Electrically evoked compound action potential measures for virtual channels versus physical electrodes

OBJECTIVES

The number of distinct pitch percepts for cochlear implant (CI) listeners is somewhat limited by the number of physical electrodes in the array. Newer-generation CIs have the capability to potentially increase this number by stimulating areas of the cochlea between the physical electrodes. Currently, this is achieved by electrically coupling adjacent electrodes or by simultaneously activating two electrodes with independent current sources (i.e., current steering). Presumably, either type of dual-electrode stimulation will generate neural excitation patterns that are intermediate to those generated by either physical electrode alone (henceforth termed virtual channel). However, it is not clear whether virtual-channel stimulation yields neural recruitment patterns with similar shapes and rates of growth as compared with each physical electrode alone. The purpose of this study was to compare basic electrically evoked compound action potential (ECAP) measures for physical electrodes and virtual channels to determine whether properties of the respective excitation patterns were similar.

DESIGN

Data were collected for 12 adult CI recipients (six Nucleus Freedom CI24RE, two Advanced Bionics HiResolution 90K, and four Advanced Bionics CII). ECAP responses were measured for a set of three adjacent physical electrodes and two corresponding intermediate virtual channels (e.g., physical electrodes 4, 5, and 6 and virtual channels 4 + 5 and 5 + 6) at three positions along the electrode array (basal, middle, and apical). Virtual channels for Nucleus subjects were produced via electrical coupling of adjacent electrode pairs (dual-electrode mode). For Advanced Bionics subjects, virtual channels were produced via simultaneous, in-phase stimulation of adjacent electrode pairs with 50% of the total current delivered to each electrode in the pair. Specific ECAP measures were as follows: (1) threshold and slope of the input/output functions, (2) amplitude for a masker-probe interval of 1500 μsecs (measure of refractory recovery), and (3) relative location of spread of excitation (SOE) functions among virtual channels and adjacent physical electrodes. Measures for virtual channels were compared with those for the flanking physical electrodes using a multivariate analysis of variance.

RESULTS

There were no statistically significant differences between physical electrodes and virtual channels for ECAP thresholds, slope of the input/output function, or refractory recovery. On average, SOE functions for the virtual channels were spatially located approximately halfway between SOE functions for the adjacent physical electrodes.

CONCLUSIONS

Results from this study suggest that virtual channels produce neural recruitment patterns with properties similar to those elicited by the adjacent physical electrodes.

Correlation of early auditory potentials and intracochlear electrode insertion properties: an animal model featuring near real-time monitoring

OBJECTIVE

The goal of this work was to assess electrophysiologic response changes to acoustic stimuli as an intracochlear electrode impacted cochlear structures in an animal model of hearing preservation cochlear implantation. The ultimate goal is to develop efficient procedures for assessing the status of cochlear physiology for intraoperative use.

METHODS

Sixteen gerbils and 18 ears were tested. A rigid electrode was inserted through a basal turn cochleostomy and directed toward the basilar membrane/osseous spiral lamina complex. We recorded acoustically evoked early auditory potentials including cochlear microphonics (CMs) and compound action potentials (CAPs) to a short stimulation sequence consisting of one stimulus frequency and intensity as the electrode was advanced. A microendoscope was used to visualize the electrode insertion progress and to identify the site of electrode impact. After each experiment, the site of intracochlear trauma was confirmed using whole mount preparations.

RESULTS

Electrophysiologic changes correlated well with the degree and location of trauma. We observed four distinct patterns. In addition, the endoscope in conjunction with the short recording sequence allowed for the detection of response changes that were reversible when the electrode was retracted. These cases were associated with less than full-thickness damage on histology.

CONCLUSION

The short recording sequence to obtain acoustically evoked intracochlear potentials and the microendoscope allowed us to detect various levels of cochlear trauma including minor and reversible damage. Recordings of this type are potentially available using current implant technology. Future improvements in the measurements can be expected to improve the efficiency of the recording paradigm to produce a clinically useful tool.

