Effects of chronic electrical stimulation on patients using a cochlear prosthesis

A longitudinal study of speech perception skills and device characteristics of adolescent cochlear implant users

BACKGROUND

For pediatric cochlear implant (CI) users, CI processor technology, map characteristics, and fitting strategies are known to have a substantial impact on speech perception scores at young ages. It is unknown whether these benefits continue over time as these children reach adolescence.

PURPOSE

To document changes in CI technology, map characteristics, and speech perception scores in children between elementary grades and high school, and to describe relations between map characteristics and speech perception scores over time.

RESEARCH DESIGN

A longitudinal design with participants 8-9-yr-old at session 1 and 15-18-yr-old at session 2.

STUDY SAMPLE

Participants were 82 adolescents with unilateral CIs, who are a subset of a larger longitudinal study. Mean age at implantation was 3.4 yr (range: 1.7-5.4), and mean duration of device use was 5.5 yr (range: 3.8-7.5) at session 1 and 13.3 yr (range: 10.9-15) at session 2.

DATA COLLECTION AND ANALYSIS

Speech perception tests at sessions 1 and 2 were the Lexical Neighborhood Test (LNT) presented at 70 dB SPL (LNT-70) and Bamford-Kowal-Bench sentences in quiet (BKB-Q) presented at 70 dB SPL. At session 2, the LNT was also administered at 50 dB SPL (LNT-50), and BKB sentences were administered in noise with a +10 dB SNR (BKB-N). CI processor technology type and CI map characteristics (coding strategy, number of electrodes, threshold levels, and comfort levels) were obtained at both sessions. Electrical dynamic range was computed, and descriptive statistics, correlations, and repeated-measures ANOVAs were employed.

RESULTS

Participants achieved significantly higher LNT and BKB scores, at 70 dB SPL, at ages 15-18 than at ages 8-9 yr. Forty-two participants had 1-3 electrodes either activated or deactivated in their map between test sessions, and 40 had no change in number of active electrodes (mean change: -0.5; range: -3 to +2). After conversion from arbitrary clinical map units to charge-per-phase in nanocoulombs (nC), no significant difference was found for T levels across time. Average comfort levels (C levels) decreased by 19 nC. Seventy-three participants (89%) upgraded their CI processor technology type. At both sessions, significant correlations were found between electrical dynamic range (EDR) and all speech perception measures except LNT-50 (r range: .31 to .47; p < 0.01). Similarly, significant correlations were also found between C levels and all speech perception measures (r range: .29 to .49; p < 0.01). At session 2, a significant correlation was found between processor technology type and the LNT-50 scores (r = .38; p < 0.01).

CONCLUSIONS

Significant improvement in speech scores was observed between elementary grades and high school for children who had used a CI since preschool. On average, T levels (nC) and electrode function remained stable for these long-term pediatric users. Analyses of maps did not allow for the determination of the exact cause of C level reductions, though power limitations in new processor systems and changes in perceived loudness over time are possible. Larger EDRs and higher C levels were associated with better speech scores. Newer speech processor technology was associated with better speech scores at a softer level.

Synaptic plasticity after chemical deafening and electrical stimulation of the auditory nerve in cats

The effects of deafness on brain structure and function have been studied using animal models of congenital deafness that include surgical ablation of the organ of Corti, acoustic trauma, ototoxic drugs, and hereditary deafness. This report describes the morphologic plasticity of auditory nerve synapses in response to ototoxic deafening and chronic electrical stimulation of the auditory nerve. Normal kittens were deafened by neonatal administration of neomycin that eliminated auditory receptor cells. Some of these cats were raised deaf, whereas others were chronically implanted with cochlear electrodes at 2 months of age and electrically stimulated for up to 12 months. The large endings of the auditory nerve, endbulbs of Held, were studied because they hold a key position in the timing pathway for sound localization, are readily identifiable, and exhibit deafness-associated abnormalities. Compared with those of normal hearing cats, synapses of ototoxically deafened cats displayed expanded postsynaptic densities, a 35.4% decrease in synaptic vesicle (SV) density, and a reduction in the somatic size of spherical bushy cells (SBCs). In comparison with normal hearing cats, ototoxically deafened cats that received cochlear stimulation had endbulbs that expressed postsynaptic densities (PSDs) that were statistically identical in size, showed a 48.1% reduction in SV density, and whose target SBCs had a 25.5% reduction in soma area. These results demonstrate that electrical stimulation via a cochlear implant in chemically deafened cats preserves PSD size but not other aspects of synapse morphology. This determination further suggests that the effects of ototoxic deafness are not identical to those of hereditary deafness.

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 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- an d C-levels weremeasured 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 samemeasures 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 werestable 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.

