Effects of preoperative electrical stimulability and historical factors on performance with multichannel cochlear implant

Evaluation of auditory pathway excitability using a pre-operative trans-tympanic electrically evoked auditory brainstem response under local anesthesia in cochlear implant candidates

OBJECTIVE

Subjective promontory stimulation is used to evaluate cochlear implant (CI) candidacy, but the test reliability is low. Electrically evoked auditory brainstem response (EABR) can verify the function of the auditory system objectively. This study's procedure uses a trans-tympanic rounded bent-tip electrode to perform pre-operative EABR under local anaesthesia (LA-TT-EABR) using MED-EL Software and Hardware. This study aimed to determine usability and effectiveness for CI candidates.

DESIGN

We hypothesised that LA-TT-EABR waveforms of good quality would be related to successful hearing outcomes. We assumed that the duration of hearing loss/deafness was a confounding factor to study outcomes.

STUDY SAMPLE

19 borderline CI candidates.

RESULTS

Positive LA-TT-EABR results were confirmed in 14 patients. LA-TT-EABR's mean latency was 2.05 ± 0.31 ms (eII/eIII) and 4.24 ± 0.39 ms (eIV/eV). Latencies weren't statistically different from intra-operative EABR elicited by basal CI contacts. All positive LA-TT-EABR patients benefitted from CI and speech performance improved one year after implantation. One patient with negative LA-TT-EABR was cochlear-implanted and had no hearing sensation.

CONCLUSIONS

LA-TT-EABR is a tool in the frame of pre-operative objective testing the auditory pathway. It seems useful for clinical testing CI candidacy. Based on this study's outcomes, LA-TT-EABR should be recommended for uncertain CI candidates.

Use of auditory evoked potentials with electrical stimulation at the round window niche pre-operatively on a brain-injured patient: A case study

OBJECTIVES

To highlight the effectiveness of using PS testing in conjunction with electrically evoked auditory evoked potentials (eAEPs) to help guide treatment plans in patients with limited behavioural responses.

METHODS

Case report on a 59-year-old male with traumatic brain injury. Electrophysiological measurements in conjunction with PS were performed.

RESULTS

eAEPs were obtained up to the thalamo-cortical region, supporting the viability of a CI in the non-implanted ear.

DISCUSSION & CONCLUSION

Use of PS in conjunction with electrically evoked auditory evoked potentials can provide valuable information to guide clinical decisions regarding implantation.

Effects of Residual Hearing on the Auditory Steady State Response for Cochlear Implantation in Children

Background and Objectives

We aim to explore the effects of residual auditory steady state response (ASSR) on cochlear implantation (CI) outcomes in children lacking auditory brainstem responses (ABRs).

Subjects and Methods

We retrospectively reviewed the data of child CI recipients lacking ABRs. All ears were divided into two groups: with residual ASSR and without ASSR. For each frequency, the T- and C-levels and the electrical dynamic ranges of postoperative 3-month and 1-year mappings were compared between the groups. To evaluate speech perception, patients who received simultaneous bilateral CIs were divided into two groups: group 1 exhibited responses at all frequencies in both ears; in group 2, at least one ear evidenced no response. The Categories of Auditory Perception (CAP) and Infant-Toddler Meaningful Auditory Integration Scale (IT-MAIS) scores were compared between the groups.

Results

We enrolled 16 patients. At 2 kHz, the postoperative 3-month and 1-year T-levels of patients with residual hearing were lower than those of hearing loss group (p=0.001, p=0.035). In residual hearing group, the ASSR threshold correlated positively with the postoperative 1-year T-level (p=0.012, R² =0.276) and C-level (p=0.002, R² =0.374). Of 10 simultaneous bilateral CI recipients, 5 exhibited ASSRs at all frequencies and the other 5 showed no response at ≥1 frequency. The latter had higher CAP scores at the postoperative 1-year (p=0.018).

Conclusions

In children exhibiting hearing loss in ABR testing, residual hearing at 2 kHz ASSR correlated positively with the post-CI T-level. Those with ASSRs at all frequencies had significantly lower CAP scores at the postoperative 1year. CI should not be delayed when marginal residual hearing is evident in ASSR.

Utilization of Nerve Integrity Monitor for Promontory Stimulation Testing Prior to Cochlear Implant

OBJECTIVE

To demonstrate the feasibility of a nerve integrity monitor as a tool for promontory stimulation testing in patients with profound sensorineural hearing loss considering cochlear implantation.

PATIENTS

Adult patients considered for cochlear implantation with no auditory response on audiometric testing INTERVENTION:: Promontory stimulation testing using the nerve integrity monitor.

MAIN OUTCOME MEASURE

By using a facial nerve stimulator and the nerve integrity monitor, transtympanic promontory stimulation testing was performed to assess auditory nerve function and determine candidacy for cochlear implantation. Patients indicated if they heard the stimulus.

RESULTS

Of the four patients completing the promontory stimulation tests, three patients heard the stimulus and one patient did not hear the stimulus. Of the three patients with a positive stimulation test, two patients have a history of progressive profound sensorineural hearing loss and one patient had a history of severe blunt temporal bone trauma. Two of these patients proceeded with cochlear implantation. The patient who had a negative promontory stimulation test has a history of neurofibromatosis type 2.

CONCLUSION

The nerve integrity monitor is a convenient tool that can be used in the clinic setting to perform promontory stimulation tests and aid in determining cochlear implant candidates, specifically in those patients who require verification of auditory nerve function. This tool is a feasible and reasonable method for promontory stimulation testing.

Development of implantable medical devices: from an engineering perspective

From the first pacemaker implant in 1958, numerous engineering and medical activities for implantable medical device development have faced challenges in materials, battery power, functionality, electrical power consumption, size shrinkage, system delivery, and wireless communication. With explosive advances in scientific and engineering technology, many implantable medical devices such as the pacemaker, cochlear implant, and real-time blood pressure sensors have been developed and improved. This trend of progress in medical devices will continue because of the coming super-aged society, which will result in more consumers for the devices. The inner body is a special space filled with electrical, chemical, mechanical, and marine-salted reactions. Therefore, electrical connectivity and communication, corrosion, robustness, and hermeticity are key factors to be considered during the development stage. The main participants in the development stage are the user, the medical staff, and the engineer or technician. Thus, there are three different viewpoints in the development of implantable devices. In this review paper, considerations in the development of implantable medical devices will be presented from the viewpoint of an engineering mind.

