INTRODUCTION

The conventional subjective tests, such as the pure-tone audiometry, are difficult to be performed in patients who can not or do not want to respond to stimulation, once such tests depend on the patient's active participation. Thus, the objective methods of heating evaluation have been more and more required in the audiological clinic practice. The Auditory Brainstem Response (ABR) integrates the group of audiological tests and represents the objective exam of choice when the aim is to recognize the hearing level. The bioelectrical potential analysis produced by nervous impulse transmission by acoustic stimulation, in the hearing via, bring the electrophysiological threshold which, in the absence of retrococlear modifications, provide valuable information for the hearing level inference. Such potentials may be recorded in an objective way, with non-invasive techniques, with no harm for the patient, and frequently, without the need of anesthesia or sedation, reasons which make its applicability better(1).

Nevertheless, the electrophysiological hearing threshold for clicks is not strictly accurate in terms of frequency or intensity related to auditory acuity. Some studies have investigated the relation among the electrophysiological threshold and the hearing level, although they have not pointed out the need to standardized such parameter (2,3,4,5,6,7). In general, the results suggest that the objective thresholds obtained on ABR represent an estimation of the subjective threshold, without, however, quantifying the differences between them (8,9,10,11,12). Consequently, this study aims at estimating and comparing the differences, in dBNA, between the electrophysiological threshold and the subjective auditory threshold average in three bands of frequency, 2-3-4 kHz , 3-4-6 kHz e 3-4 kHz, in a group of normal hearing adults.


MATERIAL AND METHOD

This is a descriptive study on quantitative basis, conducted by a specialized clinic and made of convenience sample population. This research project has been approved by the Comitê de Ética em Pesquisa da União Metropolitana de Educação e Cultura - (Research Ethics Committee of the Metropolitan Union of Culture and Education) - UNIME (171990/2005-11). 20 normal hearing adults aging from 20 to 40 years old have been selected, who agreed on participating on the study. For the research subjects selection, the following elective criteria have been considered: Not having known diseases, not using medicine or drugs, not smoking or drinking, not having hearing, otologic, neurologic or otoneurologic complaints, presenting normal meatoscopy, tone auditory threshold between 0 and 15 dBNA in the frequencies from 250 to 8000 Hz, percentage index of speech recognition equal to or higher than 96%, tympanometric curves of A kind and ipsi/contralateral acoustic reflexes compatible to auditory threshold (between 70 and 90 dBNA above the respective tone thresholds) (12). In the second phase, at the ABR test, the previously selected patients also needed to present normal nervous conduction in hearing vias, verified through the suitability to normality standards for the absolute latencies I, III and V, interpeaks I-III, III-V and I-V, bilaterally, as well as for the interaural differentiation.

The data gathering has been done in the Audiology sector of a clinic specialized on Otorrhinolaryngology. The selected subjects were undertaken to audiological and otorrhinolaryngological evaluation, which included anamneses and otorrhinolaryngological clinical examination, tone auditory threshold investigation by aerea via, speech reception threshold evaluation and percentage index of speech recognition, tympanometry and acoustic-stapediane contra and ipsilateral reflexes. The audiological tests have been conducted with Interacoustics, audiometer model AD 229, acoustically treated cabin, and Interacoustics, imitanciometer model AZ 7.

After the basic audiological evaluation, verifying the adaptation to the election criteria, the ABR investigation has been performed by through the use of Interacoustics Eclypse EP 15 equipment of two channels, in silent environment, without acoustic treatment, with the patient positioned in dorsal cubit, without the use of sedation nor any kind of medication. In order to analyze patients' adaptation in this phase, the equipment standard limits have been considered and previously investigated according to the criteria defined by American EEG Society (13), the subjects who presented absolute latencies and interpeaks in the interval described by the average of +/- 2,5 DP have been considered holders of normal nervous conduction, specifically for the test, considered as normal standard and determined in the same equipment (13).

