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ORIGINAL ARTICLE
Year : 2020  |  Volume : 23  |  Issue : 3  |  Page : 296-302
 

Normative data for cortical evoked response audiometry of a heterogeneous Indian population and its comparison with behavioral audiometry


1 Department of ENT Unit 4, Christian Medical College, Vellore, Tamil Nadu, India
2 Department of Audiology, Christian Medical College, Vellore, Tamil Nadu, India
3 Department of Statistics, Christian Medical College, Vellore, Tamil Nadu, India

Date of Submission11-Sep-2018
Date of Acceptance15-Jan-2019
Date of Web Publication10-Jun-2020

Correspondence Address:
Dr. Priyanka Misale
Department of ENT-4, Christian Medical College, Vellore, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/aian.AIAN_397_18

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   Abstract 


Objective: The objective of the study is to establish normative values of cortical evoked response audiometry (CERA) in a heterogeneous Indian population and correlate CERA threshold with pure tone audiometric (PTA) threshold values. Materials and Methods: A prospective study was carried out on 31 volunteers (n = 62) who had no otological or neurological complaints. Two study groups were formed; Group 1 with individuals from 20 to 40 years (mean age of 29.1 years) and Group 2 with individuals from 41 to 60 years (mean age of 46.2 years). The latencies and amplitudes of the waves of P1, N1, and P2 at threshold and 70 dBnHL were measured. Results: Twenty-nine participants (94% of the ears) had CERA threshold within 20 dB of true behavioral threshold with only 6% having a difference of >20 dB with their PTA thresholds. There was a significant difference (P < 0.05) at 70 dB in amplitudes for waves P1, N1, and P2 at 2 kHz and additionally at 1 kHz for N1 between the two groups. Conclusion: Normative values for CERA in a heterogeneous Indian population at 70 dB nHL using tone burst stimulus was found to have an average latency of 46.5, 90.1, and 155.5 ms for P1, N1, and P2, respectively. The average amplitude of P1 at 70 dB nHL was 4.3 μV, N1, was 6.5 μV and P2 was 3.2 μV. Hearing threshold obtained with CERA gave a good indication of the actual behavioral hearing threshold of the normal controls, and the age of an individual had a significant effect on the values obtained during CERA testing with N1 being significantly larger at 1 kHz and 2 kHz in older adults when compared to young adults.


Keywords: Aging and cortical potentials, auditory evoked potentials, cortical auditory evoked potential threshold estimation in adults, cortical auditory evoked potentials


How to cite this article:
Misale P, Lepcha A, Thomas P, Sebastian S, Sebastian T. Normative data for cortical evoked response audiometry of a heterogeneous Indian population and its comparison with behavioral audiometry. Ann Indian Acad Neurol 2020;23:296-302

How to cite this URL:
Misale P, Lepcha A, Thomas P, Sebastian S, Sebastian T. Normative data for cortical evoked response audiometry of a heterogeneous Indian population and its comparison with behavioral audiometry. Ann Indian Acad Neurol [serial online] 2020 [cited 2020 Jul 12];23:296-302. Available from: http://www.annalsofian.org/text.asp?2020/23/3/296/257024





   Introduction Top


An acoustic stimulus presented to the ear generates impulses which are carried from the cochlea to the auditory cortex. The responses generated in the cortex can be picked up by electrodes placed on the scalp and are known as the cortical evoked responses or the late latency responses. Different types of cortical evoked responses have been studied like P1-N1-P2 response which signals the first stage of neural detection of sound, followed by discriminatory potentials such as the mismatch negativity (MMN), P300, N400, and P600 which reflect higher order auditory discrimination processing.[1] Cortical evoked response audiometry (CERA) has been studied widely in literature and has been put to use in many fields such as otology, audiology, neuropsychiatry, and neurology. It has been used to estimate hearing threshold, to assess suspected functional hearing loss, for the neural detection of sound and its acoustic phonetic components at more central levels.[2] Its increasing role in hearing sciences is being studied in noise-induced hearing loss,[3] fitting of hearing aid and cochlear implant,[4],[5] studying the maturation of the auditory system and the effects of plasticity.[6],[7],[8],[9] CERA being an objective test has a distinct advantage over pure tone audiometric (PTA) (which is a subjective test) in certain scenarios, for example, to help rule out noise-induced hearing loss claims. It may also help in detecting nonorganic hearing loss and prognosis for auditory neuropathy after cochlear implant.