Predictors of audiological outcome following cochlear implantation in adults

The objective of this study was to examine variables that may predict open set speech discrimination following cochlear implantation. It consisted of a retrospective case review conducted in a tertiary referral centre with a cochlear implant programme. The patients were 117 postlingually deafened adult cochlear implant recipients. The main outcome measures were Bench, Kowal, Bamford (BKB) sentence scores recorded nine months following implant activation. The variables studied were age at the time of surgery, sex, duration of hearing loss, aetiology of hearing loss, residual hearing, implant type, speech processor strategy, number of active electrodes inserted. Variables found to have a significant effect on BKB following univariate analysis were entered into a multivariate analysis to determine independent predictors. Multivariate ordinal regression analysis gave an odds ration of 1.09 for each additional year of deafness prior to implantation (confidence interval 1.06-1.13; p < 0.001). Duration of deafness prior to implantation is an independent predictor of implant outcome. It accounted for 9% of the variability. Other factors must influence implant performance.

The convergence of cochlear implantation with induced pluripotent stem cell therapy

According to 2010 estimates from The National Institute on Deafness and other Communication Disorders, approximately 17% (36 million) American adults have reported some degree of hearing loss. Currently, the only clinical treatment available for those with severe-to-profound hearing loss is a cochlear implant, which is designed to electrically stimulate the auditory nerve in the absence of hair cells. Whilst the cochlear implant has been revolutionary in terms of providing hearing to the severe-to-profoundly deaf, there are variations in cochlear implant performance which may be related to the degree of degeneration of auditory neurons following hearing loss. Hence, numerous experimental studies have focused on enhancing the efficacy of cochlear implants by using neurotrophins to preserve the auditory neurons, and more recently, attempting to replace these dying cells with new neurons derived from stem cells. As a result, several groups are now investigating the potential for both embryonic and adult stem cells to replace the degenerating sensory elements in the deaf cochlea. Recent advances in our knowledge of stem cells and the development of induced pluripotency by Takahashi and Yamanaka in 2006, have opened a new realm of science focused on the use of induced pluripotent stem (iPS) cells for therapeutic purposes. This review will provide a broad overview of the potential benefits and challenges of using iPS cells in combination with a cochlear implant for the treatment of hearing loss, including differentiation of iPS cells into an auditory neural lineage and clinically relevant transplantation approaches.

When do we choose the 'better balance' ear for cochlear implants?

OBJECTIVES

In cochlear implant planning, the ear with poorer vestibular function, as determined through electronystagmography (ENG), is often selected as the site for implantation since surgery carries a low risk of iatrogenic labyrinthine injury. We sought to determine reasons for placing a cochlear implant in the 'better balance' ear.

METHODS

A retrospective cohort study of patients implanted with a cochlear implant at a tertiary care center from 1984 to June 2009 was performed. Based on ENG results, patients with asymmetric caloric reduction were identified. Of these patients, those who were implanted in the 'better balance' ear were selected for chart review. The charts were reviewed to determine rationale for ear selection.

RESULTS

Of the 724 cochlear implant patients implanted from 1984 to June 2009, ENG tests demonstrated that 130 (18%) had asymmetric abnormal responses. Thirty five (27%) of the patients with asymmetric abnormal responses were implanted in the 'better balance' ear. Review of these 35 patient charts revealed that reasons for selection of the 'better balance' ear fell into four categories: anatomical contraindications, attempting to attain binaural hearing, avoiding implantation of an ear with marked auditory deprivation, and patient preference.

DISCUSSION

Based on our current practice, we have identified four situations in which patients were implanted in the 'better balance' ear, and subsequently developed an algorithm to aid surgeons in side selection for cochlear implantation. Further study and validation of this algorithm is recommended.

Performance and benefit as outcome measures following cochlear implantation in non-traditional adult candidates: a pilot study

OBJECTIVES

To measure the reported benefit in quality of life (QoL) and audiological performance in a group of 'non-traditional' patients undergoing cochlear implantation, and compare the results with those of patients satisfying established criteria for implantation.

DESIGN

Longitudinal retrospective observational study.

PATIENTS

Ten consecutive early-deafened, late-implanted patients, here termed 'non-traditional' patients, with a mean age of 31.3 years (range 15-52 years) and implant experience from 8 to 76 months (mean 40.9 months).

METHODS

The Glasgow Benefit Inventory (GBI) was used to evaluate benefit in QoL. Auditory performance tests consisted of: (a) recognition of environmental sounds using the UCL/RNID lists; (b) speech pattern perception using the Three Alternative Forced Choice Test; (c) speech discrimination using modified CUNY sentences in two conditions: lip-reading alone and lip-reading with the implant. Lip-reading enhancement scores were extrapolated.

RESULTS

GBI scores varied widely from 3 to 67, with a mean of 38.9, which is comparable to the mean score of 40 reported for 'traditional' candidates. Half the subjects were totally satisfied with their implant (scoring 100), and the mean satisfaction rating was 82.7. The mean scores obtained in environmental sound discrimination (40.5%) were a little below those of conventional, 'traditional' candidates, while those for speech pattern perception (73%) were within a comparable range. In contrast all ten subjects scored zero on open-set speech perception tests after implantation. The mean lip-reading enhancement score (7.6%) was significantly below that achieved by 'traditional' cochlear implant candidates.

CONCLUSION

This group of 'non-traditional' adults demonstrates measured benefit in QoL, which is comparable to that achieved by 'traditional' candidates, but is not accompanied by a parallel improvement in performance on speech perception tasks. These results suggest that factors other than auditory performance may be equally important from the patient's perspective, and that some patients who might be considered poor candidates may still derive considerable benefit. It is suggested that benefit and performance be viewed as separate outcomes in this 'non-traditional' population.

