|Year : 2015 | Volume
| Issue : 1 | Page : 14-18
Mapping of pediatric cochlear implant recipients using electrical auditory brainstem responses as a tool
Ruchika Mittal, AV Ramesh, SS Panwar, Ajith Nilkanthan, VR Sinha, Satish Nair, Poonam Raj
Department of ENT, Army Hospital (Research and Refferal), New Delhi, India
|Date of Web Publication||10-Mar-2015|
Dr. Ruchika Mittal
M-37C Rajouri Garden, New Delhi - 110 027
Source of Support: None, Conflict of Interest: None
Objective: With expanding cochlear implant (CI) candidacy criteria young children, children with additional co-morbidities and abnormal cochlea are being implanted. In many of these cases, the child's behavioral or cognitive limitations may make doing the task of behavioral mapping difficult or impossible in pediatric CI recipients (CIR). Therefore, the present study was aimed to determine whether behavioral thresholds (T) could be predicted from objective measurements of electrical auditory brainstem responses (EABR). Methodology: Seventy-five pediatric CIR using nucleus 24 R (ST) implant and operated and visiting the CI center of a tertiary care hospital in New Delhi, India were prospectively enrolled in the study. The correlation between postoperative thresholds-EABR (t-EABR) with behaviorally obtained T and C levels was studied for CIR in the age range of 5-9 years. Statistical Analysis: The significance of the correlation was calculated using Karl Pearson's correlation. Results: Mean age of CIR was 9 ± 1.6 years, with male: female: 38:37. Of the 75 recipients, one had Mondini deformity, and two subjects had auditory neuropathy spectrum disorder as per their medical records. The group correlation coefficient was found to be r = 0.989 at P < 0.0001 for the "T" level and r = 0.885 at P < 0.0001 for "C" level versus t-EABR. Conclusion: t-EABR was found to be more correlated with T levels than with C levels, and it was always between T and C levels.
Keywords: Cochlear implants, Mapping, Pediatric, Threshold and comfortable levels, Threshold electrical auditory brainstem responses
|How to cite this article:|
Mittal R, Ramesh A V, Panwar S S, Nilkanthan A, Sinha V R, Nair S, Raj P. Mapping of pediatric cochlear implant recipients using electrical auditory brainstem responses as a tool. Indian J Otol 2015;21:14-8
|How to cite this URL:|
Mittal R, Ramesh A V, Panwar S S, Nilkanthan A, Sinha V R, Nair S, Raj P. Mapping of pediatric cochlear implant recipients using electrical auditory brainstem responses as a tool. Indian J Otol [serial online] 2015 [cited 2022 Sep 26];21:14-8. Available from: https://www.indianjotol.org/text.asp?2015/21/1/14/152852
| Introduction|| |
Cochlear implantation (CI) has become an effective and efficient method to treat patients with severe to profound hearing loss. The implantation at young age has been demonstrated to have better results.  In order to obtain optimal results postimplantation the implant needs to be programmed accurately.
The programming, that is, mapping of the nucleus (Cochlear Pvt., Australia) CI is performed by an audiologist by measuring electrical threshold (T) levels and most comfortable (C) levels of stimulus on different electrodes of CI. Programming for the Advanced Bionics and Med-El devices has not necessitated acquiring of T levels, as these levels have been obtained through automatic software manipulation. Electrical T is typically the softest sound that can be reliably identified by the patient 100% of the time. Electrical comfort level is defined as the loudest sound that can be listened comfortably for a sustained period of time. 
The gold standard in mapping is to use the patient's subjective responses to adjust the CI fitting parameters. Nowadays, with expanded CI candidacy criteria young children, children with additional co-morbidities and abnormal cochlea are being implanted. In many of these cases, the child's behavioral or cognitive limitations may make doing the task of behavioral mapping difficult or impossible. Various objective tools have been proposed in the literature for an objective definition of psychophysical T and C levels like electrically evoked stapedius reflex T (ESRT),  electrically evoked compound action potential (ECAP)  and electrically evoked auditory brainstem responses (EABR). 
Electrical auditory brainstem responses had been correlated with T and C levels in the literature previously. ,, However, the studies performed were on small number of adult patients and had reported the use of intra-operative EABR recordings for the purpose of correlation.
