Electric compound action potentials (ECAPs) and impedances in an open and closed operative site during cochlear implantation

INTRODUCTION

In patients undergoing cochlear implantation, intraoperative measures of impedance and electrically evoked compound action potentials (ECAPs) are used to confirm device integrity and electrode array position. However, these electrophysiological parameters have been shown to decrease over time, with a small decrement observable as early as 24 h post implantation and becoming more apparent after 6 months. Whether the intraoperatively measured impedances and ECAPs recorded immediately after electrode insertion versus later in the operation or in an open versus closed operative site vary has not been documented. Such variation in measurement procedure may affect the ultimate operative outcome.

PATIENTS AND METHODS

Between February and October 2016, 38 patients received a cochlear implant (Cochlear^®), with half receiving a CI 522 device and the other half receiving a CI 512 device. These patients were distributed into three groups. In the first (group A; n = 21), the impedance and threshold neural response telemetry (tNRT) measures were taken before (M1) and after cutaneous suture (M2), whereas in the second group (group B; n = 11) they were taken twice in the open operative site, once at the time of electrode insertion (M1) and then again 10 min later (M2). The last group (group C; n = 6) was measured only once after a 10 min waiting time before closing the operative site.

RESULTS

tNRTs of both group A and B were significantly higher at M1 than measured at M2. The magnitude of change in tNRT did vary significantly by group (P = .027) with group A having a bigger decrease than group B. For impedances there was evidence for a significant difference in M2 between the three groups (P = .012), with group C having significantly higher values compared to group A and B.

CONCLUSION

Intraoperative tNRT measures change significantly over time, including within the first 10 min of implantation. One underlying etiology of this phenomenon for tNRTs seems to be the condition of the surgical site whereas changes of impedances can be best explained by the 'electrochemical cleaning' theory associated with the first stimulation of the electrode. However, for both impedances and tNRTs there also is an important impact of time as well as of acute perioperative changes in electrical conductivity.

Cochlear implantation in children with meningitis related deafness: The influence of electrode impedance and implant charge on auditory performance - A case control study

OBJECTIVES

Bacterial meningitis can cause a labyrinthitis. Consequences often are intracochlear soft tissue neoformation (cochlear obliteration) or intracochlear osteoneogenesis (cochlear ossification) and deafness. Cochlear implantation becomes challenging and hearing rehabilitation is complicated. This retrospective case-control-study aimed to find correlations between morphologic, electric and functional parameters.

METHODS

The study group included children, who lost hearing due to a bacterial meningitis (n = 35 cases). Using preoperative computed tomography and intraoperative findings we grouped into 'unaltered cochleae', 'obliterated cochleae' and 'ossified cochleae'. Control group children suffered from deafness (n = 16) of other aetiology and presented with radiologically unchanged cochleae. Postoperative routine controls documented impedances, stimulation charge and hearing tests a various time points, which all were analysed.

RESULTS

Control group patients showed a mean impedance of 6.3 kΩ and the mean charge applied was 19 nC. The study group averaged at 7.9 kΩ and 24.6 nC respectively. Patients with ossified cochleae had increased values of 8.6 kΩ and 29.7 nC. The control group reached a monosyllabic word understanding of 74% and the study group of 58%. Patients with ossified cochleae reached 36%.

CONCLUSIONS

Impedances and stimulation charge influence each other. Increased charge is necessary for higher cochlear implant output. Despite higher charges, patients with obliterated and patients with ossified cochleae significantly perform worse in hearing rehabilitation. Reduced audiological outcome in study group patients without morphologic cochlear changes furthermore hints at additional factors besides cochlear tissue neogenesis like postinflammational changes at the neural pathway.