There are few published normative data from India, and very few centers use CERA for diagnostic work. Hence, it becomes imperative to have normative data from the subcontinent. Any additional data will not only help in further research in this new area but can be the basis for diagnostic tests in the field of audiology. Research on CERA is still evolving and future uses and applications may change the practice of audiological medicine. This study was carried out to establish standards for normal values of CERA in a heterogeneous adult Indian population and to compare thresholds obtained by PTA with CERA threshold values in normal adults.


   Materials and Methods Top


A prospective study was carried out at a tertiary referral center in India between February 2016 and April 2017 on normal volunteers who had no otological or neurological complaints. This study was approved by the Institutional Review Board (IRB Min No: 9843 dated 7.1.2016).

Patient profile

A total of 31 adults in the age group of 20–60 years were included in this study. The participants were divided into two groups based on their age. Group 1 had 20 volunteers who fell in the age group between 20 and 40 years and had a mean age of 29.1 years. There were 8 males (40%) and 12 females (60%) in this group. Group 2 had 11 volunteers in the age group between 41 and 60 years with a mean age of 46.2 years. There were 5 males (45.5%) and 6 females (54.5%). We studied each ear separately and thus Group 1 and 2 had 40 and 22 ears each, respectively.

All individuals had normal ear, nose, throat, and neurological examination. The PTA revealed normal hearing in all frequencies for all individuals. Group 1 had average hearing of 11.8 dB, and Group 2 had average hearing of 13.6 dB. The volunteers underwent CERA and latency, and amplitude of the waves P1, N1, and P2 for different frequencies at 0.5, 1, and 2 kHz at 70 decibel and threshold were measured. The mean with standard deviation and the range of the normal values are shown in the following tables. Latency and amplitude were measured in ms and microvolt (μV), respectively.

Parameters studied

Normative values of CERA were obtained with respect to latency and amplitude for Indian nationals. Latency and amplitudes of the waves P1, N1, and P2 were studied at 0.5, 1, and 2 kHz at 70 dB nHL and individual threshold. dB nHL (normal hearing level) is the most commonly used intensity unit in auditory evoked response measurements. It was determined by measuring the threshold of tone burst stimuli (the intensity level on the evoked response system at which tone burst is just audible) in the clinic facility for a group of 10–15 normal hearing young adults. The average of these threshold levels in dB is referred to as 0 dB nHL. The lowest level of stimulation at which the waves were clearly discernable was described as the threshold. This threshold obtained with CERA was compared with the threshold obtained by PTA and a relation between them was studied.

Statistical calculation

Data analysis was made using Statistical Package for Social Services (SPSS) software Version 16.0 (Armonk, NY: IBM Corp.). Descriptive statistics were used for presenting the collected variables of waves P1, N1, and P2 for their latency and amplitude at different frequencies and stimulus intensity. The latency and amplitude among the two groups were compared using independent sample t-test. The statistical significance was set at P < 0.05.

We divided the patients into two groups based on a preset “cut-off” value (i.e., Group 1: 20–40 years and Group 2: 41–60 years) and statistically studied the values obtained. Statistical calculations with age as a continuous variable have not been carried out in the current study.

Testing procedure and instruments

Pure tone audiometry

All individuals were tested with calibrated (GSI Audiostar, GSI 61 or Piano Inventis) audiometers in sound-treated rooms. We followed the American-Speech-Language-Hearing Association[10] recommended procedure for PTA using a modified version of the Hughson and Westlake procedure.