Glutamatergic neuronal differentiation of mouse embryonic stem cells after transient expression of neurogenin 1 and treatment with BDNF and GDNF: in vitro and in vivo studies

Differentiation of the pluripotent neuroepithelium into neurons and glia is accomplished by the interaction of growth factors and cell-type restricted transcription factors. One approach to obtaining a particular neuronal phenotype is by recapitulating the expression of these factors in embryonic stem (ES) cells. Toward the eventual goal of auditory nerve replacement, the aim of the current investigation was to generate auditory nerve-like glutamatergic neurons from ES cells. Transient expression of Neurog1 promoted widespread neuronal differentiation in vitro; when supplemented with brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF), 75% of ES cell-derived neurons attained a glutamatergic phenotype after 5 d in vitro. Mouse ES cells were also placed into deafened guinea pig cochleae and Neurog1 expression was induced for 48 h followed by 26 d of BDNF/GDNF infusion. In vivo differentiation resulted in 50-75% of ES cells bearing markers of early neurons, and a majority of these cells had a glutamatergic phenotype. This is the first study to report a high percentage of ES cell differentiation into a glutamatergic phenotype and sets the stage for cell replacement of auditory nerve.

Stem cell transplantation for auditory nerve replacement

The successful function of cochlear prostheses depends on activation of auditory nerve. The survival of auditory nerve neurons, however, can vary widely in candidates for cochlear implants and influence implant efficacy. Stem cells offer the potential for improving the function of cochlear prostheses and increasing the candidate pool by replacing lost auditory nerve. The first phase of studies for stem cell replacement of auditory nerve has examined the in vitro survival and differentiation as well as in vivo differentiation and survival of exogenous embryonic and tissue stem cells placed into scala tympani and/or modiolus. These studies are reviewed and new results on in vivo placement of B-5 mouse embryonic stem cells into scala tympani of the guinea pig cochleae with differentiation into a glutamatergic neuronal phenotype are presented. Research on the integration and connections of stem cell derived neurons in the cochlea is described. Finally, an alternative approach is considered, based on the use of endogenous progenitors rather than exogenous stem cells, with a review of promising findings that have identified stem cell-like progenitors in cochlear and vestibular tissues to provide the potential for auditory nerve replacement.

Value of the Promontory Stimulation Test in Predicting Speech Perception After Cochlear Implantation

BACKGROUND AND OBJECTIVES

To determine the role of the promontory stimulation test (PST) in cochlear implantation (CI), we evaluated the correlations between PST parameters and the results of speech perception tests after CI.

SUBJECTS AND METHOD

With use of a retrospective review of the medical records of 58 patients, the GAP50, GAP100, TDL50, and TDL100 parameters of the PST were compared with the results of speech perception tests at fixed time points after CI. Spearman's rank-order correlation coefficient was used in statistical analyses.

RESULTS

GAP100 scores were correlated with all results of speech perception tests at all time points after CI (r = 0.309-0.509, P = .001-0.010), and TDL100 scores were correlated with the results of open set one- and two-syllabic word tests at 1 month after CI (r = 0.288, P = .028); no other significant correlations were found. The GAP100 scores for postlingually deaf patients, especially for gap intervals up to 100 ms, were correlated with all results of speech perception tests after CI. A stepwise multiple regression analysis revealed that the GAP100 score is the only variable that was significantly correlated with the results of speech perception tests. The duration and onset age of deafness did not significantly affect the results.

CONCLUSION

The PST may be a useful tool for predicting the prognosis of CI. The GAP100 score, especially for gap intervals up to 100 ms, might be useful in predicting the results of speech perception tests after CI as well as in determining the optimal site for CI in postlingually deaf patients without residual hearing.

Performance of patients using different cochlear implant systems: effects of input dynamic range

OBJECTIVE

To determine, for patients who had identical levels of performance on a monosyllabic word test presented in quiet, whether device differences would affect performance when tested with other materials and in other test conditions.

DESIGN

For Experiment 1, from a test population of 76 patients, three groups (N = 13 in each group) were created. Patients in the first group used the CII Bionic Ear behind-the-ear (BTE) speech processor, patients in the second group used the Esprit3G BTE speech processor, and patients in the third group used the Tempo+ BTE speech processor. The patients in each group were matched on (i) monosyllabic word scores in quiet, (ii) age at testing, (iii) duration of deafness, and (iv) experience with their device. Performance of the three groups was compared on a battery of tests of speech understanding, voice discrimination, and melody recognition. In Experiments 2 (N = 10) and 3 (N = 10) the effects of increasing input dynamic range in the 3G and CII devices, respectively, was assessed with sentence material presented at conversational levels in quiet, conversational levels in noise, and soft levels in quiet.

RESULTS

Experiment 1 revealed that patients fit with the CII processor achieved higher scores than Esprit3G and Tempo+ patients on tests of vowel recognition. CII and Tempo+ patients achieved higher scores than Esprit3G patients on difficult sentence material presented in noise at +10 and +5 dB SNR. CII patients achieved higher scores than Esprit3G patients on difficult sentence material presented at a soft level (54 dB SPL). Experiment 2 revealed that increasing input dynamic range in the Esprit3G device had (i) no effect at conversational levels in quiet, (ii) degraded performance in noise, and (iii) improved performance at soft levels. Experiment 3 revealed that increasing input dynamic range in the CII device improved performance in all conditions.

CONCLUSIONS

Differences in implant design can affect patient performance, especially in difficult listening situations. Input dynamic range and the method by which compression is implemented appear to be the major factors that account for our results.