It has been proposed previously that postimplant EABR T may have better correlation with T and C levels.  Therefore, the present study was undertaken to record EABR T postoperatively in pediatric nucleus CI recipients (CIR) and to find a correlation with behaviorally obtained T and C levels and to determine whether we can predict behavioral T from objective measurements.
| Methodology|| |
Pediatric CIR operated and visiting the CI center of Department of Otolaryngology of a Tertiary Care Hospital in New Delhi, India were prospectively enrolled in the study after taking an informed consent [Table 1]. Each of the recipients was evaluated and considered for CI on the basis of the candidacy guidelines laid by the Department of ENT, Army Hospital (Research and Referral). All the recipients were implanted with posterior tympanotomy with cochleostomy surgical approach by various CI surgeons. All the children were implanted with nucleus 24 CI 24 RST (Cochlear Pvt., Australia) device.
Electrical auditory brainstem responses measurement protocol
Electrical auditory brainstem responses were recorded at electrodes 5 (apical), 11 (middle) and 18 (basal) using Custom Sound EP® software of Nucleus CI company and GSI Audera® software in two different laptops. Both the systems were connected through cable. Custom Sound EP® was used to set the stimulus parameters. The generated stimulus was sent to the implant via a speech processor and the response generated in the form of EABR waveforms was recorded in GSI Audera® software. The electrodes were placed in the contralateral ear (reference electrode), lower forehead (ground electrode) and high forehead (active electrode) to record EABR waveforms. Stimulus parameters used are mentioned in [Table 2]. EABR recording was started at 160 current levels as it is likely to generate a C sound to the recipients. Wave V was traced and marked manually. If wave V was absent, the current level was increased in steps of ten current levels if wave V was present current level was reduced by five current levels. The minimum current level at which wave V was recorded in EABR was marked as thresholds-EABR (t-EABR). EABRs were performed in sound-treated rooms, and the patients were to sleep during the testing. The uncooperative children were sedated with an appropriate dosage of syrup triclofos oral solution BP (0.5 ml/kg).
Thresholds and comfortable level measurement
The T and C levels were measured using the Custom Sound 3.3 programming software (Cochlear Ltd., Australia). Monopolar stimulation with a rate of 1200 Hz was used to create the maps. As a part of custom sound streamlined programming method, 3 electrodes (5, 11, and 18) spaced along the array were selected to measure T and C levels. The T levels were based on consistent responses to the conditioning activities: However, the C levels were measured by loudness- scaling procedure. The children were trained for conditioning and the loudness scaling procedure in order to establish the reliability of T and C levels.
The data were entered and analyzed using standard SPSS 11.0 for windows. The significance of the correlation was calculated using Karl Pearson's correlation at P < 0.05.
| Results|| |
Seventy-five CIR were enrolled in the study with the mean age of 9 ± 1.6 years, male:female ratio of 38 males:37 females. Of the 75 recipients, one had Mondini deformity, and two subjects had auditory neuropathy spectrum disorder as per their medical records. All were fitted with the Sprint speech processor 3-4 weeks after implantation.
Electrical auditory brainstem responses waveform
Wave I was absent and waves III and V could be recorded in all the subjects. The first milliseconds following the stimulus was an artifact. [Figure 1] shows a sample EABR waveform recorded on electrode 11. The amplitude of waves reduced with a decrease in stimulus level.
|Figure 1: Electrical Auditory Brainstem Evoked Response recorded on electrode 11 at different current levels|
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One of the primary goals of the study was to determine how EABR T related to the variables (T and C) used to program Nucleus speech processor. [Figure 2] depicts the average t-EABR and behaviorally obtained T and C levels for 75 recipients using nucleus 24 CI. t-EABR levels typically lie between the T and C levels. The mean t-EABR was found to be 175.28 ± 4.72 while the mean T and C levels were 171.03 ± 5.00 and 211.6 ± 2.74, respectively, as depicted in [Table 3].
|Figure 2: The average t-EABR and behaviourally obtained T and C level for a group of 75 cochlear implantees. X-axis showing electrode number whereas Y-axis depicting current levels|
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|Table 3: Mean valves of t - EABR, threshold (T), and comfort (C) levels at specified electrodes |
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The group correlation coefficient was found to be r = 0.989 at P < 0.0001 for the "T" level and r = 0.885 at P < 0.0001 for "C" level versus t-EABR. t-EABR was found to be more correlated with T levels than with C levels [Table 4] and [Figure 3].