Temporal changes in impedance of implanted adults for various cochlear segments

Electrode impedance (EI) is the first objective assessment carried out during the surgical procedure and follow-up of cochlear implanted patients. This measure provides information on the integrity of electrodes and on the surrounding environment. It is one of the main factors responsible for energy consumption of the cochlear implant (CI). The aim of our study is to investigate changes over time in EI in adult recipients implanted with the perimodiolar array by comparing differences in various cochlear segments. In addition, we explore the relationship between these objective measures and subjective measures such as T-level and C-level. We studied 28 adult patients. Impedance values (IVs) were calculated in "common-ground" (CG) and in monopolar (M1+2) mode for electrode groups in basal middle and apical segments. We found significant decreases in IVs between activation and 1 month. We obtained higher values for basal impedance, whereas lower IVs were found for apical electrodes at all observation times. Statistical pairing over time between impedance and T/C values showed significant correlation for both global impedance (GI) and T-C levels at CG and M1+2 mode up to 6 months. Segregated statistical analysis also showed a significant and prolonged correlation of basal IVs and fitting parameters. The higher basal impedance over time can be explained by the higher proportion of newly formed tissue in this region. The linear correlation of impedances with the fitting parameters become not significant after 3/6 months for the apical and middle segments and remained significant only for the basal region over time. This behaviour underlines the importance of persistence in intra-cochlear factors in influencing fitting parameters in the basal segment.

Using Electrically-evoked Compound Action Potentials to Estimate Perceptive Levels in Experienced Adult Cochlear Implant Users

HYPOTHESIS

The cochlear implant (CI) fitting level prediction accuracy of electrically-evoked compound action potential (ECAP) should be enhanced by the addition of demographic data in models.

INTRODUCTION

No accurate automated fitting of CI based on ECAP has yet been proposed.

METHODS

We recorded ECAP in 45 adults who had been using MED-EL CIs for more than 11 months and collected the most comfortable loudness level (MCL) used for CI fitting (prog-MCL), perception thresholds (meas-THR), and MCL (meas-MCL) measured with the stimulation used for ECAP recording. Linear mixed models taking into account cochlear site factors were computed to explain prog-MCL, meas-MCL, and meas-THR.

RESULTS

Cochlear region and ECAP threshold were predictors of the three levels. In addition, significant predictors were the ECAP amplitude for the prog-MCL and the duration of deafness for the prog-MCL and the meas-THR. Estimations were more accurate for the meas-THR, then the meas-MCL, and finally the prog-MCL.

CONCLUSION

These results show that 1) ECAP thresholds are more closely related to perception threshold than to comfort level, 2) predictions are more accurate when the inter-subject and cochlear regions variations are considered, and 3) differences between the stimulations used for ECAP recording and for CI fitting make it difficult to accurately predict the prog-MCL from the ECAP recording. Predicted prog-MCL could be used as bases for fitting but should be used with care to avoid any uncomfortable or painful stimulation.

Delayed changes in auditory status in cochlear implant users with preserved acoustic hearing

This retrospective review explores delayed-onset hearing loss in 85 individuals receiving cochlear implants designed to preserve acoustic hearing at the University of Iowa Hospitals and Clinics between 2001 and 2015. Repeated measures of unaided behavioral audiometric thresholds, electrode impedance, and electrically evoked compound action potential (ECAP) amplitude growth functions were used to characterize longitudinal changes in auditory status. Participants were grouped into two primary categories according to changes in unaided behavioral thresholds: (1) stable hearing or symmetrical hearing loss and (2) delayed loss of hearing in the implanted ear. Thirty-eight percent of this sample presented with delayed-onset hearing loss of various degrees and rates of change. Neither array type nor insertion approach (round window or cochleostomy) had a significant effect on prevalence. Electrode impedance increased abruptly for many individuals exhibiting precipitous hearing loss; the increase was often transient. The impedance increases were significantly larger than the impedance changes observed for individuals with stable or symmetrical hearing loss. Moreover, the impedance changes were associated with changes in behavioral thresholds for individuals with a precipitous drop in behavioral thresholds. These findings suggest a change in the electrode environment coincident with the change in auditory status. Changes in ECAP thresholds, growth function slopes, and suprathreshold amplitudes were not correlated with changes in behavioral thresholds, suggesting that neural responsiveness in the region excited by the implant is relatively stable. Further exploration into etiology of delayed-onset hearing loss post implantation is needed, with particular interest in mechanisms associated with changes in the intracochlear environment.