Changes in pitch with a cochlear implant over time

In the normal auditory system, the perceived pitch of a tone is closely linked to the cochlear place of vibration. It has generally been assumed that high-rate electrical stimulation by a cochlear implant electrode also evokes a pitch sensation corresponding to the electrode's cochlear place ("place" code) and stimulation rate ("temporal" code). However, other factors may affect electric pitch sensation, such as a substantial loss of nearby nerve fibers or even higher-level perceptual changes due to experience. The goals of this study were to measure electric pitch sensations in hybrid (short-electrode) cochlear implant patients and to examine which factors might contribute to the perceived pitch. To look at effects of experience, electric pitch sensations were compared with acoustic tone references presented to the non-implanted ear at various stages of implant use, ranging from hookup to 5 years. Here, we show that electric pitch perception often shifts in frequency, sometimes by as much as two octaves, during the first few years of implant use. Additional pitch measurements in more recently implanted patients at shorter time intervals up to 1 year of implant use suggest two likely contributions to these observed pitch shifts: intersession variability (up to one octave) and slow, systematic changes over time. We also found that the early pitch sensations for a constant electrode location can vary greatly across subjects and that these variations are strongly correlated with speech reception performance. Specifically, patients with an early low-pitch sensation tend to perform poorly with the implant compared to those with an early high-pitch sensation, which may be linked to less nerve survival in the basal end of the cochlea in the low-pitch patients. In contrast, late pitch sensations show no correlation with speech perception. These results together suggest that early pitch sensations may more closely reflect peripheral innervation patterns, while later pitch sensations may reflect higher-level, experience-dependent changes. These pitch shifts over time not only raise questions for strict place-based theories of pitch perception, but also imply that experience may have a greater influence on cochlear implant perception than previously thought.

Effects of antioxidants on auditory nerve function and survival in deafened guinea pigs

Based on in vitro studies, it is hypothesized that neurotrophic factor deprivation following deafferentation elicits an oxidative state change in the deafferented neuron and the formation of free radicals that then signal cell death pathways. This pathway to cell death was tested in vivo by assessing the efficacy of antioxidants (AOs) to prevent degeneration of deafferented CNVIII spiral ganglion cells (SGCs) in deafened guinea pigs. Following destruction of sensory cells, guinea pigs were treated immediately with Trolox (a water soluble vitamin E analogue)+ascorbic acid (vitamin C) administered either locally, directly in the inner ear, or systemically. Electrical auditory brainstem response (EABR) thresholds were recorded to assess nerve function and showed a large increase following deafness. In treated animals EABR thresholds decreased and surviving SGCs were increased significantly compared to untreated animals. These results indicate that a change in oxidative state following deafferentation plays a role in nerve cell death and antioxidant therapy may rescue SGCs from deafferentation-induced degeneration.

Regeneration of human auditory nerve. In vitro/in video demonstration of neural progenitor cells in adult human and guinea pig spiral ganglion

Time lapse video recordings of cultured adult human and guinea pig spiral ganglion (hSG and gpSG) show that mitogen responsive progenitor/stem cells develop in the form of spheres that proliferate and differentiate into mature neurons and glia cells. Neurospheres, cultured with EGF and bFGF showed expression of nestin and incorporation of 5'-Bromo-2-deoxyuridine (BrdU). Newly formed BrdU labelled cells were positive for beta-tubulin, and also for GFAP demonstrating that neuronal cells were derived from a dividing population of progenitor cells. Dissociated spheres cultured either with glia cell line-derived neurotrophic factor (GDNF) or brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), induced differentiation of the progenitor cells. Video microscopy showed that neurons develop from subcultured spheres maintained for up to four weeks. Neurons showed fasciculation and migration with a speed of 10-30 microm/h, and some cells had up to 6 mm long neurites coexpressing TrkB and TrkC receptors. Precise dissection suggests that the neurons formed are cochlea-specific. The results suggest that the mammalian auditory nerve has the capability for self-renewal and replacement. Transplantation of progenitor cells together with established means to induce neural differentiation and fiber growth may facilitate strategies for better repair and treatment of auditory neuronal damage.

Psychophysical metrics and speech recognition in cochlear implant users

Intersubject variability in perception is a prominent characteristic of people with cochlear implants. This study characterized intersubject differences using simple metrics based on psychophysical measures: maximum comfortable loudness levels (C levels) and dynamic ranges (DRs). In a group of 17 subjects, we assessed across-site variation (ASV) and across-site mean (ASM) values of C levels and DRs for bipolar (BP) and monopolar (MP) stimulation, and examined the relation of these metrics to speech recognition across subjects. Significant negative correlations with speech recognition were found for ASVs of C levels for BP stimulation; i.e., subjects with high ASVs of BP C levels had poor speech recognition. Positive correlations with speech recognition were found for ASMs of C levels and ASMs of DRs for both BP and MP stimulation; i.e., subjects with high mean C levels and large mean DRs had better speech recognition. Thus, these psychophysical metrics are effective for diagnosis of individual differences in performance of subjects with cochlear implants. Furthermore, they point to some potentially useful treatment procedures.

Deafness-related plasticity in the inferior colliculus: gene expression profiling following removal of peripheral activity

The inferior colliculus (IC) is a major center of integration in the ascending as well as descending auditory pathways, where both excitatory and inhibitory amino acid neurotransmitters play a key role. When normal input to the auditory system is decreased, the balance between excitation and inhibition in the IC is disturbed. We examined global changes in gene expression in the rat IC 3 and 21 days following bilateral deafening, using Affymetrix GeneChip arrays and focused our analysis on changes in expression of neurotransmission-related genes. Over 1400 probe sets in the Affymetrix Rat Genome U34A Array were identified as genes that were differentially expressed. These genes encoded proteins previously reported to change as a consequence of deafness, such as calbindin, as well as proteins not previously reported to be modulated by deafness, such as clathrin. A subset of 19 differentially expressed genes was further examined using quantitative RT-PCR at 3, 21 and 90 days following deafness. These included several GABA, glycine, glutamate receptor and neuropeptide-related genes. Expression of genes for GABA-A receptor subunits beta2, beta3, and gamma2, plus ionotropic glutamate receptor subunits AMPA 2, AMPA 3, and kainate 2, increased at all three times. Expression of glycine receptor alpha1 initially declined and then later increased, while alpha2 increased sharply at 21 days. Glycine receptor alpha3 increased between 3 and 21 days, but decreased at 90 days. Of the neuropeptide-related genes tested with qRT-PCR, tyrosine hydroxylase decreased approximately 50% at all times tested. Serotonin receptor 2C increased at 3, 21, and 90 days. The 5B serotonin receptor decreased at 3 and 21 days and returned to normal by 90 days. Of the genes tested with qRT-PCR, only glycine receptor alpha2 and serotonin receptor 5B returned to normal levels of expression at 90 days. Changes in GABA receptor beta3, GABA receptor gamma2, glutamate receptor 2/3, enkephalin, and tyrosine hydroxylase were further confirmed using immunocytochemistry.