Intracochlear recordings of electrophysiological parameters indicating cochlear damage

OBJECTIVE

: The pathophysiologic mechanisms resulting in hearing loss during electrode implantation are largely unknown. To better understand the functional implications of electrode implantation, we recorded the effects of cochlear damage on acoustically evoked intracochlear measurements using normal-hearing gerbils.

METHODS

: A metal electrode was placed on the surface of the round window, and recordings of the cochlear microphonic (CM) and compound action potential (CAP) were made in response to stimulation with tone-bursts at various frequencies in 1-octave intervals and at intensities of 15 to 72 dB sound pressure level. The electrode was then advanced incrementally, with CM and CAP measurements obtained at each step. These data were compared with data obtained at the round window, and the electrode was withdrawn when a significant change was observed. After electrophysiological analysis, the cochlea was examined histologically.

RESULTS

: Results show that on electrode insertion, loss of amplitude in the CM and CAP occurs after damage to cochlear structures. Loss of activity was typically first apparent in the CAP rather than the CM.

CONCLUSION

: These results suggest that a reduction of the CAP can be an early marker of interaction of the electrode with cochlear structures. Such measurements are potentially available with slight modifications to current cochlear implant technology.

Use of "phantom electrode" technique to extend the range of pitches available through a cochlear implant

OBJECTIVE

The range of pitch sensations available in cochlear implants (CIs) is conventionally thought to be limited by the location of the most apical and basal electrodes. However, partial bipolar stimulation, in which current is distributed to two intracochlear electrodes and one extracochlear electrode, can produce "phantom electrode" (PE) pitch percepts that extend beyond the pitch range available with physical electrodes. The goals of this study were (1) to determine the PE configuration that generated the lowest pitch relative to monopolar (MP) stimulation of the most apical electrode and (2) to determine the amount of pitch shift produced by different PE configurations.

DESIGN

Ten Advanced Bionics CI users (9 unilateral and 1 bilateral), implanted with the CII or HiRes 90k implant and the HiFocus 1, HiFocus 1j, or Helix electrode arrays participated in this study. PEs were created by simultaneously stimulating the primary and compensating electrodes in opposite phase. To test different PE configurations, the proportion of current delivered to the compensating electrode (sigma) and the electrode separation between the primary and compensatory electrode (D) were varied. To estimate the relative pitch of PEs, the lowest pitched PEs with primary electrodes 4 and 8 were compared with subsets of MP electrodes (1, 2, 3, 4, 5 and 5, 6, 7, 8, 9, respectively).

RESULTS

In all subjects, it was possible to identify sigma and D values that produced a PE that was lower in pitch than the MP stimulation of the primary electrode. In some subjects, increasing sigma and/or D produced progressively lower pitch percepts, whereas in others, PE pitch changed nonmonotonically with sigma and/or D. The amount of PE pitch shift could be estimated only for 14 cases; in seven cases, the pitch shift was <1 MP electrode, and in seven other cases, the pitch shift was between 1 and 2 MP electrodes.

CONCLUSIONS

PE stimulation can elicit pitch percepts lower than that of the most apical MP electrode; the PE pitch is lower by the equivalent of 0.5 to 2 MP electrodes.

Probing the electrode-neuron interface with focused cochlear implant stimulation

Cochlear implants are highly successful neural prostheses for persons with severe or profound hearing loss who gain little benefit from hearing aid amplification. Although implants are capable of providing important spectral and temporal cues for speech perception, performance on speech tests is variable across listeners. Psychophysical measures obtained from individual implant subjects can also be highly variable across implant channels. This review discusses evidence that such variability reflects deviations in the electrode-neuron interface, which refers to an implant channel's ability to effectively stimulate the auditory nerve. It is proposed that focused electrical stimulation is ideally suited to assess channel-to-channel irregularities in the electrode-neuron interface. In implant listeners, it is demonstrated that channels with relatively high thresholds, as measured with the tripolar configuration, exhibit broader psychophysical tuning curves and smaller dynamic ranges than channels with relatively low thresholds. Broader tuning implies that frequency-specific information intended for one population of neurons in the cochlea may activate more distant neurons, and a compressed dynamic range could make it more difficult to resolve intensity-based information, particularly in the presence of competing noise. Degradation of both types of cues would negatively affect speech perception.