Evolution of impedance field telemetry after one day of activation in cochlear implant recipients

OBJECTIVES

Changes in impedance between 24 hours and one month after cochlear implantation have never been explored due to the inability to switch on within one day. This study examined the effect of early activation (within 24 hours) on the evolution of electrode impedance with the aim of providing information on the tissue-to-electrode interface when electrical stimulation was commenced one day post implantation.

METHODS

We performed a retrospective review at a single institution. Patients who received a Nucleus 24RECA implant system (Cochlear, Sydney, Australia) and underwent initial switch-on within 24 hours postoperatively were included. Impedance measurements were obtained intraoperatively and postoperatively at 1 day, 1 week, 4 weeks, and 8 weeks.

RESULTS

A significant drop in impedance was noted 1 day after an initial activation within 24 hours followed by a significant rise in impedance in all channels until 1 week, after which the impedance behaved differently in different segments. Basal and mid-portion electrodes revealed a slight increase while apical electrodes showed a slight decrease in impedance from 1 week to 8 weeks postoperatively. Impedance was relatively stable 4 weeks after surgery.

CONCLUSIONS

This is the first study to report the evolution of impedance in all channels between initial mapping 1 day and 1 month after cochlear implantation. The underlying mechanism for the differences in behavior between different segments of the electrode may be associated with the combined effect of dynamics among the interplay of cell cover formation, electrical stimulation, and fibrotic reaction.

Stimulation parameters differ between current anti-modiolar and peri-modiolar electrode arrays implanted within the same child

OBJECTIVE

To compare stimulation parameters of peri-modiolar and anti-modiolar electrode arrays using two surgical approaches.

METHODS

Impedance, stimulation thresholds, comfortably loud current levels, electrically evoked compound action potential thresholds and electrically evoked stapedial reflex thresholds were compared between 2 arrays implanted in the same child at 5 time points: surgery, activation/day 1, week 1, and months 1 and 3. The peri-modiolar array was implanted via cochleostomy in all children (n = 64), while the anti-modiolar array was inserted via a cochleostomy in 43 children and via the round window in 21 children.

RESULTS

The anti-modiolar array had significantly lower impedance, but required higher current levels to elicit thresholds, comfort, electrically evoked compound action potential thresholds and electrically evoked stapedial reflex thresholds than the peri-modiolar array across all time points, particularly in basal electrodes (p < 0.05). The prevalence of open electrodes was similar in anti-modiolar (n = 5) and peri-modiolar (n = 3) arrays.

CONCLUSION

Significant but clinically acceptable differences in stimulation parameters between peri-modiolar and anti-modiolar arrays persisted four months after surgery in children using bilateral cochlear implants. The surgical approach used to insert the anti-modiolar array had no overall effect on outcomes.

Multichannel cochlear implant for selective neuronal activation and chronic use in the free-moving Mongolian gerbil

BACKGROUND

Animal models for chronic multichannel cochlear implant stimulation and selective neuronal activation contribute to a better understanding of auditory signal processing and central neural plasticity.

NEW METHOD

This paper describes the design and surgical implantation of a multichannel cochlear implant (CI) system for chronic use in the free-moving gerbil. For chronic stimulation, adult-deafened gerbils were connected to a multichannel commutator that allowed low resistance cable rotation and stable electric connectivity to the current source.

RESULTS

Despite the small scale of the gerbil cochlea and auditory brain regions, final electrophysiological mapping experiments revealed selective and tonotopically organized neuronal activation in the auditory cortex. Contact impedances and electrically evoked auditory brainstem responses were stable over several weeks demonstrating the long-term integrity of the implant and the efficacy of the stimulation.

COMPARISON WITH EXISTING METHODS

Most animal models on multichannel signal processing and stimulation-induced plasticity are limited to larger animals such as ferrets, cats and primates. Multichannel CI stimulation in the free-moving rodent and evidence for selective neuronal activation in gerbil auditory cortex have not been previously reported.