Delayed neurotrophic treatment preserves nerve survival and electrophysiological responsiveness in neomycin-deafened guinea pigs

Benefits of cochlear prostheses for the deaf are dependent on survival and excitability of the auditory nerve. Degeneration of deafferented auditory nerve fibers is prevented and excitability maintained by immediate replacement therapy with exogenous neurotrophic factors, in vivo. It is important to know whether such interventions are effective after a delay following deafness, typical for the human situation. This study evaluated the efficacy of brain-derived neurotrophic factor (BDNF) and ciliary neurotrophic factor axokine-1 analogue (CNTF Ax1) application, 2 or 6 weeks postdeafening, in preventing further degeneration and a decrease in excitability. Guinea pigs were deafened and implanted with intracochlear stimulating electrodes, a scala tympani cannula-osmotic pump system, and auditory brainstem response (ABR) recording electrodes. Subjects received BDNF + CNTF Ax1 or artificial perilymph (AP) treatment for 27 days, beginning at 2 or 6 weeks following deafening. Electrical (E) ABR thresholds increased following deafening. After 1 week, in the 2-weeks-delayed neurotrophic factor treatment group, EABR thresholds decreased relative to AP controls, which were statistically significant at 2 weeks. In the 6-week delay group, a tendency to enhanced EABR sensitivity began at 2 weeks of treatment and increased thereafter, with a significant difference between neurotrophic factor- and AP-treated groups across the treatment period. A clear, statistically significant, enhanced survival of spiral ganglion cells was seen in both neurotrophic factor treatment groups relative to AP controls. These findings demonstrate that BDNF + CNTF Ax1 can act to delay or possibly even reverse degenerative and, likely apoptotic, processes well after they have been activated. These survival factors can rescue cells from death and enhance electrical excitability, even during the period of degeneration and cell loss when the spiral ganglion cell population is reduced by >50% (6 weeks). It is noteworthy that this same degree of ganglion cell loss, secondary to receptor damage, is typically observed after a period equivalent to some years of deafness in humans.

Across-site threshold variation in cochlear implants: relation to speech recognition

Functional implications of across-site variation in detection thresholds in subjects with cochlear implants were evaluated by comparing thresholds to speech recognition performance. Detection thresholds for bipolar (BP) and monopolar (MP) stimulation of all available stimulation sites were assessed in 21 subjects with Nucleus CI24M and CI24R(CS) implants. We found significant negative correlations between speech recognition and within-subject across-site threshold variance for both BP and MP stimulation, but no significant correlation of speech recognition with mean threshold levels. These results suggest that across-site variance of detection thresholds could provide a useful early indication of the prognosis for speech recognition and might serve as an indicator for specific therapeutic approaches in individual subjects.

Delayed neurotrophin treatment supports auditory neuron survival in deaf guinea pigs

As key factors in the development and maintenance of the auditory system, neurotrophins can prevent auditory neuron degeneration when applied within three to five days of deafening. We tested each of the neurotrophins BDNF, NT-3, NT-4/5 and NGF for their ability to support auditory neuron survival following a two-week period of deafness in guinea pigs, when approximately 15% auditory neuron degeneration has already occurred. Although delayed, the treatment with each neurotrophin prevented further degeneration with similar efficacy.

Choice of ear for cochlear implantation: the effect of history and residual hearing on predicted postoperative performance

OBJECTIVE

This study used mathematical formulas predicting cochlear implant outcomes to investigate the effects of implantation in the poorer ear on postoperative speech recognition.

DESIGN

Retrospective cohort study with mathematical and statistical analyses. This study used the University of Iowa formula for predicting outcomes derived from implantation of the better ear on the basis of duration of deafness and preoperative speech understanding, applying this predictive model to a cohort of patients undergoing implantation in the poorer ear at The Johns Hopkins Medical Center.

SETTING

Tertiary referral center with active cochlear implant program.

PATIENTS

Postlingually deafened adults (n = 58) with preoperative Central Institute for the Deaf sentence scores less than or equal to 40%.

INTERVENTION

Cochlear implantation with all three Food and Drug Administration-approved devices.

MAIN OUTCOME MEASURE

Postoperative monosyllabic word recognition scores and correlations between actual and predicted results.

RESULTS

There was good statistical correlation between the predicted postoperative performance using the University of Iowa formula and the actual performance of our cohort of patients undergoing implantation in the poorer ear (r = 0.50, p < 0.0001). In addition, as a population, our cohort had a mean postoperative consonant-nucleus-consonant word score of 41.8%, which was statistically the same as that predicted by the University of Iowa formula (43.6%).

CONCLUSIONS

The postoperative performance of cochlear implant patients is most closely correlated with duration of deafness. However, our results indicate that this measure may not be ear specific and is more reflective of the total auditory receptivity of the patient. These observations help to form guidelines for choice of ear for implantation.

A review of cochlear implantation in mitochondrial sensorineural hearing loss

OBJECTIVE

Mitochondrial sensorineural hearing loss (SNHL) may be nonsyndromic (occurring in isolation), associated with the A1555G mutation in the MTRNR1 gene. Mitochondrial SNHL may also be syndromic, associated with the A3243G point mutation in the MTTL1 gene. In syndromic cases-mitochondrial encephalopathy, lactic acidosis, and strokelike episodes (MELAS), maternally inherited diabetes and deafness, Kearns-Sayre syndrome, and chronic progressive external ophthalmoplegia-the SNHL compounds already existing disabilities. The genetic basis for mitochondrial SNHL and postulated sites of pathologic changes are discussed.