Pneumococcal meningitis post-cochlear implantation: Preventative measures

Objective

Both clinical data and laboratory studies demonstrated the risk of pneumococcal meningitis post-cochlear implantation. This review examines strategies to prevent post-implant meningitis.

Data Sources

Medline/PubMed database; English articles after 1980. Search terms: cochlear implants, pneumococcus meningitis, streptococcus pneumonia, immunization, prevention.

Review Methods

Narrative review. All articles relating to post-implant meningitis without any restriction in study designs were assessed and information extracted.

Results

The presence of inner ear trauma as a result of surgical technique or cochlear implant electrode array design was associated with a higher risk of post-implant meningitis. Laboratory data demonstrated the effectiveness of pneumococcal vaccination in preventing meningitis induced via the hematogenous route of infection. Fibrous sealing around the electrode array at the cochleostomy site, and the use of antibiotic-coated electrode array reduced the risk of meningitis induced via an otogenic route.

Conclusion

The recent scientific data support the U.S. Food and Drug Administration recommendation of pneumococcal vaccination for the prevention of meningitis in implant recipients. Nontraumatic cochlear implant design, surgical technique, and an adequate fibrous seal around the cochleostomy site further reduce the risk of meningitis.

Consensus panel on a cochlear coordinate system applicable in histologic, physiologic, and radiologic studies of the human cochlea

HYPOTHESIS

An objective cochlear framework, for evaluation of the cochlear anatomy and description of the position of an implanted cochlear implant electrode, would allow the direct comparison of measures performed within the various subdisciplines involved in cochlear implant research.

BACKGROUND

Research on the human cochlear anatomy in relation to tonotopy and cochlear implantation is conducted by specialists from numerous disciplines such as histologists, surgeons, physicists, engineers, audiologists, and radiologists. To allow accurate comparisons between and combinations of previous and forthcoming scientific and clinical studies, cochlear structures and electrode positions must be specified in a consistent manner.

METHODS

Researchers with backgrounds in the various fields of inner ear research as well as representatives of the different manufacturers of cochlear implants (Advanced Bionics Corp., Med-El, Cochlear Corp.) were involved in consensus meetings held in Dallas, March 2005, and Asilomar, August 2005. Existing coordinate systems were evaluated, and requisites for an objective cochlear framework were discussed.

RESULTS

The consensus panel agreed upon a 3-dimensional, cylindrical coordinate system of the cochlea using the "Cochlear View" as a basis and choosing a z axis through the modiolus. The zero reference angle was chosen at the center of the round window, which has a close relationship to the basal end of the Organ of Corti.

CONCLUSION

Consensus was reached on an objective cochlear framework, allowing the outcomes of studies from different fields of research to be compared directly.

Force of cochlear implant electrode insertion performed by a robotic insertion tool: comparison of traditional versus Advance Off-Stylet techniques

OBJECTIVE

Robotic cochlear implant electrode array insertion offers substantial potential advantages, namely repeatability and minimization of insertion forces, leading to decreased intracochlear trauma. Using such a robotic insertion tool, we sought to analyze force profiles during deployment of stylet-containing electrode arrays using either traditional insertion, in which the stylet is withdrawn after complete insertion of the electrode, or Advance Off-Stylet (AOS) insertion, in which the stylet is withdrawn simultaneous with electrode array insertion.

STUDY DESIGN

Prospective.

SETTING

Tertiary referral center.

INTERVENTIONS

A robotic cochlear implant insertion tool coupled with a force-sensing carriage was used to perform electrode array insertions into an anatomically correct, three-dimensional scala tympani model during either straight insertion (n = 4) or AOS insertion (n = 4).

MAIN OUTCOME MEASURES

Both insertion techniques begin with a 7-mm straight insertion during which forces were similar averaging approximately 0.006 N. For insertion from 7 to 17 mm, traditional insertion forces averaged 0.046 ± 0.027 N, with a peak of 0.093 N, and AOS insertion forces averaged 0.008 ± 0.006 N, with a peak of 0.034 N. Beyond 9.74 mm, the difference between traditional and AOS insertion forces was highly significant.