CONCLUSIONS

Overall, our results show that the gerbil is a robust rodent model for selective and tonotopically organized multichannel CI stimulation. We anticipate that this model provides a useful tool to develop and test both passive stimulation and behavioral training strategies for plastic reorganization and restoration of degraded unilateral and bilateral central auditory signal processing in the hearing impaired and deaf central auditory system.

Electrically evoked compound action potential (ECAP) in cochlear implant children: Changes in auditory nerve response in first year of cochlear implant use

AIM

Recording of the electrically evoked compound action potential (ECAP) of the auditory nerve in cochlear implant (CI) patients represents an option to assess changes in auditory nerve responses and the interaction between the electrode bundle and the neural tissue over time. The aim of the present work is to assess ECAP changes during the first year of cochlear implant for the purpose of predicting thresholds and adjustment of the patients' programs over time.

METHOD

Data were collected from 25 children using Cochlear Nucleus 24 implants. ECAP thresholds were examined at the time of surgery, at initial stimulation, and 3, 6 and 12 months post-stimulation. Five electrodes located at basal, middle, and apical positions in the cochlea were tested at each time interval and ECAP thresholds were analyzed and compared.

RESULTS

There was a significant decrease in ECAP thresholds between the intraoperative measure and fitting time at all electrode sites. Mean ECAP thresholds measured at 3, 6 and 12 months post-stimulation remained similar to initial stimulation levels. Although there was no significant difference in ECAP thresholds recorded at fitting time and 12 months follow up session, there was significant increase in behavioral T and C levels from initial stimulation to the 12 months' time point.

CONCLUSION

Most electrodes undergo non-significant change in ECAP thresholds over time, and therefore thresholds obtained on the day of initial stimulation can be used to estimate the patients' map levels at any time. On the other hand, intraoperative thresholds demonstrated significant change relative to postoperative recording times, limiting the ability to use intraoperatively recorded ECAP thresholds to predict postoperative measurements.

Long-term electrode impedance changes and failure prevalence in cochlear implants

OBJECTIVE

This study assessed the prevalence of electrode failures and electrode impedance measures in Nucleus cochlear implants around initial activation (an average of 16 days after surgery) and after 8 to 12 years of device use.

DESIGN

Retrospective data from the Melbourne Cochlear Implant Clinic was collated and analysed.

STUDY SAMPLE

Included in this study were 232 adults, all of whom were implanted at the clinic between March 1998 and August 2005.

RESULTS

Overall 0.5% of electrodes failed over the entire test period, with 5.6% of devices showing one or more electrode failure. The majority of these failures were recorded by initial activation. The numbers of electrode failures have decreased over time with array type, such that no failures were recorded with the currently available Contour Advance array. Array type was shown to affect electrode impedance at both time points, with the Contour and Contour Advance arrays having significantly higher absolute values than the Banded array. However, the Banded array had significantly higher area-normalized impedances at initial and final measures than the Contour and Contour Advance array.

CONCLUSIONS

A relatively low incidence of electrode failures were recorded for the Nucleus devices of these recipients. Electrode impedance dropped for all array types after 8 to 12 years of device use.

Impedance, neural response telemetry, and speech perception outcomes after reimplantation of cochlear implants in children

OBJECTIVE

To compare mean impedance levels, neural response telemetry (NRT), and auditory perception after initial and explant-reimplant pediatric cochlear implants.

STUDY DESIGN

Retrospective case review.

SETTING

Tertiary referral hospital and cochlear implant program.

PATIENTS

Children 0 to 16 years inclusive who have undergone explant-reimplant of their cochlear implant.

INTERVENTION

Impedance levels, NRT, and speech perception performance.

MAIN OUTCOME MEASURES

Impedance, NRT, and auditory perception at switch on, 3 months, 12 months, 3 years, and 5 years after initial cochlear implant and reimplantation.