DATA SOURCES

Sources used were relevant clinical and basic science publications.

STUDY SELECTION

A search of the entire databases of Medline and Web of Science, using various subject headings and free-text terms, was used to identify patients with mitochondrial disease having cochlear implants.

DATA EXTRACTION

The data from publications were critically reviewed and tabulated to assess implantation outcomes.

DATA SYNTHESIS

The data were not amenable to formal meta-analysis or valid data summarization, other than descriptive statistics.

CONCLUSIONS

There is an increasing awareness of the prevalence of mitochondrial SNHL and its progressive nature. High-risk candidates warrant genetic testing and family screening. Correlating the data for mitochondrial SNHL as a treatable entity is important, and the authors present an overview of these patients successfully rehabilitated by cochlear implantation.

Does choosing the "worse" ear for cochlear implantation affect outcome?

OBJECTIVE

To determine whether choosing the "better" ear or the "worse" ear for cochlear implantation impacts performance outcome.

DESIGN

Retrospective cohort study.

SETTING

University teaching hospital-cochlear implant program.

METHODS

Two groups of cochlear implantees were selected and matched based on clinical parameters, including duration of deafness/age at implantation, implant types, and processing strategies. Nineteen patients received an implant in his or her "better" ear of the two that had been amplified. An equal number of patients received an implant in the "worse" ear--an ear that was not amplified or was chosen to avoid causing oscillopsia; or if the patient was not willing to relinquish his or her hearing aid in the "better" ear based on subjective or objective criteria. Standard speech perception testing was performed.

RESULTS

The average open-set speech perception responses at 1 year after implantation were as follows: word recognition score 40.4% and sentence recognition score 81% in the aided subjects (better ears); word recognition score 41.5% and sentence recognition score 84.5% in the unaided group (worse ears).

CONCLUSION

No differences were found between the two groups of implantees. Choosing the "worse" ear for implantation did not appear to have a negative impact on performance outcome in this match-paired study.

The prognostic value of promontory electric auditory brain stem response in pediatric cochlear implantation

OBJECTIVE

To test the hypothesis that children with clear promontory electrically evoked auditory brain stem responses (prom-EABRs) would outperform, after cochlear implantation, children who had no prom-EABR preoperatively.

DESIGN

A prospective study was undertaken on 47 implanted children assigning them to two groups (group A: 35 children with a clear wave e-V in the preoperative prom-EABR and group B: 12 children with no prom-EABR). Speech perception and speech intelligibility were assessed annually up to 3 yr after implantation with the IOWA sentence test (level A and level B), Connected Discourse Tracking, Categories of Auditory Performance, and Speech Intelligibility Rating. t-test and Mann-Whitney U test were used to compare the above outcome measures in the two groups.

RESULTS

There was no statistically significant difference between the two groups on any of the outcome measures at any interval. Moreover, the small differences observed showed no consistent trend toward either group of children. Further analysis revealed that the outcomes have not been affected by possible confounding factors (age at implantation, duration of deafness, preoperative unaided pure-tone thresholds, and number of inserted electrodes).

CONCLUSIONS

The results suggest that children with no prom-EABR performed at levels comparable with children who had clear promontory responses preoperatively. The prognostic value of prom-EABR is limited and absence of a prom-EABR is not, by itself, a contraindication for cochlear implantation. However, in selected cases (congenital malformations, cochlear nerve dysplasia or suspected aplasia, narrow internal auditory canal, etc.) the presence of a prom-EABR is a positive finding in the assessment of candidates for cochlear implantation as it confirms the existence of intact auditory neurones.

Predictors of cochlear implant performance

Open set speech understanding with cochlear implants, without speechreading, is nowadays a common finding. However, there is a large variation in speech understanding between cochlear implant users. We tried to find pre-operative parameters which predicted the post-operative results. Thirty-seven adult post-lingually deafened Nucleus cochlear implant users with a mean age of 46 years (range 16 68) and a mean duration of deafness of 15 years (range 1.5-47) were studied. Pre-operatively, we performed pure-tone audiometry, round window and ear canal electrical stimulation, psychological tests and imaging. Additionally, we measured pre-operatively speech understanding in the auditory, the visual and the audiovisual conditions with several tests which were also administered after 6 and 12 months' implant experience. Correlation analysis between the pre-operative variables and the post-operative factors showed that duration of deafness and residual hearing are the most important predictors. The temporal difference limen in pre-operative round window electrical stimulation is a secondary predictor.

Auditory-nerve integrity after middle-fossa acoustic-tumor removal

We sought to investigate the functional integrity of the auditory nerve in patients with postoperative hearing loss after middle cranial fossa acoustic-tumor removal in a case-series descriptive study. The study setting was a tertiary referral center, a private otologic practice. The study population comprised seven patients who underwent a middle-cranial-fossa approach for unilateral acoustic-tumor resection and sustained postoperative anacusis with an anatomically intact auditory nerve. Four were men and three women; they ranged in age from 30 to 60 years; all underwent surgery between 1990 and 1994 and agreed to return to the center to participate in the study during 1995. Diagnostic electrical promontory stimulation was used to determine the functional integrity of the auditory nerve. Our main outcome measures were the presence or absence of discrete tone perception, electrical threshold, maximum acceptable level and dynamic range, gap detection and temporal difference limen during electrical promontory stimulation. Three of the seven patients demonstrated positive responses to electrical promontory testing (e.g., discrete tone perception). All three were able to perform the gap-detection and temporal difference limen tests. None of the preoperative characteristics was related to performance on promontory stimulation testing. We conclude that the middle cranial fossa approach permits anatomic--and, in some cases, functional auditory-nerve preservation. These data suggest that auditory rehabilitation in some patients who sustain anacusis after the middle cranial fossa approach to acoustic tumors, as in neurofibromatosis 2, may be provided with cochlear implantation.

Can event-related potentials be evoked by extra-cochlear stimulation and used for selection purposes in cochlear implantation?