|Figure 3: Correlation between threshold electrical auditory brainstem audiometry (EABR) Current levels in Y axis and (a) threshold (T) current levels and (b) comfort (C) levels current levels in X axis|
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|Table 4: Pearson's correlation between t - EABR and threshold (T) and comfort (C) levels |
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Thresholds-electrical auditory brainstem responses and T and C levels recorded in one child with Mondini deformity and two children with auditory neuropathy are depicted in [Table 5].
|Table 5: t - EABR and threshold (T) and comfort (C) levels in Mondini and auditory neuropathy cases |
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| Discussion|| |
Electrical auditory brainstem responses are a measurement of the ABR using an electrical stimulus. EABR's are potential differences generated when a patient's auditory brainstem is stimulated with an electrical pulse, bypassing the outer and the middle ear. In CIR, the electrical stimulation is provided directly by the implant.
Electrical auditory brainstem responses are an efficient objective tool for programming the speech processor of CIR. The strong relationship between T of detection and ABR T for acoustic stimuli has been well-documented.  A strong relationship between EABR T and T of detection for the same stimuli has also been reported.  In our work, EABR T obtained were always within the subject's behavior dynamic range and showed a correlation to both psychophysical T and C levels. Behavioral "T" levels approximate most to t-EABR rather than "C" levels. However, the extent of the intrasubject variability was substantial. Brown et al. 1994  showed in her study that mean EABR T corresponded to a level that was approximately two-thirds of the way between behavioral T and maximum comfort level. However, considerable intrasubject variability was seen by her too while examining the individual data. She attributed this variability to differences in temporal integration across subjects. Therefore, we suggest a conservative clinical approach in which EABR T can be used as a level at which one knows that the subject can hear the tone and will not be un C. This level can be a useful point to begin conditioning a young, congenitally deaf child to respond to initial electrical stimulation, thus facilitating rather than replacing traditional behavioral programming of the device. We recommend starting electrical stimulation from 10 to 15 current levels below EABR T and then slowly increasing the level of stimulus with constant observation of behavior of the child. If the child is not overtly responding to 15-20 current levels above the t-EABR we set that level as C level for that electrode and set 10 current levels below t-EABR as T level. With a minimum of three recording at different electrodes at different positions of the electrode array, one can interpolate the T and C levels for the rest of the electrodes.
Any comparison between evoked potential T and behavioral T will be dependent upon the definition of T and the specific conditions of measurement. For example, both EABR T and behavioral T will be dependent on the rate of stimulus presentation.  In addition, EABR T can be affected by factors such as the number of sweeps recorded and subject state.  As such, the conclusions of this and other studies addressing this issue are dependent to a large degree on the parameters of measurement. Therefore, in our study, we have deliberately chosen EABR stimulation and recording parameters that are reasonably standard, and can be reproduced and used efficiently in a clinical situation. Similarly, the MAPs used in this study were obtained using standard clinical techniques, routine stimulation parameters, and were the actual MAPs used by the subjects for everyday communication.
In comparison to other evoked potentials, ECAP is routinely used as an intra-operative objective tool to measure response of the auditory nerve in many CI centers. Most of the clinical uses of EABR can also be accomplished with ECAP. However, there are situations in which EABR is preferable to ECAP, e.g., in auditory neuropathy cases or in cases in which integrity of the auditory pathway through brainstem is in question, cases in which device function needs to be checked and when ECAPs are not recordable especially in inner ear malformations. ESRT are very difficult to record and generally not used as a clinical tool.
Electrical auditory brainstem responses have several preoperative, intra-operative and postoperative applications. It can also be recorded in auditory neuropathy cases where by definition, acoustically evoked ABR is absent  and also reported to be most applicable objective test technique when compared with ECAP and ESRT in CIR with inner ear malformations.  Cochlear malformations have reported to occur in approximately 20% of children with congenital sensorineural hearing loss. 
In summary, we have demonstrated the use of t-EABR as one of the electrophysiological tests in determining fitting and programming parameters for an individual CI user. This has more use in pediatric CI than in adult implantees.
Electrical auditory brainstem responses can be used to MAP the speech processor for young and difficult CIR and can also be recorded efficiently in children with inner ear malformations and auditory neuropathy. However, our study includes small number of subjects with auditory neuropathy and Mondini deformity, and the above findings need to be corroborated on a large number of similar subjects.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]