RESULTS

The explant-reimplant group receiving Cochlear contour array had significantly (p < 0.001) raised impedance at switch on, 3 months, 12 months, and 3 years, compared with their initial implant. The explant-reimplant group receiving Cochlear straight array had marginally significant (p = 0.045) raised impedance at switch on, 3 months, 12 months, and 3 and 5 years. Infection was associated with greater increases in impedance in the reimplant Contour group. NRT was increased in the explant-reimplant group but not significantly (p = 0.06). Auditory perception returned to preexplant levels within 6 months in 61% of children.

CONCLUSION

Impedance is higher after explant-reimplant and remains increased for years after explant-reimplant with Cochlear contour and to a lesser degree the straight array device.

Insertion site and sealing technique affect residual hearing and tissue formation after cochlear implantation

Tissue formation around the electrode array of a cochlear implant has been suggested to influence preservation of residual hearing as well as electrical hearing performance of implanted subjects. Further, inhomogeneity in the electrical properties of the scala tympani shape the electrical field and affect current spread. Intracochlear trauma due to electrode insertion and the insertion site itself are commonly seen as triggers for the tissue formation. The present study investigates whether the insertion site, round window membrane (RWM) vs. cochleostomy (CS), or the sealing material, no seal vs. muscle graft vs. carboxylate cement, have an influence on the amount of fibrous tissue and/or new bone formation after CI implantation in the guinea pig. Hearing thresholds were determined by auditory brainstem response (ABR) measurements prior to implantation and after 28 days. The amount of tissue formation was quantified by evaluation of microscopic images obtained by a grinding/polishing procedure to keep the CI in place during histological processing. An insertion via the round window membrane resulted after 28 days in less tissue formation in the no seal and muscle seal condition compared to the cochleostomy approach. Between these two sealing techniques there was no difference. Sealing the cochlea with carboxylate cement resulted always in a strong new bone formation and almost total loss of residual hearing. The amount of tissue formation and the hearing loss correlated at 1-8 kHz. Consequently, the use of carboxylate cement as a sealing material in cochlear implantation should be avoided even in animal studies, whereas sealing the insertion site with a muscle graft did not induce an additional tissue growth compared to omitting a seal. For hearing preservation the round window approach should be used.

Impedance changes in chronically implanted and stimulated cochlear implant electrodes

OBJECTIVES

Electrode impedance increases following implantation and undergoes transitory reduction with onset of electrical stimulation. The studies in this paper measured the changes in access resistance and polarization impedance in vivo before and following electrical stimulation, and recorded the time course of these changes.

DESIGN

Impedance measures recorded in (a) four cats following 6 months of cochlear implant use, and (b) three cochlear implant recipients with 1.5-5 years cochlear implant experience.

RESULTS

Both the experimental and clinical data exhibited a reduction in electrode impedance, 20 and 5% respectively, within 15-30 minutes of stimulation onset. The majority of these changes occurred through reduction in polarization impedance. Cessation of stimulation was followed by an equivalent rise in impedance measures within 6-12 hours.

CONCLUSIONS

Stimulus-induced reductions in impedance exhibit a rapid onset and are evident in both chronic in vivo models tested, even several years after implantation. Given the impedance changes were dominated by the polarization component, these findings suggest that the electrical stimulation altered the electrode surface rather than the bulk tissue and fluid in the cochlea.

Real-time measurement of electrode impedance during intracochlear electrode insertion

OBJECTIVES/HYPOTHESIS

This pilot study details the use of a software tool that uses continuous impedance measurement during electrode insertion, with the eventual potential to assess and optimize electrode position and reduce insertional trauma.

STUDY DESIGN

Software development and experimental study with human cadaveric cochleae and two live surgeries.

METHODS

A prototype program to measure intracochlear electrode impedance and display it graphically in real time has been developed. The software was evaluated in human cadaveric temporal bones while simultaneously making real-time fluoroscopic recordings and in two live surgeries during intracochlear electrode insertion.

RESULTS

Impedance changes were observed with various scalar positions, and values were consistent with those obtained using clinically available software. Using Contour Advance electrodes, impedance values increased after stylet removal, particularly when using the monopolar mode.