To investigate whether electrically evoked event-related responses (P300) could be elicited by extra-cochlear stimulation, measurements were performed on a group of adults fitted with the single-channel extra-cochlear implant. To optimize measurement conditions, and because of the low number of subjects still using an extra-cochlear device in our cochlear implant programme, measurements were also performed on a group of experienced users fitted with the intra-cochlear Nucleus multichannel device. For reference purposes, subjects with normal hearing (control group) were also included in the study. Reproducible late latency responses (N1 and P2 peaks) were found in the five extra-cochlear implant users, while P300s were present in four out of these five subjects. The latencies were longer than those of the control group, but were similar to those obtained in the intra-cochlear implant group. Significant correlations were found for most N1, P2 and P300 measurements evoked by the tonal stimuli and by speech stimuli. The P300 amplitudes, evoked by either tonal or speech stimuli, appeared to be related to speech perception ability. This led to the conclusion that N1, P2 and P300 measurements may have potential as a clinical tool for preoperative prediction and postoperative evaluation of sound processing on a cortical level.

The future of cochlear implants

Remarkable progress has been made in recent years in the design and application of processing strategies for cochlear implants. Most notably, use of the new spectral peak (SPEAK) and continuous interleaved sampling (CIS) strategies have provided large improvements in speech reception performance compared with prior strategies (NIH Consensus Statement, 1995; Skinner et al., 1994a; Wilson et al., 1991). All major manufacturers of multichannel implant systems, including Advanced Bionics Corp., Bionic Systems, Cochlear Pty. Ltd., and Med El, now offer CIS or CIS-like strategies in their speech processors. The SPEAK strategy was developed by Cochlear Pty. Ltd and continues to be one of the options available in that company's devices. The principal purpose of this editorial is to present some of the many possibilities for further improvements in performance. To the extent that such possibilities are realized, implant systems of the future may be quite different from present systems, with different processing strategies, electrode designs, telemetry features, and fitting procedures.

Effects of chronic high-rate electrical stimulation on the cochlea and eighth nerve in the deafened guinea pig

This study was undertaken to examine the effects of chronic high-rate stimulation on the eighth nerve and cochlea. Fifty-four male pigmented guinea pigs were deafened and implanted with single ball electrodes in scala tympani. Four groups of animals received chronic electrical stimulation at a level of 5 microCol/cm2/ph for 1000 h as follows: Group A: 1000 Hz, 100 microseconds/ph duration, 100 microA peak; Group B: 250 Hz, 100 microseconds/ph duration, 100 microA peak; Group C: 2750 Hz, 36 microseconds/ph duration, 250 microA peak; Group D: 250 Hz, 400 microseconds/ph duration, 25 microA peak. Also, two control groups received 20 min stimulation during weekly electrically evoked auditory brainstem response (eABR) measurement (Group E) and about 5 s stimulation (Group F) during a brief eABR 3 day postimplantation and at perfusion. On Day 50, animals were perfused, midmodiolar sections cut and a quantitative assessment of spiral ganglion cells (SGC) performed. All stimulated subjects showed a similar decrease in eABR thresholds and dynamic range over time. No stimulation conditions induced pathology. All stimulation conditions enhanced survival of SGCs compared to unimplanted ears and implanted non-stimulated ears (Group F). There were no statistically significant differences in SGC survival between any stimulated groups, including Group E stimulated once a week. In conclusion, high-rate stimulation, under the conditions of this study, provides no additional risks and the same benefits to SGC survival as low-rate stimulation.

Effects of pulse separation on detection thresholds for electrical stimulation of the human cochlea

Effects of pulse separation on detection of electrical stimulation of the cochlea were studied in 12 profoundly deaf human subjects with Nucleus 22 cochlear implants. Biphasic symmetric pulses were used. Pulse separation is the time from offset of one biphasic pulse to the onset of the next biphasic pulse in the train. Effects of pulse separation were studied in the context of different covariables in four stages of the experiment. Effects of pulse separation seen in the different stages were similar, despite the different covariables. Both pulse separation and the total number of pulses per stimulus seem to be important variables affecting stimulus detection. For 0.5 ms/phase pulses, thresholds were lowest at the shortest pulse separations tested (0.2-1.1 ms) and increased as a function of pulse separation. For 2 ms/phase pulses, detection thresholds were lowest at pulse separations around 7.5 ms, in most cases, and higher at both longer and shorter pulse separations. These results suggest that interactions among adjacent pulses can either hinder or facilitate detection of the signal depending on the magnitudes of pulse separation and phase duration. Pulse separations at which thresholds measured for 2 ms/phase pulses were minimum were fairly consistent across subjects and did not correlate well with speech recognition scores. However, significant variation in this measure across species has been seen.

Electrically evoked whole nerve action potentials in Ineraid cochlear implant users: responses to different stimulating electrode configurations and comparison to psychophysical responses

Electrically evoked whole nerve action potentials (EAP) have been recorded from 20 ineraid cochlear implant users in response to bipolar and/or monopolar electrical stimulation of the cochlea. EAP growth functions and refractory recovery functions were obtained for a variety of different stimulating conditions. Where possible, parallel psychophysical experiments were conducted that measured the just detectable increment in stimulus current level (JND), forward-masking functions, and detection thresholds for a range of different stimuli. Variations in EAP threshold, slope of the EAP growth function, and the rate of recovery of the EAP from the refractory state were observed both across subjects for a given place and by mode of stimulation (bipolar versus monopolar), as well as across electrodes within a subject. A poor correlation between slope of the EAP growth functions and current JNDs was obtained. However, good correlations were observed between EAP threshold and psychophysical detection threshold and between the EAP refractory recovery functions and the psychophysical forward-masking functions. Our interpretation of these findings is that these particular physiological measures are related to the excitability of the auditory nerve to electrical stimulation and further that these measures are related to performance of cochlear implant patients on listening tasks. Consequently, such information may prove to be useful in adjusting the stimulation parameters of the cochlear implant speech processor in order to maximize an individual's performance with the device.

