Carolyn J. Brown

Summary of Results Using the Nucleus CI24M implant to record the electrically evoked compound action potential

OBJECTIVE This study outlines a series of experiments using the neural response telemetry (NRT) system of the Nucleus CI24M cochlear implant to measure the electrically evoked compound action potential (EAP). The goal of this investigation was to develop a protocol that allows successful recording of the EAP in a majority of CI24M cochlear implant users. DESIGN Twenty-six postlingually deafened adults participated in this study. A series of experiments were conducted that allowed us to examine how manipulation of stimulation and recording parameters may affect the morphology of the EAP recorded using the Nucleus NRT system. RESULTS Results of this study show consistent responses on at least some electrodes from all subjects. Cross-subject and cross electrode variations in both the growth of the response and the temporal refractory properties of the response were observed. The range of stimulus and recording parameters that can be used to record the EAP with the Nucleus NRT system is described. CONCLUSIONS Using the protocol outlined in this study, it is possible to reliably record EAP responses from most subjects and for most electrodes in Nucleus CI24M cochlear implant users. These responses are robust and recording these responses does not require that the subject sleep or remain still. Based on these results, a specific protocol is proposed for measurement of the EAP using the NRT system of the CI24M cochlear implant. Potential clinical implications of these results are discussed.

The relationship between EAP and EABR thresholds and levels used to program the Nucleus 24 speech processor : Data from adults

Objective: The objective of this study was to determine the relationship between electrically evoked whole nerve action potential (EAP) and electrical auditory brain stem response (EABR) thresholds and MAP threshold (T‐level) and maximum comfort level (C‐level) for subjects who use the Nucleus 24 cochlear implant system. Design: Forty‐four adult Nucleus 24 cochlear implant users participated in this study. EAP thresholds were recorded using the Neural Response Telemetry System developed by Cochlear Corporation. EABR thresholds were measured for a subset of 14 subjects using standard evoked potential techniques. These physiologic thresholds were collected on a set of five electrodes spaced across the cochlea, and were then compared with behavioral measures of T‐level and C‐level used to program the speech processor. Results: EAP thresholds were correlated with MAP T‐ and C‐levels; however, the correlation was not strong. A technique for improving the correlation by combining measures of T‐ and C‐levels made on one electrode with the EAP thresholds was presented. Correlations between predicted and measured T‐ and C‐levels using this technique were 0.83 and 0.77, respectively. Similar results were obtained using the EABR thresholds for a smaller set of subjects. In general, EABR thresholds were recorded at levels that were approximately 4.7 programming units lower than EAP thresholds. Conclusions: Either EAP or EABR thresholds can be used in combination with a limited amount of behavioral information to predict MAP T‐ and C‐levels with reasonable accuracy.

Electrically evoked whole-nerve action potentials : data from human cochlear implant users

This study describes a method for recording the electrically evoked, whole-nerve action potential (EAP) in users of the Ineraid cochlear implant. The method is an adaptation of one originally used by Charlet de Sauvage et al. [J. Acoust. Soc. Am. 73, 615-627 (1983)] in guinea pigs. The response, recorded from 11 subjects, consists of a single negative peak that occurs with a latency of approximately 0.4 ms. EAP input/output functions are steeply sloping and monotonic. Response amplitudes ranging up to 160 micro V have been recorded. Slope of the EAP input/output function correlates modestly (approximately 0.6-0.69) with results of tests measuring word recognition skills. The refractory properties of the auditory nerve were also assessed. Differences across subjects were found in the rate of recovery from the refractory state. These findings imply that there may be difference across subjects in the accuracy with which rapid temporal cues can be coded at the level of the auditory nerve. Reasonably strong correlations (approximately 0.74-0.85) have been found between the magnitude of the slope of these recovery curves and performance on tests of word recognition.

Comparison of EAP thresholds with MAP levels in the nucleus 24 cochlear implant: data from children.

Objective: The purpose of this study is to examine the relationship between the electrically evoked compound action potential (EAP) thresholds and the MAP thresholds (T‐levels) and maximum comfort levels (C‐levels) in children implanted with the Nucleus 24 device. Design: EAP thresholds were measured using the Neural Response Telemetry system of the Nucleus 24 device. Twenty children implanted with the Nucleus 24 cochlear implant participated in this study. EAP thresholds were compared with the behavioral measures of T‐ and C‐level used to construct the MAP these children used on a daily basis. For these subjects, both EAP and MAP T‐ and C‐levels were obtained the same visit, which occurred at 3 to 5 mo postconnection. Results: EAP thresholds were shown to fall between MAP T‐ and C‐level for 18 of 20 subjects tested; however, considerable variability across subjects was noted. On average, EAP thresholds fell at 53% of the MAP dynamic range. Correlations between EAP threshold and MAP T‐ and C‐level improved substantially when combined with behavioral measures obtained from one electrode in the array. Conclusions: Moderate correlations were found between EAP thresholds and MAP T‐ and C‐levels for the children participating in this study. However, a technique is described for improving the accuracy of predictions of MAP T‐ and C‐levels based on EAP data combined with a small amount of behavioral information.