Cortical auditory evoked potentials or cortical evoked response audiometry

CERAs were recorded (IHS Smart EP; Intelligent Hearing Systems, Miami, FL, USA) using the standard protocol (active electrode-vertex; reference electrode-ipsilateral mastoid; ground electrode-lower forehead. Bandpass filter was kept between 1 and 30 Hertz (Hz). Stimuli at 500, 1000, and 2000 Hz in alternating polarity at a rate of 1.1 tone bursts per se cond was presented through ER 3A insert earphones. Time window for recording the CAEP was 500 ms. Electrode impedance was kept < 5 kOhms and inter-electrode impedance of < 2 kOhms. Intensity was varied from 70 dB nHL to threshold. A step size of 20 dB was used at higher intensities and 10 dB near the threshold. P1-N1-P2 complex latencies and amplitude were checked for each response [Figure 1]. Peak to peak amplitude of P1-N1, N1-P2, and P2-N2 was measured. For convenience, we have defined these waves as P1, N1, and P2. Amplitude of P1 was measured as the amplitude difference between P1-N1, N1 amplitude was measured as the amplitude difference between N1-P2, P2 amplitude was measured as the amplitude difference between P2-N2. The direct visual identification of peaks was used for analyzing the waves. The lowest intensity at which, a clear P1-N1-P2 complex was identified was considered to be the threshold. The test was carried out by junior audiologists under the guidance of two senior audiologists (PT/SS). The first author and the senior experienced audiologists (PT/SS) identified the waves and made all the measurements. Only those waves and measures on which there was 100% agreement between the three researchers were considered for analysis. This test required approximately 1 h for every ear, and thus every volunteer had to sit for nearly 2 h for the test to be completed.
Figure 1: Depicts the cortical evoked response audiometry waves in a volunteer for both the right (R) and left (L) ear. The waves P1, N1, P2, and N2 have been marked in the tracing. In this particular case, the waves are last discernable at 20 dB nHL for both the right and left ear. The abscissa (X-axis) denotes various stimuli level used, i.e., 70, 50, 30, 20, and 10 dB nHL and the ordinate (Y-axis) denotes the time interval in ms from 0 to 500

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   Results Top


[Table 1] and [Table 2] show latency and amplitude of both Groups 1 and 2 (n = 62) (age group between 20 and 60 years, mean age 35.2 years) combined at 70 dB nHL and threshold, respectively. There was no statistical difference between the male and female gender. It is observed that at threshold the latency of all the waves increases and the amplitude decreases as compared to the values obtained at 70 dB, and this is statistically significant (the P value for all the comparisons was < 0.001).
Table 1: Latency and amplitude of P1, N1, and P2 for adult patients (age group 20-60 years) (n=62) at 70 dB

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Table 2: Latency and amplitude of P1, N1, and P2 for adult patients (age group 20-60 years) (n=62) at threshold

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The effect of aging on CERA values was studied by dividing the individuals into two groups as described previously. [Table 3] shows the latency and amplitude of P1, N1, and P2 waves at 70 dB separately for Group 1 (n = 40) and Group 2 (n = 22). There was no statistically significant difference in latency between the two groups. The amplitude of the waves P1, N1, and P2 was also compared and there was a statistically significant difference for P2 wave at 0.5 kHz (P2 amplitude being smaller in the older adults), for N1 wave at 1 kHz (N1amplitude being larger in older adults), and for all the three waves at 2 kHz (P1 and N1 amplitudes being larger and P2 amplitude being smaller in older adults). The N1 wave was larger at all the frequencies in the older adults (group 2) as compared to Group 1, but it was statistically significant at 1 and 2 kHz.
Table 3: Latency and amplitude of P1, N1, and P2 at 70 dB for Group 1 (n=40) and Group 2 (n=22)