Deafness induced cell size changes in rostral AVCN of the guinea pig

The right cochleae of 250-350 g guinea pigs were lesioned by topical administration of neomycin in the middle ear cavity. Eight weeks after the lesion, the cochleae and cochlear nuclei were analyzed. Cochlear hair cell loss was assessed, and cell areas of spherical bushy cells in the rostral anteroventral cochlear nucleus (AVCN) were compared between the lesioned and normal hearing sides for each animal. In five animals with both inner and outer hair cell loss in the lesioned cochlea, the average area of neuronal somata in the rostral AVCN in the lesioned side was 22% smaller than the average area of these cells in the normal hearing side. In two animals with outer hair cell loss but inner hair cells remaining, there was no difference in cell size between the lesioned and non-lesioned AVCN. These results provide evidence that there is significant shrinkage in AVCN cell size in the mature mammal after hearing loss associated with inner hair cell loss.

Chronic electrical stimulation reverses deafness-related depression of electrically evoked 2-deoxyglucose activity in the guinea pig inferior colliculus

The [14C]-2-deoxyglucose (2-DG) autoradiographic technique was used to study how auditory-related metabolic activity changes with deafness, and how chronic electrical stimulation of the deafened system may modify these changes. Guinea pigs were deafened by administration of kanamycin and ethacrynic acid. After nine weeks of deafness, the basal unstimulated uptake of 2-DG in the inferior colliculus (IC) was lower than in normal hearing control animals. 100 microA of acute cochlear electrical stimulation significantly increased 2-DG uptake in normal hearing animals but did not evoke a significant increase in four or nine week deafened animals. Electrically elicited 2-DG uptake in the IC is therefore depressed by prolonged deafness. In a second series of experiments, after four weeks of deafness, animals were chronically electrically stimulated via a cochlear implant 2.5-3.5 h a day, five days a week for five weeks at 100 microA. Acute cochlear electrical stimulation following this chronic stimulation significantly increased 2-DG uptake in the contralateral IC over unstimulated levels. This suggests that some depressive effects of profound deafness on the auditory brain stem may be reduced or reversed with chronic electrical stimulation by a cochlear implant.

Preoperative electrical stimulation for cochlear implant selection. The use of ear canal electrodes versus transtympanic electrodes

Preoperative electrostimulation tests were performed on 43 postlingually deaf and 20 prelingually deaf cochlear implant (CI) candidates. The stimulating electrode was placed at three different locations, i.e. the round window, the promontory and the ear canal and the results were compared. The evoked sensations were reported to be of auditory origin by most of the postlingually deaf CI candidates. Prelingually deaf subjects could not always distinguish clearly and reliably between "hearing" and "feeling". The percentage of stimulated ears of postlingually deaf subjects in whom hearing sensations were evoked was almost identical for the three locations of the stimulating electrode. However, in 5 out of the 7 ears without hearing sensations during ear canal stimulation (ECS), hearing sensations could be evoked during either promontory stimulation (PS) or round window stimulation (RWS). The mean threshold level for "hearing" at a stimulation frequency of 62 Hz was lowest during RWS, 7.7 dB higher during PS and 35.8 dB higher during ECS. The mean electrical dynamic range at 62 Hz was most favourable during RWS (23.9 dB), smaller during PS (15.6 dB) and smallest during ECS (10.0 dB). All differences were statistically significant. Placement of the ear canal electrode was easier, less frightening for the patient and required less time than insertion of the needle electrode for PS or RWS. Therefore we recommend the use of ECS to examine whether the CI candidate can be stimulated, and of RWS if more detailed information is required.

Cochlear wall titanium implants for auditory nerve stimulation

Genetically deaf dalmatian dogs and ototoxically deafened macaque monkeys were implanted with electrodes housed in cochlear wall titanium implants to assess long-term stability, tolerance, and performance. Short-term human implantation, followed by trials of stimulation, was performed in 4 unilaterally deaf patients. In the dog experiments, cochlear wall electrode stimulation produced consistent electrophysiologic thresholds that were higher, by approximately 6 dB, than those obtained with bipolar scala tympani stimulation. Clinical testing revealed electrically evoked middle latency response, auditory brain stem response, and/or behavioral detection responses in 3 of 4 patients, at levels below those for facial nerve activation and pain sensation. Electrode place discrimination studies, with controls for loudness cues, revealed near-perfect discrimination in a monkey subject. Eleven of the 12 animal implants were found to be rigidly fixed in the cochlear bone, with direct contract between bone and implant over 8% to 23% of the implant surface for the 6 implants examined in detail. These results suggest that long-term fixation of titanium cochlear wall implants occurs by virtue of intimate implant-bone contact in restricted areas. This approach to prosthetic stimulation demonstrates encouraging performance characteristics in achieving auditory activation.

Effects of phase duration on detection of electrical stimulation of the human cochlea

Detection thresholds for biphasic symmetric pulses were measured in fourteen human subjects implanted with the Cochlear Corporation Nucleus 22 Implant. The effects of phase duration on thresholds were studied using single pulses, and 500 ms pulse trains at 100 pps. Psychophysical detection thresholds decreased as a function of phase duration with a change in slope at approximately 0.5 ms/phase. Mean single-pulse and pulse-train slopes were -3.60 and -4.25 dB/doubling of phase duration for pulse durations of less than about 0.5 ms/phase. For pulse durations greater than 0.5 ms/phase, mean slopes were -5.71 and -7.54 dB/doubling for single pulses and pulse trains, respectively. Thresholds for pulse trains decreased as a function of stimulus duration for durations up to at least 300 ms, with the rate of decrease being dependent on the phase duration of the pulse. Effects of stimulus duration were greater for longer phase duration signals. We hypothesize that the longer phase duration pulses activate multiple spikes in a single fiber and/or more effective patterns of spikes across fibers, which may explain why slopes of psychophysical threshold functions are steeper than those of functions for single auditory nerve fibers for longer duration pulses. Thresholds were compared to respective speech perception scores (CID sentences) since thresholds for long phase duration signals have been shown previously to be correlated with nerve survival patterns, and nerve survival patterns may affect speech perception. Correlation coefficients ranged from -0.59 to -0.81, depending on stimulus parameters and subject selection.