CONCLUSIONS

Impedance values seem systematically affected by electrode position, with higher values being associated with proximity to the cochlear wall. The new software is capable of acquiring impedance measurements during electrode insertion, and these data may be useful to guide surgeons to achieve optimal and atraumatic electrode insertion, to guide robotic electrode insertion, and to provide insights about electrode position in the cochlea.

Electrical stimulation causes rapid changes in electrode impedance of cell-covered electrodes

Animal and clinical observations of a reduction in electrode impedance following electrical stimulation encouraged the development of an in vitro model of the electrode-tissue interface. This model was used previously to show an increase in impedance with cell and protein cover over electrodes. In this paper, the model was used to assess the changes in electrode impedance and cell cover following application of a charge-balanced biphasic current pulse train. Following stimulation, a large and rapid drop in total impedance (Z(t)) and access resistance (R(a)) occurred. The magnitude of this impedance change was dependent on the current amplitude used, with a linear relationship determined between R(a) and the resulting cell cover over the electrodes. The changes in impedance due to stimulation were shown to be transitory, with impedance returning to pre-stimulation levels several hours after cessation of stimulation. A loss of cells over the electrode surface was observed immediately after stimulation, suggesting that the level of stimulation applied was creating localized changes to cell adhesion. Similar changes in electrode impedance were observed for in vivo and in vitro work, thus helping to verify the in vitro model, although the underlying mechanisms may differ. A change in the porosity of the cellular layer was proposed to explain the alterations in electrode impedance in vitro. These in vitro studies provide insight into the possible mechanisms occurring at the electrode-tissue interface in association with electrical stimulation.

Electrical compound action potentials recorded with automated neural response telemetry: threshold changes as a function of time and electrode position

OBJECTIVE

Since the introduction of neural response telemetry (NRT) for the Nucleus 24 cochlear implant (CI24), researchers and clinicians have investigated the feasibility of using the electrically evoked compound action potential (ECAP) threshold to objectively predict psychophysical measurements that are used in the programming of the speech processor. The ability to substitute objective for behavioral measurements, particularly measurements made at the time of surgery, would greatly facilitate programming the MAP for young children and other individuals who are not able to provide reliable behavioral data required for MAP programming. There have been a number of studies that have examined characteristics of the ECAP measured at the time of surgery and postoperatively; however, all the available published data are based on the CI24. With the introduction of the Nucleus Freedom device, an automated NRT (AutoNRT) program became available, which was capable of measuring ECAP thresholds at lower levels than was previously possible with NRT software associated with the CI24 device. It was hypothesized that the enhancements to the NRT program may improve the predictability of postoperative measurements from intraoperatively recorded ECAP thresholds. The purpose of this study was to track ECAP thresholds obtained using AutoNRT as a function of time and electrode position.

DESIGN

ECAP thresholds were recorded from 71 children and adults implanted with the Nucleus Freedom device using the AutoNRT test protocol. ECAP thresholds were obtained at the time of surgery, at initial stimulation, and 3 mos poststimulation. Five electrodes located at basal, middle, and apical positions in the cochlea were tested at each time interval and thresholds were compared.

RESULTS

Significant differences were found in ECAP thresholds measured with AutoNRT as a function of both time and electrode position. Basal electrodes had higher ECAP thresholds than apical electrodes and that relationship was consistent for each time period. Thresholds for all electrodes decreased between surgery and initial stimulation and remained relatively stable at 3 mos poststimulation. ECAP thresholds were consistently lower for children compared with adults at each time point. Mid-array electrodes (11 and 16) showed the least amount of change over time.

CONCLUSIONS

AutoNRT thresholds demonstrated significant change over time, limiting the ability to use intraoperatively recorded ECAP thresholds to predict postoperative measurements. In this study, electrodes 11 and 16 showed the least amount of change in ECAP threshold over time and therefore would be the best choices for estimating postoperative ECAP thresholds. Although not an ideal solution, mid-array ECAP thresholds obtained intraoperatively may prove to be helpful in creating a first MAP when no other behavioral or electrophysiological data are available.