CONCLUSION

With the use of a robotic insertion tool, which minimizes operator variability and maximizes repeatability, we have shown that cochlear implant electrode insertion via AOS is associated with lower average and maximum insertion forces compared with traditional insertion. These findings support the use of AOS over traditional, straight insertion.

Flexible cochlear microendoscopy in the gerbil

OBJECTIVES/HYPOTHESIS

To validate the scientific utility of flexible cochlear microendoscopy in the gerbil. This model is currently being developed to study the effects of intracochlear electrode positioning on functional parameters.

STUDY DESIGN

Animal experiments.

METHODS

A flexible fiberoptic microendoscope featuring a light channel and an outer diameter of 0.4 mm was specially modified to allow intracochlear visualization. Specifically, the focus distance was reduced to 1 mm and the optical properties were modified so that visualization was adequate when submerged in perilymphatic fluid. This endoscope was used to view intracochlear contents and monitor the progress of electrode insertions in 11 gerbils. The endoscopic data estimating the site of damage were compared to postmortem microdissections.

RESULTS

The endoscope allowed for adequate visualization of intracochlear content in all animals. The site of electrode contact seen in the endoscope was confirmed in the microdissected cochleae in 10 of 11 cases, indicating the endoscope's ability to correctly identify the site of intracochlear trauma in this animal model.

CONCLUSIONS

The current report demonstrates the feasibility of intracochlear microendoscopy in an animal model of hearing preservation cochlear implantation.

The value of digital volume tomography in assessing the position of cochlear implant arrays in temporal bone specimens

OBJECTIVES

Radiological evaluation of the position of cochlear implant (CI) devices is an upcoming method for quality control after CI surgery. First, results of imaging of the middle and inner ear with digital volume tomography (DVT) show considerable advantages such as exceptional image quality, thin slice thickness, and low radiation dose. The aim of this study was to evaluate whether DVT is an appropriate method for postoperative imaging of CI patients and to identify the exact position of the implant array within the cochlear by multiple measurements.

DESIGN

Thirteen formalin-fixed temporal bone specimens were implanted with a CI array and scanned in DVT. To determine the exact electrode position, these specimens were ground and stained for microscopic measurements. The measurements on grindings acted as a referee and were compared with the measurements in DVT scans. The statistical analysis between the two measurement protocols was performed using the Bland-Altman method.

RESULTS

Best achievable agreement between DVT scans and histological reference was shown. Mean differences between DVT and grindings from -1.55 to -65.40 microm were calculated. All means are within the region of accuracy. General positioning of the implant into the cochlea could be verified in all specimens. The exact position of the implanted array within the cochlear scalae could be recognized correctly in 11 of 13 cases in DVT. It was possible to identify shiftings between the tympanic and vestibular scalae in all cases.

CONCLUSION

DVT seems to be a convenient technique for postoperative position control after cochlear implantation.

Identifying cochlear implant channels with poor electrode-neuron interface: partial tripolar, single-channel thresholds and psychophysical tuning curves

OBJECTIVE

The goal of this study was to evaluate the ability of a threshold measure, made with a restricted electrode configuration, to identify channels exhibiting relatively poor spatial selectivity. With a restricted electrode configuration, channel-to-channel variability in threshold may reflect variations in the interface between the electrodes and auditory neurons (i.e., nerve survival, electrode placement, and tissue impedance). These variations in the electrode-neuron interface should also be reflected in psychophysical tuning curve (PTC) measurements. Specifically, it is hypothesized that high single-channel thresholds obtained with the spatially focused partial tripolar (pTP) electrode configuration are predictive of wide or tip-shifted PTCs.

