Do Impedance Changes Correlate With a Delayed Hearing Loss After Hybrid L24 Implantation?

The association between electrode impedance and short-term outcomes in cochlear implant recipients of slim modiolar and slim straight electrode arrays

OBJECTIVES

Electrode impedance measurements from cochlear implants (CI) reflect the status of the electrode array as well as the surrounding cochlear environment, and could provide a clinical index of functional changes with the CI. The goals of this study were to examine (1) the impact of electrode array type on electrode impedance, and (2) the relationship between electrode impedance and short-term hearing preservation and speech recognition outcomes.

METHODS

Retrospective study of 115 adult hearing preservation CI recipients of a slim modiolar or slim straight array. Common ground electrode impedances, pre- and post-operative hearing thresholds and CNC word recognition scores were retrieved.

RESULTS

Electrode impedances were significantly higher for recipients of the straight electrode array. Within individuals, electrode impedances were stable after the first week post-activation. However, increased standard deviation of electrode impedances was associated with greater loss of low frequency hearing at initial activation, and with poorer speech recognition at 6 months post-implantation.

CONCLUSIONS

Results demonstrate that electrode impedances depend on the type of implanted array. Findings also suggest that there may be a role for the variability in electrode impedance across electrodes as an indicator of changes in the intracochlear environment that contribute to outcomes with a CI.

Impedance development after implantation of hybrid-L24 cochlear implant electrodes

OBJECTIVE

Shorter and thinner electrodes were developed for preserving residual hearing after cochlear implantation by minimising trauma. As trauma is regarded as one of the causes of fibrous tissue formation after implantation, and increase in impedance is considered to be connected to fibrous tissue formation, the aim of the current study was to evaluate impedance development after implantation of Hybrid-L electrodes.

DESIGN

Impedance values were retrospectively collected from our clinical database and evaluated for all active contacts and basal, middle and apical contacts separately for up to 10 years.

STUDY SAMPLES

All 137 adult patients received a Hybrid-L electrode and had to be implanted for at least 1 year.

RESULTS

On average impedances increased to 13 kOhm before first fitting and dropped to 5-7 kOhm under electrical stimulation with lower values measured on apical contacts. Mean values remained stable over years, but variability increased. Values before first fitting were independent of age at implantation whereas lower values were found later in patients of higher age at implantation.

CONCLUSION

Despite smaller contacts, impedance values after start of electrical stimulation were comparable to published values of Contour electrodes. This might suggest less tissue growth with the Hybrid-L electrode array.

Cochlear Implant Electrode Impedance as Potential Biomarker for Residual Hearing

Introduction and Objectives

Among cochlear implant candidates, an increasing number of patients are presenting with residual acoustic hearing. To monitor the postoperative course of structural and functional preservation of the cochlea, a reliable objective biomarker would be desirable. Recently, impedance telemetry has gained increasing attention in this field. The aim of this study was to investigate the postoperative course of the residual acoustic hearing and clinical impedance in patients with long electrode arrays and to explore the applicability of impedance telemetry for monitoring residual hearing.

Methods

We retrospectively analyzed records of 42 cochlear implant recipients with residual hearing covering a median postoperative follow-up of 25 months with repeated simultaneous pure tone audiometry and impedance telemetry. We used a linear mixed-effects model to estimate the relation between clinical electrode impedance and residual hearing. Besides the clinical impedance, the follow-up time, side of implantation, gender, and age at implantation were included as fixed effects. An interaction term between impedance and follow-up time, as well as subject-level random intercepts and slopes, were included.

Results

Loss of residual hearing occurred either during surgery or within the first 6 post-operative months. Electrode contacts inserted further apically (i.e., deeper) had higher impedances, independent of residual hearing. The highest impedances were measured 1 month postoperatively and gradually decreased over time. Basal electrodes were more likely to maintain higher impedance. Follow-up time was significantly associated with residual hearing. Regardless of the time, we found that a 1 kΩ increase in clinical impedance was associated with a 4.4 dB deterioration of residual hearing (p < 0.001).

Conclusion

Pure tone audiometry is the current gold standard for monitoring postoperative residual hearing. However, the association of clinical impedances with residual hearing thresholds found in our study could potentially be exploited for objective monitoring using impedance telemetry. Further analysis including near-field related impedance components could be performed for improved specificity to local immune responses.

Access and Polarization Electrode Impedance Changes in Electric-Acoustic Stimulation Cochlear Implant Users with Delayed Loss of Acoustic Hearing

Acoustic hearing can be preserved after cochlear implant (CI) surgery, allowing for combined electric-acoustic stimulation (EAS) and superior speech understanding compared to electric-only hearing. Among patients who initially retain useful acoustic hearing, 30-40 % experience a delayed hearing loss that occurs 3 or more months after CI activation. Increases in electrode impedances have been associated with delayed loss of residual acoustic hearing, suggesting a possible role of intracochlear inflammation/fibrosis as reported by Scheperle et al. (Hear Res 350:45-57, 2017) and Shaul et al. (Otol Neurotol 40(5):e518-e526, 2019). These studies measured only total impedance. Total impedance consists of a composite of access resistance, which reflects resistance of the intracochlear environment, and polarization impedance, which reflects resistive and capacitive properties of the electrode-electrolyte interface as described by Dymond (IEEE Trans Biomed Eng 23(4):274-280, 1976) and Tykocinski et al. (Otol Neurotol 26(5):948-956, 2005). To explore the role of access and polarization impedance components in loss of residual acoustic hearing, these measures were collected from Nucleus EAS CI users with stable acoustic hearing and subsequent precipitous loss of hearing. For the hearing loss group, total impedance and access resistance increased over time while polarization impedance remained stable. For the stable hearing group, total impedance and access resistance were stable while polarization impedance declined. Increased access resistance rather than polarization impedance appears to drive the increase in total impedances seen with loss of hearing. Moreover, access resistance has been correlated with intracochlear fibrosis/inflammation in animal studies as observed by Xu et al. (Hear Res 105(1-2):1-29, 1997) and Tykocinski et al. (Hear Res 159(1-2):53-68, 2001). These findings thus support intracochlear inflammation as one contributor to loss of acoustic hearing in our EAS CI population.