A longitudinal study of electrode impedance, the electrically evoked compound action potential, and behavioral measures in nucleus 24 cochlear implant users.

Objective The primary goal of this study was to examine changes that may occur in electrode impedance, electrically evoked compound action potential (EAP) threshold and slope of the EAP growth function, and behavioral measures of threshold (MAP T-level) and maximum comfort (C-level) over time in both adult and child cochlear implant users. Secondary goals were to determine whether changes in these measures are consistent between children and adults, and to determine whether behavioral measures (MAP T- and C-levels) and electrophysiologic measures (EAP thresholds) exhibit the same trends over time. Design Thirty-five children and 33 adults implanted with the Nucleus CI24M between November 1996 and August 1999 participated in this study. Subjects were included in this study if 1) they had used their implant for at least 1 yr after device connection, and 2) they had participated in the necessary data collection at a minimum number of the time intervals assessed in this study. EAP threshold, slope of the EAP growth function, and common ground electrode impedance measures were collected intraoperatively, at initial stimulation, and at several subsequent visits up to 2 yr post initial stimulation. MAP T- and C-levels were measured at initial stimulation and at the same time intervals as described above. Results Changes in electrode impedance, EAP thresholds, and slope of the EAP growth function from measures made intraoperatively, at initial stimulation, and at 1 to 2 mo post initial stimulation were similar in both children and adults. Beyond the 1- to 2-mo visit, children exhibited significant increases in electrode impedance, EAP thresholds, slope, and MAP T-levels, whereas these same measures in adults remained relatively stable. EAP thresholds in children stabilized by the 3- to 8-mo visit, and electrode impedance stabilized by the 6- to 8-mo visit, while slope of the EAP growth function, MAP T-levels, and MAP C-levels were stable by 1 yr post initial stimulation. C-levels in adults increased up to 1 yr post initial stimulation; however, the amount of increase was much smaller than that seen in children. In both children and adults, longitudinal trends in EAP thresholds mirrored T-level more closely than C-level. Conclusions The results of this study suggest that peripheral changes occur in many children that do not generally occur in adults within the first year of cochlear implant use. One implication of these results is that if EAP thresholds are to be used to assist in programming the speech processor for children, it is best to make those measures at the same time interval as device programming rather than using measures made intraoperatively or at the initial programming session to set MAP levels at later visits.

Binaural cochlear implants placed during the same operation

Objective To evaluate the binaural listening advantages for speech in quiet and in noise and to localize sound when independently programmed binaural cochlear implants are used, and to determine whether ears with different hearing ability and duration of profound deafness perform differently with cochlear implants as well as to what extent preimplant psychophysical and physiologic assessment could be predictive of performance. Study Design Prospective study in which patients were prospectively selected to undergo bilateral implantation during a single surgical procedure at a tertiary referral center. All testing was performed with patients using their right, left, or both cochlear implants. Preimplant and intraoperative measures used electrical stimulation at the round window and stimulation through the cochlear implant. Results Bilateral implantation during the same operation did not cause any postoperative problems such as severe vertigo or ataxia. At 1 year, results of speech testing in quiet demonstrated a binaural advantage for 2 of 10 subjects. Speech-in-noise testing demonstrated that two implants were beneficial for two individuals. All subjects benefited from a head shadow effect when an ear with a better signal-to-noise ratio was available. The ability to localize sound was improved with binaural implants in all subjects. Preimplant psychophysical or physiologic measures were not predictive of eventual speech perception performance. Conclusion Binaural cochlear implants can assist in the localization of sounds and have the potential in some individuals to improve speech understanding in quiet and in noise.

Auditory neuropathy characteristics in children with cochlear nerve deficiency.