DESIGN

Data were collected from five cochlear implant listeners implanted with the HiRes90k cochlear implant (Advanced Bionics Corp., Sylmar, CA). Single-channel thresholds and most comfortable listening levels were obtained for stimuli that varied in presumed electrical field size by using the pTP configuration for which a fraction of current (sigma) from a center-active electrode returns through two neighboring electrodes and the remainder through a distant indifferent electrode. Forward-masked PTCs were obtained for channels with the highest, lowest, and median tripolar (sigma = 1 or 0.9) thresholds. The probe channel and level were fixed and presented with either the monopolar (sigma = 0) or a more focused pTP (sigma > or = 0.55) configuration. The masker channel and level were varied, whereas the configuration was fixed to sigma = 0.5. A standard, three-interval, two-alternative forced choice procedure was used for thresholds and masked levels.

RESULTS

Single-channel threshold and variability in threshold across channels systematically increased as the compensating current, sigma, increased and the presumed electrical field became more focused. Across subjects, channels with the highest single-channel thresholds, when measured with a narrow, pTP stimulus, had significantly broader PTCs than the lowest threshold channels. In two subjects, the tips of the tuning curves were shifted away from the probe channel. Tuning curves were also wider for the monopolar probes than with pTP probes for both the highest and lowest threshold channels.

CONCLUSIONS

These results suggest that single-channel thresholds measured with a restricted stimulus can be used to identify cochlear implant channels with poor spatial selectivity. Channels having wide or tip-shifted tuning characteristics would likely not deliver the appropriate spectral information to the intended auditory neurons, leading to suboptimal perception. As a clinical tool, quick identification of impaired channels could lead to patient-specific mapping strategies and result in improved speech and music perception.

Spread of excitation measurements for the detection of electrode array foldovers: a prospective study comparing 3-dimensional rotational x-ray and intraoperative spread of excitation measurements

OBJECTIVE

The optimal positioning of electrode arrays in the cochlea is extremely important. Our standard approach is to use a 3-dimensional rotational x-ray for the intraoperative determination of the position of the electrode array. We wanted to see if spread of excitation (SOE) is useful for determining the electrode array position within the cochlea.

STUDY DESIGN

Prospective blind study design.

SETTING

Tertiary University Referral Center (Cochlear Implantation Center Amsterdam-Academic Medical Center, University of Amsterdam).

PATIENTS

Seventy-two implanted ears with a Cochlear Freedom device.

INTERVENTION

After cochlear implantation, we compared the 3-dimensional rotational x-ray imaging and SOE measurements. The investigators were blinded for the intraoperative surgeon findings and also for the imaging findings.

OUTCOME MEASURE(S)

Electrode array foldovers within the cochlea and the reliability of the SOE measurements.

RESULTS

We placed implants in 72 ears in this study, and all procedures seemed to be surgically uneventful. To our surprise, we discovered 4 electrode foldovers in this group. Of the 4 foldovers, 3 were corrected intraoperatively.

CONCLUSION

We found that intraoperative imaging and/or electrophysiologic measurements such as the SOE provide very useful information regarding electrode position within the cochlea. Spread of excitation is effective in detecting electrode array foldovers if the audiologist is experienced. Some software modifications are suggested.

Role of electrode placement as a contributor to variability in cochlear implant outcomes

HYPOTHESIS

Suboptimal cochlear implant (CI) electrode array placement may reduce presentation of coded information to the central nervous system and, consequently, limit speech recognition.

BACKGROUND

Generally, mean speech reception scores for CI recipients are similar across different CI systems, yet large outcome variation is observed among recipients implanted with the same device. These observations suggest significant recipient-dependent factors influence speech reception performance. This study examines electrode array insertion depth and scalar placement as recipient-dependent factors affecting outcome.

METHODS

Scalar location and depth of insertion of intracochlear electrodes were measured in 14 patients implanted with Advanced Bionics electrode arrays and whose word recognition scores varied broadly. Electrode position was measured using computed tomographic images of the cochlea and correlated with stable monosyllabic word recognition scores.

RESULTS

Electrode placement, primarily in terms of depth of insertion and scala tympani versus scala vestibuli location, varies widely across subjects. Lower outcome scores are associated with greater insertion depth and greater number of contacts being located in scala vestibuli. Three patterns of scalar placement are observed suggesting variability in insertion dynamics arising from surgical technique.