Effects of time after deafening and implantation on guinea pig electrical detection thresholds

Changes in detection threshold level as a function of time after deafening and implantation have been described previously in macaque [Pfingst, 1990] and human [Skinner et al., 1995] cochlear implant subjects. Characterization of the mechanisms underlying these changes will contribute to our understanding of the anatomical and physiological factors affecting electrical stimulus detection. In addition, understanding the time course of early threshold changes is essential to the interpretation of acute physiological studies of cochlear implants. To better characterize time-dependent threshold changes, we monitored changes in guinea pig psychophysical electrical detection thresholds with time after deafening and cochlear implantation. Threshold levels for 100 Hz sinusoidal bursts were initially unstable over the first 30 days post-surgery (DPS), after which thresholds stabilized. At longer intervals (>100 DPS), increases (>10 dB) in threshold level were observed for 100 Hz sinusoids in three of 11 cases. These changes were transient in one case and long-term in two cases. The time course of threshold change, both early and late, could not be explained on the basis of changes in spiral ganglion cell survival. The guinea pig seems to be an ideal preparation for studies of this nature, because threshold changes are similar in type, but accelerated in time course, relative to those observed in primates.

Effect of speech processor program modifications on cochlear implant recipients' threshold and maximum acceptable loudness levels

This study's purpose was to determine whether or not modifications in speech processor electrical stimulation levels were associated with changes in five Nucleus 22 cochlear implant recipients' thresholds or maximum acceptable loudness levels (MALs). These modifications in minimum and maximum stimulation levels were made to optimize hearing in everyday life. One threshold and one MAL were obtained on each active electrode during six, weekly test sessions, three before and three after program modification. Only one participant had a significant change in threshold after program modification; this participant and four others had significant changes in MAL. Participants' threshold variability was the same, but MAL variability was higher than that observed in other studies. Because these participants had no experience making MAL judgments prior to this study, this result suggests that implant recipients should be given sufficient practice in making MAL judgments to provide a stable clinical estimate of the upper boundary of the electrical dynamic range.

Across-species comparisons of psychophysical detection thresholds for electrical stimulation of the cochlea: I. Sinusoidal stimuli

Several species have been, and continue to be, used as subjects in studies of electrical stimulation of the cochlea. Few attempts, however, have been made to determine if data obtained from different species are quantitatively or qualitatively similar. The present work compares psychophysical absolute detection threshold vs. frequency functions for sinusoidal stimuli obtained from humans, nonhuman primates, cats, and guinea pigs. Threshold data for monopolar and bipolar electrode configurations from both previously published and unpublished studies are compared. In general, within all four species, significant intersubject variation in detection threshold level was found, but slopes of threshold vs. frequency functions were relatively well conserved within a species, under the conditions studied. With one exception (cat bipolar stimulation), threshold functions reached a minimum at or near 100 Hz across species and electrode configurations. In all cases, thresholds were significantly lower for monopolar, as compared with bipolar, configurations. Statistically, there were no significant differences in absolute threshold level across species. Threshold levels increased with frequency above 100 Hz at a rate of 3.0-7.9 dB/octave, depending on both electrode configuration and species. Slopes were steeper for monopolar than for bipolar configurations. When slopes were averaged between 200 and 2000 Hz, no statistically significant differences in overall slopes were found, nor was there a significant interaction between electrode configuration and species. There were, however, consistent species differences within more restricted regions of the function. Human functions for both monopolar and bipolar stimulation were steeper than all animal functions in the range of 100-300 Hz. Within this range, the differences between slopes for human and nonhuman subjects were statistically significant. In addition, differences were noted in the frequency at which slope decreased, with slopes for nonhuman subjects showing the decrease at higher frequencies than did those for human subjects. These differences may be true species differences, or may reflect the influence of confounding variables associated with each experimental-subject model.

Auditory plasticity in cochlear implant patients

Dynamic range (DR) of cochlear implant electrodes and speech recognition scores (SRS) were chronologically studied in 23 post-lingually deaf subjects over a period of one year. DR significantly increased over one year (ANOVA, p = 0.03). When DR was compared among three groups of electrodes which were located in the inferior, ascending and superior segments of the cochlear basal turn, that in the superior segment showed the largest increase (p = 0.001), followed by those in the ascending and inferior segments. Vowel and consonant recognition scores with auditory cue were best 6 months after operation and were maintained at the level for one year. Although the time course of improvement differed between DR and SRS, a significant positive correlation was observed at 12 months after operation. Improvement of SRS could be a result of a DR increase in the superior segment of the cochlea, which should convey pitch information over the mid-frequency region.

Functional responses from guinea pigs with cochlear implants. II. Changes in electrophysiological and psychophysical measures over time

This study, the second of a two-part investigation, assessed changes over time in functional measures of the electrically stimulated auditory system following ototoxic deafening. Guinea pigs were trained to respond behaviorally to threshold level acoustic stimuli and then unilaterally deafened and implanted with a bipolar pair of electrodes within the cochlea and a single extracochlear electrode. Using pulsatile stimuli, thresholds for the electrically evoked auditory brainstem response (EABR) and psychophysical detection were repeatedly collected from the same animals over 3-month post-implantation periods. Thresholds were obtained as a function of stimulus phase duration primarily using bipolar intracochlear stimulation. As in earlier studies, the threshold measures exhibited both intra- and intersubject variability. Analysis of group data failed to show any statistically significant changes over time in either EABR or psychophysical threshold at any fixed pulse duration. However, significant changes over time were found in the slopes of the strength-duration functions for both measures. Slopes became shallower with time, suggesting a reduction in the efficiency of stimulus current integration, a trend presumed to occur with neural degeneration. This result suggests that strength-duration functions could be useful as a clinical diagnostic measure.