Objective: To describe a group of children exhibiting electrophysiologic responses characteristic of auditory neuropathy (AN) who were subsequently identified as having absent or small cochlear nerves (i.e., cochlear nerve deficiency). Design: A retrospective review of the clinical records, audiological testing results, and magnetic resonance imaging (MRI) studies. Fifty-one of 65 children with AN characteristics on auditory brain stem response (ABR) testing had MRI available for review. Nine (18%) of these 51 children with ABR characteristic of AN have been identified as having small (N = 2; 4%) or absent (N = 7; 14%) cochlear nerves on MRI. Results: Of the nine children with cochlear nerve deficiency, five (56%) were affected unilaterally and four (44%) bilaterally. Eight of nine presented after failing a newborn infant hearing screening, whereas one presented at 3 yr of age. On diagnostic ABR testing, all 9 children (9 of 13 affected ears; 69%) had evidence of a cochlear microphonic (CM) and absent neural responses in at least one ear. In the unilateral cases, AN characteristics were detected in all affected ears. In bilateral cases, at least one of the ears in each child demonstrated the AN phenotype, whereas the contralateral ear had no CM identified. Only one ear with cochlear nerve deficiency had present otoacoustic emissions as measured by distortion-product otoacoustic emissions. In children with appropriate available behavioral testing results, all ears without cochlear nerves were identified as having a profound hearing loss. Only 4 (31%) of the 13 ears with cochlear nerve deficiency had a small internal auditory canal on MRI. Conclusions: Children with cochlear nerve deficiency can present with electrophysiologic evidence of AN. These children frequently refer on newborn screening examinations that use ABR-based testing methods. Similar to other causes of AN, diagnostic ABR testing will show a CM with absent neural responses. Given that 9 (18%) of 51 children with available MRI and electrophysiologic characteristics of AN in our program have been identified as having cochlear nerve deficiency makes this a relatively common diagnosis. These findings suggest that MRI is indicated for all children diagnosed with AN. Moreover, electrophysiologic evidence of unilateral AN in association with a profound hearing loss should make the clinician highly suspicious for this problem. Although children with cochlear nerve deficiency who have a small nerve may benefit from cochlear implantation or amplification, these interventions are obviously contraindicated in children with completely absent cochlear nerves.

Cochlear Implantation in Children with Congenital Inner Ear Malformations

Objective/Hypothesis: To assess the audiologic and surgical outcomes for pediatric cochlear implant patients with inner ear malformations.

An improved method of reducing stimulus artifact in the electrically evoked whole-nerve potential.

Objective: Recording a compound action potential in response to electrical stimulation requires attention to minimize contamination due to electrical stimulus artifact. In patients implanted with the Nucleus® 24 device, the electrically evoked whole‐nerve potential (EAP) is recorded using a neural response telemetry (NRT) system. This system employs a forward‐masking technique that greatly reduces stimulus artifact. However, theoretical considerations and experimental animal data suggest that the technique may distort the acquired EAP waveform under some situations. We proposed and evaluated a modification to the forward‐masking technique that addresses this concern, particularly during collection of refractory recovery data. Design: We first examined neural responses of the electrically stimulated auditory nerve using cat preparations. Through single‐fiber recordings from cats, we demonstrated underlying physiological limitations likely encountered with the “standard” forward masking technique. We then recorded feline EAP waveforms using both the standard technique and our proposed, modified, technique. Finally, we collected EAP data from human cochlear implant patients using both artifact reduction methods. These comparisons allowed us to evaluate the effectiveness of our modification. Results: The cat EAP data demonstrated that the standard forward‐masking technique currently in use in the Nucleus® NRT system can distort the EAP waveform when the nerve is partially refractory. In the cat, this distortion resulted in forward‐masking recovery curves with artifactually prolonged recovery times and inaccurate latency trends. Similar effects were observed in the comparison of human recovery curves obtained using both the standard and modified techniques. In some cases, the modified technique produced EAP waveforms with more clearly defined peaks than were obtainable with the standard method. Conclusions: Consideration should be given to implementing our modified forward‐masking artifact reduction scheme, because it introduces less distortion of the EAP waveform and accordingly provides for more accurate assessment of the refractory properties of the electrically stimulated nerve.

Electrical suppression of tinnitus with high-rate pulse trains.

Hypothesis Application of high-rate pulse trains (e.g., 4800 pps) to the cochlea may represent an effective treatment of tinnitus. Background Tinnitus is a widespread clinical problem with multiple treatments but no cure. A cure for tinnitus would restore the perception of silence. One plausible hypothesis for the origin of tinnitus associated with sensorineural hearing loss is that it is due to loss or alteration of the normal spontaneous activity in the deafferented regions of the cochlea. Electrical stimulation of the cochlea with 5000-pps pulse trains can produce spontaneous-like patterns of spike activity in the auditory nerve. Methods Eleven volunteer human subjects with bothersome tinnitus and high-frequency sensorineural hearing loss underwent myringotomy and temporary placement of a round window electrode. High-rate pulse train stimuli were presented at various stimulus intensities and tinnitus, and stimulus perception were scaled by the subject. Three cochlear implant recipients with tinnitus in the implanted ear underwent similar stimulation. Results Five of 11 (45%) of transtympanic subjects showed substantial or complete tinnitus suppression with either no perception or only a transient perception of the stimulus. Three showed tinnitus suppression only in association with the perception of the stimulus. Three showed no effects on tinnitus. A similar pattern of responses was seen in the cochlear implant subjects. Conclusions Although the study lacked an ideal placebo control, the results are promising and support further research to develop a clinically useful intervention for this troubling disorder.