CONCLUSION

A significant portion of variability in word recognition scores across a broad range of performance levels of CI subjects is explained by variability in scalar location and insertion depth of the electrode array. We suggest that this variability in electrode placement can be reduced and average speech reception improved by better selection of cochleostomy sites, revised insertion approaches, and control of insertion depth during surgical placement of the array.

Revision cochlear implantation surgery in adults: indications and results

OBJECTIVE

To assess the efficacy, risks, and indications of revision cochlear implantation (RCI) and to identify the clinical, audiologic, and device-related characteristics that predict outcome.

STUDY DESIGN

Retrospective case series.

SETTING

Academic tertiary referral center.

PATIENTS

Adults (> or = 18 yr) who underwent RCI at Johns Hopkins University.

INTERVENTION

Revision cochlear implant surgery.

MAIN OUTCOME MEASURES

Speech perception by open-set testing and patient report and patient report of symptom resolution.

RESULTS

During the 16-year period of this study, 4.8% of all adults implanted at our center have required 1 or more RCI surgeries. A total of 48 RCIs have been performed. The indications for RCI included infection (12%), electrode extrusion (15%), hard failure (23%), suspected device failure (42%), and isolated facial nerve stimulation (8%). Overall, successful resolution of the implant-related or medical condition was achieved with RCI in 83% of cases. Speech perception was lower in only 1 (2.1%) of 48 cases. Satisfactory preoperative speech recognition was preserved or surpassed in 5 of 6 infection cases and 8 cases with progressive symptoms of tinnitus and facial nerve stimulation. All cases of hard failure regained or surpassed previous peak performance. Improved speech recognition was experienced by 75% of cases with suspected device failure. Of cases in which RCI failed to restore previous functional benefit, there was a significant association with advanced age (> 70 yr). Whereas an abnormal integrity test was predictive of favorable outcome after RCI, a negative test was not predictive of outcome. A similar pattern of results was observed with respect to ex vivo device analysis.

CONCLUSION

Revision cochlear implantation can be safely performed to restore lost benefit in appropriately selected cases. When properly performed after medical and audiologic options have been exhausted, RCI rarely compromises previous function and, in most cases, can resolve functional complaints and distracting symptoms. When positive, integrity testing is a useful screen for the presence of a device defect. In cases in which device integrity is uncertain, clinical judgment guided by longitudinal assessment can help determine whether RCI is likely to be beneficial.

Surgical factors in pediatric cochlear implantation and their early effects on electrode activation and functional outcomes

OBJECTIVE

To assess the impact of surgical factors on electrode status and early communication outcomes in young children in the first 2 years of cochlear implantation.

STUDY DESIGN

Prospective multicenter cohort study.

SETTING

Six tertiary referral centers.

PATIENTS

Children 5 years or younger before implantation with normal nonverbal intelligence.

INTERVENTION

Cochlear implant operations in 209 ears of 188 children.

MAIN OUTCOME MEASURES

Percent active channels, auditory behavior as measured by the Infant Toddler Meaningful Auditory Integration Scale/Meaningful Auditory Integration Scale and Reynell receptive language scores.

RESULTS

Stable insertion of the full electrode array was accomplished in 96.2% of ears. At least 75% of electrode channels were active in 88% of ears. Electrode deactivation had a significant negative effect on Infant Toddler Meaningful Auditory Integration Scale/Meaningful Auditory Integration Scale scores at 24 months but no effect on receptive language scores. Significantly fewer active electrodes were associated with a history of meningitis. Surgical complications requiring additional hospitalization and/or revision surgery occurred in 6.7% of patients but had no measurable effect on the development of auditory behavior within the first 2 years. Negative, although insignificant, associations were observed between the need for perioperative revision of the device and 1) the percent of active electrodes and 2) the receptive language level at 2-year follow-up.

CONCLUSION

Activation of the entire electrode array is associated with better early auditory outcomes. Decrements in the number of active electrodes and lower gains of receptive language after manipulation of the newly implanted device were not statistically significant but may be clinically relevant, underscoring the importance of surgical technique and the effective placement of the electrode array.