|Year : 2020 | Volume
| Issue : 2 | Page : 89-92
Electrically evoked stapedial reflex threshold assessment among malaysian cochlear implant users: Comparisons between different stimulus rates
Cheu Lih Aw1, Sook Kim Tan1, Rosninda Abdullah1, Nik Adilah Nik Othman2, Mohd Normani Zakaria1
1 Department of Audiology and Speech Pathology, School of Health Sciences, Universiti Sains Malaysia, Kubang Kerian, Kelantan, Malaysia
2 Department of Otorhinolaryngology, School of Medical Sciences, Universiti Sains Malaysia, Kubang Kerian, Kelantan, Malaysia
|Date of Submission||07-Nov-2019|
|Date of Decision||26-May-2020|
|Date of Acceptance||03-Jun-2020|
|Date of Web Publication||17-Jul-2020|
Dr. Mohd Normani Zakaria
Department of Audiology and Speech Pathology, School of Health Sciences,
Universiti Sains Malaysia, 16150 Kubang Kerian, Kelantan
Source of Support: None, Conflict of Interest: None
Background and Objective: Cochlear implant (CI) mapping can be carried out using subjective or objective assessments. Electrically evoked stapedial reflex threshold (ESRT) measurement is one of the valuable objective assessments to establish appropriate CI maps. As such, more research efforts are required to determine the most optimum recording protocols when recording ESRT. In the present study, the effect of stimulus rate on ESRT among CI users was determined. It was also of interest to know which stimulus rate would produce the optimum correlation with the behavioral most comfortable level (MCL) results. Materials and Methods: In this study that employed a repeated measures design, 12 Malaysian CI users were enrolled (mean age = 22.3 ± 13.3 years, ranging from 14 to 51 years). MCL and ESRT (measured in charge units, qu) were measured for three representative electrodes (electrode 2, electrode 6, and electrode 11) using 1000 pps and 2000 pps rates. Results: ESRT was 100% recordable for electrodes 2 and 6. For electrode 2, the mean ESRT values were 20.5 qu and 20.4 qu for 1000 pps and 2000 pps, respectively. While for electrode 6, the mean ESRT was 25.4 qu for 1000 pps and 25.7 for 2000 pps. Excellent correlations were found between ESRT and MCL for both stimulus rates (intraclass correlation ≥0.90). Conclusion: Both stimulus rates were found to produce comparably good ESRT results. Since ESRT and MCL values were highly correlated with each other, the objective ESRT testing can be used conveniently to predict the behavioral MCL, particularly among difficult-to-test patients.
Keywords: Cochlear implant, electrically evoked stapedial reflex threshold, mapping, stimulus rate
|How to cite this article:|
Aw CL, Tan SK, Abdullah R, Nik Othman NA, Zakaria MN. Electrically evoked stapedial reflex threshold assessment among malaysian cochlear implant users: Comparisons between different stimulus rates. Indian J Otol 2020;26:89-92
|How to cite this URL:|
Aw CL, Tan SK, Abdullah R, Nik Othman NA, Zakaria MN. Electrically evoked stapedial reflex threshold assessment among malaysian cochlear implant users: Comparisons between different stimulus rates. Indian J Otol [serial online] 2020 [cited 2020 Aug 12];26:89-92. Available from: http://www.indianjotol.org/text.asp?2020/26/2/89/289943
| Introduction|| |
Nowadays, more treatment options are available for individuals with hearing loss. Sophisticated hearing aids are conveniently available in the market to help those with up to severe degree of hearing loss to hear better. Cochlear implant (CI) is an electronic device that bypasses the damaged hair cells in the inner ear and stimulates the auditory nerve directly to restore hearing in patients with bilateral severe to profound hearing loss. According to the recent data from the U.S. Food and Drug Administration, approximately 324,200 people worldwide have received the implants.
After the CI surgery, subsequent procedures (i.e., CI mapping and speech therapy) will take place. In the mapping process, threshold (T) level and most comfortable level (MCL) are typically obtained for each implanted electrode either using subjective or objective measurements. In recent years, the age limit for pediatric cochlear implantation has become progressively lower., Consequently, the CI mapping with subjective measurements can be difficult, especially in young children as they could provide only minimal feedback, have shorter auditory experience and inadequate cognitive abilities. As such, the reliability of CI maps obtained behaviorally can be affected. In this regard, objective measurements for CI mapping are beneficial. Electrically evoked stapedial reflex threshold (ESRT) measurement is one of the valuable objective assessments to establish appropriate CI maps.,
ESRT is defined as the lowest amount of electrical stimulation that produces an identifiable acoustic reflex waveform (amplitude deflection of at least 0.02 mmho). Apart from its objective nature, it is also quick, easy, accurate, and can be recorded using the conventional tympanometric machine (i.e., no additional device is needed)., However, more research efforts are required to determine the most optimum recording protocols when recording ESRT. Thus, the present study was conducted to determine the effect of stimulus rate on ESRT among Malaysian CI users. In addition, it was also of interest to know which stimulus rate would produce the optimum correlation with the behavioral MCL results. These data are vital for describing the characteristics of ESRT values among Malaysian CI users, developing standards of care for the audiologists and establishing appropriate norms.
| Materials and Methods|| |
The present study employed a repeated measures design. Prior to the data collection, an ethical approval was obtained from the Institutional Ethics Committee, which is in accordance with the 1975 Declaration of Helsinki and its later amendments. All participants provided their consent form indicating the voluntary participation in the study.