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[Table 4] shows the comparison of the latency and amplitude for P1, N1, and P2 for Group 1(n = 40) and Group 2 (n = 22) separately at the threshold. There were no statistically significant differences between the two groups with respect to latency except for the 1 kHz frequency at threshold where the latency of P1 wave was significantly prolonged in Group 2 (P = 0.02). There was no statistically significant difference in the amplitudes of the P1, N1, and P2 waves except at 2 kHz for P2 wave where the amplitude in Group 2 was significantly less than Group 2 compared to Group 1 (P = 0.007).
Table 4: Latency and amplitude of P1, N1, and P2 at threshold for Group 1 (n=40) and Group 2 (n=22)

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[Table 5] shows the relationship between the PTA threshold and the threshold as obtained by CERA. It describes the proportion of CERA threshold falling within +5, 10, 15, and 20 dB of PTA of both groups separately. Overall (n = 62), in 94% ears at 2 kHz frequency, CERA threshold fell within +20 dB of PTA threshold and at 1 kHz frequency 97% ears had CERA threshold falling within +20 dB of PTA threshold.
Table 5: Relation between pure tone audiometric threshold and cortical evoked response audiometry threshold

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   Discussion Top


P1-N1-P2 responses are CERA waves recorded at the vertex, and it comprises a positive wave at about 50 ms (P1) post-stimulus onset, a large negative wave N1 occurring between 80 and 140 ms and P2 wave which is a large positive peak occurring between 140 and 250 ms.[11] The P1 wave originates from the secondary auditory cortex, the N1 has multiple generators in the primary auditory cortex, the frontal lobes and midbrain, the P2 originates from the thalamic reticular activating system, and the N2 has nonspecific subcortical origins.[12],[13] P1-N1-P2 response analysis includes the measurement of latency and amplitude of each individual peak component. As the intensity level decreases, the P1-N1-P2 responses increase in latency and decrease in amplitude.[1] This can be observed from [Table 1] and [Table 2]; as the threshold of hearing is reached the latency of the waves P1, N1, and P2 increases and the amplitude decreases.

Normative values

In a study done by Sahu et al.[14] on adults with normal hearing (using da/speech stimulus) at various intensities the latency of N1 and P2 was studied. The latency for N1 and P2 at 70 dBSL was 90.39 (8.40) and 131.10 (16.0) ms, respectively. Moreover, at 30 dBSL, it was 152.05 (15.46) and 179.14 (12.82) ms, respectively. A study by Yuvaraj et al.[15] on normal hearing individuals (using clicks evoked stimuli at 40 SL) had the P1, N1, P2 latencies at 67.03 (5.73), 116.36 (8.89), and 146.03 (9.09) ms, respectively. Using speech stimulus, they found the latencies to be 66.86 (2.39), 116.36 (−0.45), and 146.04 (9.12) ms, respectively. Narne et al.[16] used click stimuli at 80 dB nHL in normal hearing individuals and found latencies of P1, N1, and P2 to be 50 (8.1), 85 (9) and 142 (12) ms, respectively. Kumar and Jayaram[17] carried out CERA testing using stimuli unmodified/da/and synthesized/da/at 30–40 dB SL in normal hearing individuals. The latencies of P1, N1, and P2 were 69 (15.2), 120.5 (23.5), and 145.3 (25.6) ms, respectively.

Amplitudes of P1, N1, P2 on normal individuals (using click-evoked stimuli at 40SL) were found to be 2.37 (0.42), −0.45 (0.08), and 2.81 (0.29) microvolt, respectively.[15] Using speech as the stimulus, they found the amplitudes to be 2.39 (0.47), −0.45 (0.09), and 2.82 (0.28) microvolt respectively. A study by Narne and Vanaja[16] using click stimuli at 80 dBnHL in normal individuals showed the amplitude of N1/P2 to be 6.2 (1.3) microvolt. Kumar and Jayaram[17] used unmodified/da/and synthesized/da/stimuli at 30–40 dB SL in normal hearing individuals and found the amplitudes of P1, N1, and P2 to be 2.5 (0.6), −0.5 (0.5), and 2.8 (1.5) microvolt, respectively. The amplitude increases[18] with increasing stimulus intensity until a saturation point is reached and latencies decreases with increasing stimulus intensity.[19]

In our study, the latencies and amplitudes of P1, N1, and P2 were studied at 70 db and at threshold of the patient. It is to be noted that they were studied with tone bursts at 0.5, 1, and 2 kHz and this difference in the test parameters should be considered while comparing the values with other studies. Very few studies have been carried out for the normative values of the waves P1, N1, and P2 at different frequencies (0.5, 1, and 2 kHz) using tonebursts. This adds to the value of our study.

