mechanical properties of the middle and inner ears as measured by intracochlear pressure (P(IC)) and stapes velocity (Vstap).

Background: CIs designed to provide combined electroacoustic stimulation are now available. To maintain functional acoustic hearing, it is important to know if a CI electrode can alter middle or inner ear mechanics because any alteration could contribute to elevated low-frequency thresholds in electroacoustic stimulation patients.

Methods: Seven human cadaveric temporal bones were prepared, and pure-tone stimuli from 120 Hz to 10 kHz were presented at a range of intensities up to 110 dB sound pressure level. P(IC) in the scala vestibuli (P(SV)) and tympani (PST) were measured with fiber-optic pressure sensors concurrently with VStap using laser Doppler vibrometry. Five CI electrodes from two different manufacturers with varying dimensions were inserted via a round window approach at six different depths (16-25 mm).

Results: The responses of P(IC) and VStap to acoustic stimulation were assessed as a function of stimulus frequency, normalized to sound pressure level in the external auditory canal, at baseline and electrode-inserted conditions. Responses measured with electrodes inserted were generally within approximately 5 dB of baseline, indicating little effect of CI electrode insertion on P(IC) and VStap. Overall, mean differences across conditions were small for all responses, and no substantial differences were consistently visible across electrode types.

Conclusion: Results suggest that the influence of a CI electrode on middle and inner ear mechanics is minimal despite variation in electrode lengths and configurations.

Figure 1

Baseline stapes velocity (V_Stap), and scala vestibuli and tympani pressure (P_SV/ST) transfer function magnitudes (A) and phases (B). Responses recorded in the six specimens meeting inclusion criteria are shown normalized to the sound pressure level recorded in the ear canal. Responses are superimposed onto the 95% CI and range of responses (gray areas) observed previously in V_Stap and P_SV/ST, respectively (23,35). Symbol size in (A) indicates the signal to noise ratio (small: > 3 dB, medium: > 6 dB, large: > 9 dB), and dots show the noise floor for each measurement (normalized as: noise/P_EC during sound stimulation).

Figure 2

An example set of recordings in one representative specimen (249L). (A.) Baseline transfer functions are represented by circles, while transfer functions recorded with various CI electrodes shown in the legend. (B.) Difference in transfer function magnitude recorded with each CI electrode inserted compared to the baseline for the same specimen as in (A.). Responses are shown in units of dB difference from baseline as a function of stimulus frequency. Symbol size indicates the signal to noise ratio (small: > 3 dB, medium: > 6 dB, large: > 9 dB), and dots show the noise floor for each measurement (normalized as: noise/P_EC during sound stimulation). Nucleus Hybrid L24 (HL24), Nucleus CI422 Slim Straight inserted at 20 and 25 mm (SS20 & SS25), Nucleus CI24RE Contour Advance (NCA), HiFocus Mid-Scala (MS), and HiFocus 1j (1j).

Figure 3

Analysis of the responses across the population of specimens is shown. (A.) The mean (± SEM; gray bands) transfer functions across specimens, recorded under baseline (circles) and CI electrode inserted conditions (symbols) for V_Stap (left), P_SV (center) and P_ST (right) magnitudes. Responses are shown superimposed over the same range of responses shown in prior reports (dark gray bands) (23,35). (B) The mean (± SEM) differences (in dB re: baseline) compared to baseline across all specimens tested. Nucleus Hybrid L24 (HL24), Nucleus CI422 Slim Straight inserted at 20 and 25 mm (SS20 & SS25), Nucleus CI24RE Contour Advance (NCA), HiFocus Mid-Scala (MS), and HiFocus 1j (1j).

Figure 4

Summary of the effects of inserting different cochlear implant electrodes into the ST with respect to the acoustic baseline. Responses were grouped within three frequency bands with relatively low (f < 1 kHz), middle (1 kHz < f < 3 kHz), and high (f > 3 kHz) frequencies. Responses are shown as the average difference within each frequency band, for each electrode condition. See the RESULTS for a description of the statistical analysis. Nucleus Hybrid L24 (HL24), Nucleus CI422 Slim Straight inserted at 20 and 25 mm (SS20 & SS25), Nucleus CI24RE Contour Advance (NCA), HiFocus Mid-Scala (MS), and HiFocus 1j (1j).

Figure 5

Mean (±SEM) effects of inserting the cochlear implant electrodes into the ST on differential intracochlear pressure, (P_SV-P_ST)/P_EAC. Nucleus Hybrid L24 (HL24), Nucleus CI422 Slim Straight inserted at 20 and 25 mm (SS20 & SS25), Nucleus CI24RE Contour Advance (NCA), HiFocus Mid-Scala (MS), and HiFocus 1j (1j).