Once all the participation criteria for this study has been complied, the electrophysiological threshold investigation through ABR has been conducted. In order to get the electrical potentials electrodes with dischargeable surfaces have been used, by MEDTRACE, placed after skin hygiene with absolute alcohol and discreet scarification with abrasive substance. The patients have been guided to keep their eyes closed during the acquisitions, avoiding any kind of body contact, mainly of eyelid, jaws and cervix. The electrode positioning corresponded to the international system 10/20, with the negative ones placed on the earlobes (A1 and A2), left and right respectively, the positive one in the forehead region closer to vertex (Cz) and the normal electrode in the frontal region (Fpz). The electrode impendency has been verified, always starting the acquisitions when values lower than 5kOhms have been identified (14,15). The acoustic stimulation has been monoaural, presented by insertion transductor, comprehending 2000 non-filtered clicks, 500 and 8000 Hz, with the duration of 100 microseconds and rarefied polarity. Intensities varied from 80 to 15 dBNA, with gradual and dependable reduction to the result obtained at the immediately previous intensity, obeying the following order: 80, 60, 40, 30, 25, 20 and 15 dBNA. For the milder intensities the reproduction ability has been verified through the new presentation of 2000 clicks. The results have been visualized in 12 ms windows. High and low filters have been used, respectively adapted in 50 and 2000 Hz. Sensitiveness varied between 40 and 80µV and clicks have been presented in the frequency of 20.1/s, starting in the right ear.

The electrophysiological threshold investigation results have been analysed by to examiners, in an independent way, and the agreement rate between both observations has been estimated. When there has been disagreement, for analysis purposes, the observation that indicated a worse threshold has been considered. For such analysis, the tone threshold averages of 2, 3 and 4 kHz (F1); of 3, 4 e 6 kHz (F2) and of 3 and 4 kHz (F3)have been calculated. For each ear, the differences in dBNA, between the electrophysiological threshold result and the averages F1, F2, and F3 have been estimated.

Such differences have been analysed as continuous data and in categories. When comparing the electrophysiological threshold value to the F1, F2 and F3 averages, the differences in dB and their variability and central tendencies measures have been estimated. In order to study such differences distribution, boxplots have been used, a kind of figure which summarizes the central tendency and dispersal measures, in which the rectangle represents the interval between the first and the third quartile, respectively corresponding to the percentages of 25 and 75 of the distribution, and the median - percentage 50 - is represented by the corresponded line. Additionally, the vertical line located above and below the quartiles represents 1.5 times the interval between the median and the respective quartile. The differences have also been analysed into three categories: Values inferior to 10 dBNA, values from 10 to 15 dBNA, and values superior to 15 dBNA, verifying in which one of them the higher proportion of cases have been found. 5% significance level has been adopted. The statistical program SAS, version 8.11, has been used for the analysis.


RESULTS

All the 20 patients selected in the first phase presented normal parameters of nervous conduction in auditory vias up to the brainstem, therefore constituting the study final population. The average global age has been 29.3 years, higher among women (31.8), when comparing to the average age among men (26.2), and there has been a higher participation of female patients (55.0%).

The agreement rate between the two independent observers for the analysis of the records referring to the electrophysiological threshold study has been of 85%, and tendency to underestimation or overestimation for such threshold identification has not been detected.

On Table 1, the values related to average and standard deviation of differences between the electrophysiological threshold and the auditory tone threshold and the auditory tone threshold (F1, F2 and F3) observed in the study population may be seen. In the comparison of the paired samples statistically significant diference between F1 and F2 in both ears, and even between F2 and F3 in the right year, have been observed Considering such pairs, the average corresponding to F2 (3-4-6 kHz) presented lower difference, in dBNA, of the electrophysiological threshold.

Graphs 1 and 2, in boxplot format, show the differences between the electrophysiological threshold and the averages F1, F2 and F3 for both ears, in order to make the visualization of distribution values in the quartiles possible (Q1 - 25%; Q2 - median 50%; Q3 - 75%). The visual analysis demonstrate higher concentration of values between Q1 and Q# for F2, Bilaterally, which suggests a higher measure accuracy when the differences for the F2 average (3-4-6 kHz) are considered.

Graph 1. Boxplot of the difference distribution in dBNA between the electrophysiological threshold and the averages 2, 3 and 4 kHz (F1), 3, 4 and 6 kHz (F2), 3 and 4 kHz (F3), for the right ear.


Graph 2. Boxplot of the difference distribution in dBNA between the electrophysiological threshold and the averages 2, 3 and 4 kHz (F2), 3, 4 and 6 kHz (F2), 3 and 4 kHz (F3), for the left ear.

On Table 2, it is verified that the differences are mostly classified in the interval between 10 and 15 dBNA, for all frequency bands (F1, F2 and F3). The differences which were mower than 10 dBNA occurred in lower proportion for F3, and in higher proportion for F2, comparing the frequency bands.

DISCUSSION

Results have demonstrated that the electrophysiological threshold have been closer to the tonal threshold average of 3, 4 and 6 kHz (F2) when compared to F1 and F3, which respectively correspond to 2, 3 and 4 kHz, and 3 and 4 kHz averages. In addition to this, the visual analysis of differences distribution made it possible to observe the appearing advantages in the measure accuracy for F2, when compared to the other ones, which demonstrated higher dispersion of values. Generally, considering all frequency bands analysed in this study, the differences are mostly found between 10 and 15 dBNA.