Correlation between cortical evoked response audiometry threshold and pure tone audiometric threshold

In a study by Mahdavi,[20] CERA thresholds were within 0–15 dB SL of true hearing thresholds in 95% of individuals and the remaining 5%, the difference between the CERA threshold and true hearing threshold was 20–25 dB. Lightfoot and Kennedy[21] observed that 94% of individual threshold estimates with CERA were within 15 dB of behavioral threshold and 80% were within 10 dB.

In our study, 94% of the ears studied had CERA threshold within 20 dB of true hearing threshold with the exception of four tested ears (6%) where there was a discrepancy of >20 dB. Coles and Mason[22] stated that a small percentage of cases (~1%) will have CERA responses that occur at levels significantly greater than their true hearing threshold. In a study by Rose et al.,[23] 8% cases had discrepancy between evoked potentials and subjective thresholds of >60db. This suggests that although CERA corroborates very well with the PTA thresholds, we still have to be cautious in relying on the data completely.

Difference in age groups

Neuromaturation of commissural axons and association fibers is still occurring by the age of 12 years,[24] and changes in the morphology of the P1-N1-P2 may be expected across the lifespan. The wave P1 is prominent in children,[8],[25] and N1 wave becomes dominant by late teenage.[25] As our study involved adult individuals, we have mainly considered N1 response and have studied the effect of aging on it. At 70 dB the amplitude of N1 was significantly increased in the subjects with advanced age at 1 and 2 kHz.

In a study done by Harris et al.,[26] the response latencies and amplitudes were found to be significantly higher at lower frequencies (0.5 kHz) in the elderly population (64–76 years) and attributed it to the age-related decline in inhibitory control in the central auditory nervous system. They noted that the increased N1 and/or P2 latencies are the result of decreased neuronal synchrony within the auditory pathway which is due to a general slowing of neuronal processing and due to changes in excitatory and inhibitory processes.[26] Several other studies have also found that the N1 and/or P2 latencies are delayed in older adults as compared to younger adults.[27],[28],[29] We did not note any significant difference in latency between the two groups at 70 dB nHL. At threshold, there was no significant difference in the latency or amplitude for N1 at 1 kHz and 2 kHz. Our study did not recruit elderly individuals when compared to the elderly population studied by Harris et al.[26]

With regard to amplitudes, some studies report similar N1-P2 amplitudes for younger and older subjects[29],[30] whereas others report an age-related increase in amplitudes.[27],[31]

Similar to the age-related changes in response latency, unusually large N1-P2 amplitudes have been ascribed to an age-related decline in inhibitory control.[26] Our study also showed significantly large N1 amplitudes at 70 dB nHL at 1 kHz and 2 kHz in the older participants which could be attributed to age-related decline in inhibitory control.

One possible source of error in our study may be the restricted number of participants in Group 2 as the procedure required 2 h for completion and thus individuals with more advanced age groups found it difficult to comfortably sit through the test.


   Conclusion Top


Normative values for CERA in a heterogeneous Indian population at 70 dB HL using tone burst stimulus were found have an average latency of 46.5 ms for P1, 90.1 ms for N1, and 155.5 ms for P2. The average amplitude of P1 at 70 dB HL was 4.3 μV, N1 was 6.5 μV, and P2 was 3.2 μV. Hearing threshold obtained with CERA gives a good indication of the actual behavioral hearing threshold of the normal control, and the age of an individual has a significant effect on the values obtained during CERA testing with N1 being significantly larger at 1 and 2 kHz in older adults when compared to young adults.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
   References Top

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    Figures

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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]



 

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