Changes of Postoperative Impedances in Cochlear Implant Patients

OBJECTIVE

The objective of this study was to investigate the effect of intraoperative application of steroid suspension and coating of the electrode contacts with a thin film of iridium oxide on the short-term, time-dependent development of the intracochlear impedance in adults implanted with the Nucleus 24 Contour electrode.

STUDY DESIGN

The time-dependent development of intracochlear impedances was investigated in four different groups of adult patients at daily and later weekly intervals until the first fitting. The four groups were as follows: 1) standard Nucleus 24 Contour (control, n = 7); 2) standard Nucleus 24 Contour with intraoperative application of steroids (Group S, n = 6); 3) iridium-coated Nucleus 24 Contour control (Group I, n = 8); and 4) iridium-coated Nucleus 24 Contour with intraoperative application of steroids (Group I + S, n = 5). All patients had postlinguistic onset of severe to profound sensorineural hearing loss and no or little benefit of conventional hearing aids. Absence of ossification or any other cochlear anomaly and also absence of signs of retrocochlear or central origin to the hearing impairment bilaterally had to be confirmed preoperatively.

RESULTS

Steroid application reduced impedances significantly (Groups S and I + S), whereas iridium coating lowered variance of the impedance among patients but did not reduce the impedance significantly. The steroid-induced reduction is more pronounced at basal electrode contacts. Furthermore, there is some indication that the tissue growth could be faster in patients having the iridium-coated Contour electrode.

CONCLUSION

Provided that the reduction of electrode impedances with application of steroids is persisting, intracochlear application of steroids can be considered on a regular basis. Iridium coating of the electrode contacts seems not to be justified to be included as standard procedure.

A longitudinal study of electrical stimulation levels and electrode impedance in children using the Clarion cochlear implant

CONCLUSIONS

Electrical stimulation levels and electrode impedance values (EIVs) in children using the Clarion cochlear implant (CI) programmed with CIS strategy stabilized after 3 months of implant use. The data presented here may be useful as a general guideline for the programming of infants and young children and may further be of help for the identification of patients who fall outside the "average" range.

OBJECTIVES

The purpose of the present study was to evaluate changes in electrical stimulation levels, i.e. threshold (T) levels, comfortable (M) levels, dynamic range (DR), and EIVs during the first 18 months of implant use, in children using the Clarion CI.

MATERIALS AND METHODS

The maps of 18 pre-lingual children (mean age at implantation 4.2 years; range 1-8), using the Enhanced Bipolar 1.2 or Bipolar standard electrode with the S-Series speech processor programmed with CIS strategy, were examined at five time points: connection, and 3, 6, 12, and 18 months post-initial stimulation. T levels, M levels, DR and EIVs were analyzed according to four cochlear segments: apical, apical-medial, medial-basal, and basal.

RESULTS

During the first 3 months of implant use T levels increased to some extent, whereas M levels and DR increased significantly. From 3 months and through the entire follow-up, T and M levels as well as DR were stable. EIVs of current carrying electrodes decreased significantly from connection to the 3-month visit; thereafter a stabilization of values was evident. Electrical stimulation levels and EIVs did not differ among the cochlear segments during the entire follow-up.

Dual electrode stimulation using the nucleus CI24RE cochlear implant: electrode impedance and pitch ranking studies

OBJECTIVE

The first aim of the study was to determine the reduction in electrode impedances using dual electrode stimulation compared with single electrode stimulation in the new Nucleus CI24RE receiver-stimulator. The CI24RE is connected to the Nucleus 22-electrode intracochlear array. Dual electrode stimulation is produced by electrically coupling two adjacent single electrodes. The second aim was to determine whether dual electrode stimulation produced pitch percepts that were intermediate to the pitch of the two adjacent single electrodes.