It is known that the electrophysiological threshold depends on the electric potential record, which has close relation to the number of stimulated fibers, synchronism and size of electric activity. Thus, the subject may detect a piece of sound in weak intensity without necessarily registering corresponding electric potentials (16). Therefore, it is essential to know the differences between the subjective and electrophysiological auditory thresholds, once its determination bears examiner's mind when conventional subjective exams do not present the expected results (17,18). The concentration of such difference in the interval of 10 to 15 dBNA for this study population may be considered lower when compared to the results obtained by BELL et al. (3), who evaluated 10 patients with normal hearing and observed difference average values for the subjective threshold average of 3, 4 and 6 kHz around 16 dBNA, and by SWANEPOEL et al. (4), who found differences between 14 and 18 dBNA between the average of 2, 3 and 4 kHz and the electrophysiological threshold when evaluating 28 patients with better tonal threshold than 25 dBNA. However, it is important to point out that the absolute values of this difference vary among equipment. Consequently, professionals must consider such parameter as part of previous standardization in the beginning of the clinical practice, as well as it is performed for standard knowledge of standard for absolute latencies and interpeak intervals (19, 20, 21). While the latter must be known for the neurophysiological integrity, the difference of the electrophysiological threshold for the subjective threshold implies in the inference accuracy of auditory level.

As it has been described in the results when comparing the three average performances, F1, F2 and F3, in the approach and accuracy for the electrophysiological threshold measure, the one which corresponds to the threshold to the frequencies of 3, 4 and 6 kHz has been better evaluated in all analysis. Several authors mention that there is relative agreement between the ABR results and the behavioral threshold for pure tones in the frequency band from 2 to 4 kHz (2,5,6,8,9,21,22). While others state a higher agreement when compared to the tonal auditory threshold of 3 to 6 kHz frequencies (3,7,16), coherently to this investigation results.

Click is a wide band stimulation, presents a spectrum with a high range of frequencies, and are energy peaks which are concentrated between 1 and 4 kHz (21), Despite such range, it presents certain response specification, with tendencies to high frequencies (from 2 kHz on). The evidences of such study refer to a group with normal hearing, thus it is not possible to generalize such results for other auditory conditions. Consequently, it is important to conduct new studies with bigger samples and with population which presents hearing loss, so that the correspondence with the frequency band averages, as well as the differences between the tonal and electrophysiological auditory threshold may be investigated in the presence of one alteration and in different audiometric configurations. The comparative analysis of equipment may also provide valuable information.


CONCLUSIONS

1. For this population with normal hearing, the threshold average of 3, 4 and 6 kHz has obtained higher agreement with the electrophysiological threshold, once it presents lower difference and higher accuracy when comparing to the average of 2, 3 and 4 kHz or of 3 and 4 kHz. The hearing level for this frequency band (3 to 6 kHz) seems to have greater influence when determining the electrophysiological threshold by clicks;

2. For the equipment used, the differences between the electrophysiological threshold and the tonal threshold of the three frequency bands are mostly found in the interval between 10 and 15 dBNA. Such results can not be generalized, once it depends on technical conditions which are peculiar to each service. Such parameter standardization by be implemented in addition to the other ones which have been traditionally investigated, once it implies in the inference accuracy in the hearing level.


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1. Especializandoem Audiology by the Metropolitan Union of Education and Culture - UNIME - Lauro de Freitas (BA), Brazil. (Fonoaudiólogo report.)
2. PhD student in Public Health of the Institute of Public Health, Federal University of Bahia - UFBA - Salvador (BA), Brazil. (Assistant Professor of Department
Speech of the Federal University of Bahia - UFBA - Salvador (BA), Brazil.)
3. Especializanda in Audiology by the Metropolitan Union of Education and Culture - UNIME - Lauro de Freitas (BA), Brazil. (Fonoaudióloga Clinic.)

Work of completing post-graduate broadly held in the Course of the Union Speech Metropolitan of Education and Culture - UNIME - Lauro de Freitas (BA), Brazil.

Grayson Amorim Tenório
Mailing address: Rua Helio de Oliveira, 598 - Ed Jordana, Apt. 104 - Luis Anselmo - Salvador / BA - CEP: 40265-020 -- Phone: (71) 3233-2077 - E-mail: grayson_fono@hotmail.com

This article was submitted in Management System Publications in the R@IO approved on 20/3/2007 and 29/9/2006 at 23:05:23