Twelve Malaysian CI users (mean age = 22.3 ± 13.3 years, ranging from 14 to 51 years) were enrolled in this study. They were selected among patients of Audiology Clinic, University Hospital using the convenience sampling method. All of them fulfilled the inclusion criteria, i.e., those who had been implanted and switched on at least 6 months prior to the study participation, and impedance telemetry showed regular electrode impedances.
Most comfortable level and evoked stapedial reflex threshold procedures
All procedures took place in the Audiology Clinic, University Hospital. After conducting brief history taking, otoscopic examination, and tympanometry were carried out to ensure clear ear canals and healthy middle ears (the presence of conductive elements such as fluid in the middle ear can affect the ESRT measurement). MCL and ESRT (measured in charge units, qu) were determined for three representative electrodes: electrode 2 (apical array), electrode 6 (middle array), and electrode 11 (basal array). The electrical stimulation was presented to CI device through the respective fitting software. By choosing one electrode, the MCL was obtained using a subjective verbal judgment method. In particular, loudness scales were used and he/she was asked to judge at which stimulation level produced the most comfortable loudness. The MCL was then measured for other electrodes.
For recording the ESRT, the tympanometric device (AT235 Middle Ear Analyzer by Interacoustics, Denmark) was set for the measurement of acoustic reflex decay (with a longer recording window). This device was calibrated in accordance with EN60645-5/ANSI S3.39 (1987). A soft recording probe was placed in the ear contralateral to the CI device. On the stimulation, the changes of middle ear impedance were measured with a 226 Hz pure tone. By using the CI fitting software, one electrode was chosen and stimulated with 500 ms bursts of stimulation pulses at 2000 pps. The stimulation level was increased until a clear acoustic reflex waveform was observed. The stimulation level was then manipulated using the standard bracketing procedure to obtain ESRT. The ESRT was taken as the lowest stimulation level that produced definite and repeatable amplitude deflection of at least 0.02 mmho. Similar procedures were used for other electrodes and repeated for 1000 pps rate. During the ESRT testing, the participants were asked to remain relatively still and quiet.
The MCL and ESRT values for each stimulus rate (1000 pps and 2000 pps) and for all electrodes (i.e., 2, 6, and 11) were computed for each participant (in charge units, qu). Both descriptive and inferential statistics were expressed as applicable. Since the data were found to be normally distributed (P > 0.05 by Shapiro–Wilk test), parametric analyses could be carried out. For each stimulus rate, paired t-test was used to compare the charge unit values between MCL and ESRT. Likewise, for each electrode, paired t-test was also employed to compare ESRT results between the two stimulus rates. To determine the correlation between MCL and ESRT for each electrode, intraclass correlation (ICC, with a two-way mixed effects and absolute agreement type) was utilized. The statistical significance level was set at P < 0.05. To support the hypothesis testing results, Cohen's effect size (d) was computed. The effect sizes were interpreted as small (d = 0.20), medium (d = 0.50), and large (d = 0.80). All data were analyzed using SPSS software version 20 (SPSS Inc., Chicago, IL, USA).
| Results|| |
Among the CI participants, 61% were males and the remaining 39% were females. The age at implantation ranged from 5 to 43 years (mean = 15.5 ± 13.0 years), while the duration of CI use was between 2 and 9 years (mean = 6.8 ± 2.8 years). As illustrated in [Figure 1], the possible causes of hearing loss among the participants were rubella (25%), 13% meningitis (13%), sepsis (12%), genetic (12%), and unknown (38%). The majority of them used CI monaurally (88%).
|Figure 1: The possible causes of hearing loss among cochlear implant users in the present study|
Click here to view
The MCL was successfully recorded from all CI users for all electrodes (i.e., 2, 6, and 11). Likewise, the ESRT was able to be measured in all participants for electrodes 2 and 6. Nevertheless, only 29% of CI users revealed measurable ESRT for electrode 11. Consequently, only the data from electrodes 2 and 6 were analyzed using the intended descriptive and inferential statistics.