DESIGN

Eight postlingually hearing-impaired adults with severe to profound loss, implanted with the CI24RE, participated in the study. Electrode impedances were measured by using the standard telemetry function of the system. A pitch ranking task was used to measure pitch for dual and single electrodes. Seven sets of three electrodes along the electrode array were tested. Each set of electrodes consisted of a dual electrode and the two adjacent single electrodes. Pitch ranking was measured using a two-alternative forced choice procedure, with the three electrodes in each set paired with each other as AB and BA pairs. The subject indicated which of the two stimuli had the higher pitch. Random variation in current level was used to remove any loudness cues.

RESULTS

The average electrode impedance was 38.6% lower for dual electrodes compared with single electrodes. Three subjects were able to successfully rank the three electrodes in each set in the expected tonotopic order for all seven sets of electrodes along the array. Three other subjects were able to rank sets of electrodes in the tonotopic order for most of the tested positions on the array. The remaining two subjects gave more variable pitch ranking across positions along the array, although successful tonotopic ranking was demonstrated for several sets of electrodes.

CONCLUSIONS

Dual electrode stimulation with the CI24RE receiver-stimulator produced systematically lower electrode impedances and was capable of producing pitch percepts that were intermediate to those produced by the corresponding adjacent single electrodes. This makes available up to 43 channels of stimulation from 22 single electrodes.

Measurement and Analysis of Access Resistance and Polarization Impedance in Cochlear Implant Recipients

BACKGROUND

Impedance measurements are commonly performed at the end of cochlear implant surgery, not only to confirm that all electrodes are working but also to monitor the impedances of the newly implanted electrodes. The current method of testing allows the determination of only the overall electrode impedance but not its components, access resistance and polarization impedance. To determine whether any longitudinal change in the electrode impedance is caused by a change in the endocochlear environment or rather caused by a change in the surface quality of the electrode, it is necessary to extract access resistance and polarization impedance.

METHODS

We applied an impedance model that enabled us to calculate access resistance and polarization impedance after measurement of electrode impedance at three points along the voltage waveform.

RESULTS

The results show that the value of the components of electrode impedance varied with time after surgery: access resistance increased slowly over time, whereas polarization impedance increased up to Week 2 but decreased after commencement of electrical stimulation at that stage. These results are consistent with the hypothesis that a layer of fibrous tissue forms around the electrode within the cochlear canal, resulting in a slow increase of access resistance, whereas a layer of proteins forms on the surface of the electrode in the early phase after implantation. Electrical stimulation appears to disperse this surface layer, thereby reducing both the polarization impedance and electrode impedance.

CONCLUSION

The method presented enables the extraction of more detailed information about the longitudinal changes in the intracochlear environment after cochlear implantation.

Anin vitromodel for investigating impedance changes with cell growth and electrical stimulation: implications for cochlear implants

The impedance of stimulating electrodes used in cochlear implants and other neural prostheses often increases post-implantation, and is thought to be due to fibrous tissue encapsulation of the electrode array. Increased impedance results in higher power requirements to stimulate target neurons at set charge densities. We developed an in vitro model to investigate the electrode-tissue interface in a highly controlled environment. This model was tested using three cell types, with and without charge-balanced biphasic electrical stimulation. Under standard tissue culture conditions, a monolayer of cells was grown over the electrode surface. Electrode impedance increased in proportion to the extent of cell coverage of the electrode. Cell type was a significant factor in the amount of impedance increase, with kidney epithelial cells (MDCK) creating the greatest impedance, followed by dissociated rat skin fibroblasts and then macrophages (J774). The application of electrical stimulation to cell-covered electrodes caused impedance fluctuations similar to that seen in vivo, with a lowering of impedance immediately following stimulation, and a recovery to pre-stimulation levels during inactive periods. Examination of these electrodes suggests that the stimulation-induced impedance changes were due to the amount of cell cover over the electrodes. This in vitro technique accurately models the changes in impedance observed with neural prostheses in vivo, and shows the close relationship between impedance and tissue coverage adjacent to the electrode surface. We believe that this in vitro approach holds great promise to further our knowledge of the mechanisms contributing to electrode impedance.