Mean and standard error of mean of MCL and ESRT for different stimulus rates for electrodes 2 and 6 is shown in [Figure 2]. For electrode 2, the MCL ranged from 12.7 qu to 37.5 qu (mean = 23.9 qu). The ESRT values ranged from 12.5 qu to 36.8 qu (mean = 20.5 qu) and from 12.5 qu to 34.8 qu (mean = 20.4 qu) for stimulus rates of 1000 pps and 2000 pps, respectively. As revealed descriptively, both stimulus rates produced mean ESRT values lower than that of MCL for electrode 2 [Figure 2]. However, comparable results were found between MCL and ESRT for both stimulus rates (P = 0.095 and P = 0.078 for 1000 pps and 2000 pps, respectively). These statistical results were supported by small effect sizes (d = 0.41 for 1000 pps and d = 0.42 for 2000 pps). Likewise, no significant effect of stimulus rate was found on ESRT for electrode 2 (P = 0.899). The correlation between MCL and ESRT was excellent for both stimulus rates (ICC values of 0.90 and 0.91 for 1000 pps and 2000 pps, respectively).
|Figure 2: Mean and standard error of mean of most comfortable level and electrically evoked stapedial reflex threshold recorded with stimulus rates of 1000 pps and 2000 pps for electrode 2 (a) and electrode 6 (b) (in charge units, qu)|
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For electrode 6, the MCL ranged from 13.5 qu to 35.8 qu (mean = 25.8 qu) [Figure 2]. For 1000 pps rate, the ESRT ranged from 12.7 qu to 47.1 qu (mean = 25.4 qu). For the stimulus rate of 2000 pps, the ESRT ranged from 13.1 qu to 47.1 qu (mean = 25.7 qu). When the MCL and the ESRT results were compared, the statistical results were insignificant for both stimulus rates (P = 0.866 and d = 0.04 for 1000 pps, P = 0.972 and d = 0.01 for 2000 pps). Similar to the result of electrode 2, no significant influence of stimulus rate was found on ESRT for electrode 6 (P = 0.180). Both MCL and ESRT results were found to be highly correlated with each other (ICC = 0.90 for both stimulus rates).
| Discussion|| |
In general, the mean ESRT values obtained in the present study are in line with those of previous studies involving CI users.,, Even though the statistical results were insignificant, these ESRT results were generally below the respective MCL values. This pattern is sensible and has been consistently demonstrated in other studies.,,,,, This indicates that if the ESRT method is used for CI mapping, an over-stimulation will be avoided. The over-stimulation is a condition where the stimulation levels exceed the amount needed to create the desired auditory perception. When it occurs, the sound quality and the auditory skills can be affected. As such, the soft sounds will be perceived as loud and consonant sounds can be distorted. This is not particularly good for children as they may acquire poor speech production skills.
In the present study, the correlations between ESRT and MCL were measured using ICC. The ICC values are interpreted as poor (<0.50), moderate (0.50–0.75), good (0.75–0.9), and excellent (>0.90). As revealed, excellent correlations were found between ESRT and MCL (≥0.90). This finding is in fact consistent with those of previous studies.,, Moreover, the ESRT results were not dependent on the stimulus rate (i.e., both 1000 pps and 2000 pps rates produced comparable ESRT results). This implies that the ESRT results can accurately predict the behavioral MCL values, which is beneficial when testing difficult-to-test patients.,,
In the present study (that compared ESRT results between 1000 pps and 2000 pps rates), the effect of stimulus rate on ESRT was not evident. This indicates that both rates were sufficient to produce the optimum neural stimulation. Theoretically, high rates of electrical stimulation would allow more information to be captured enabling CI users to achieve the optimum performance in the ESRT testing. That is, the use of high pulse rates may increase the temporal integration resulting in increased loudness. In a study by Buckler and Overstreet, the ESRT was recorded from eight CI users using approximately 3000 pps rate. The correlation value between ESRT and MCL was 0.94 (slightly higher than those of the present study due to the use of a higher stimulus rate). However, the use of too high pulse rates should be avoided, that is, increased pulse rate would decrease the amount of neural excitation due to the neural refractoriness.
Similar to the method employed in the previous studies, we recorded ESRT from the representative electrodes, i.e., electrode 2 (apical array), electrode 6 (middle array), and electrode 11 (basal array)., For electrode 11, the ESRT could not be measured in more than 50% of CI users. This is due to several possible reasons. First, since the basal electrodes are often placed within the cochleostomy or extra-cochlear area, higher artifacts would be detected affecting the ESRT recording. Second, the acoustic reflex is best elicited with low-frequency stimuli. For the high-frequency stimulation, a rapid adaptation of the auditory nerve would occur and the reflex is often absent. Thus, the absence of recordable ESRT for electrode 11 (basal array that stimulates high-frequency nerve fibers) might be due to this factor.
The present study had several limitations. First, all participants recruited in the present study were fitted with one brand of CI. In this regard, the influence of stimulus rate on ESRT among CI users from other manufacturers should also be studied. Second, the present study only gathered the ESRT data from teenagers and adults. As such, a similar study should be performed among pediatric CI users. It is worth mentioning that even though the sample size in the present study was small, the desired results (excellent correlations) had been obtained. Nevertheless, conducting studies with bigger samples is beneficial to ascertain the present study findings.
| Conclusion|| |
Among CI users, excellent correlations were found between ESRT and MCL for both stimulus rates (1000 pps and 2000 pps). This implies that the objective ESRT testing can accurately predict the behavioral MCL, which is advantageous when assessing difficult-to-test patients. Both stimulus rates were found to produce comparably good ESRT results and can be conveniently used to record ESRT in clinical settings.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]