Portuguese Version

Year:  2003  Vol. 69   Ed. 3 - (11º)

Artigo Original

Pages: 356 to 362

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Auditory brainstem response (ABR): use of masking in unilateral hearing loss patients

Author(s): Melissa M. T. Toma 1,
Carla G. Matas 2

Keywords: hearing, auditory brainstem response, masking, unilateral hearing loss

Abstract:
The need of masking in auditory brainstem response (ABR) evaluation is still considerably debated issue (Durrant and Ferraro, 2001). Aim: In addition, the present study was to investigate the need of masking in ABR with unilateral hearing loss. Study design: Clinical prospective. Material and Method: The sample was constituded of 22 persons with unilateral hearing loss, being 10 female and 12 male, ranging from 9 to 44 years old. All persons were submited to the following audiological exams: pure - tone and speech audiometry accoustic impendance tests and audiometry brainstem response in absence and presence .of masking. Results: In general, all persons had profound sensorineural unilateral hearing loss and bilateral type - A tympanometric curves ABR evaluation, it was possible to observe in 100% of the sample the presence of wave V in poor ear. As soon as the masking was introduced, these waves has not been viewed. Conclusion: As might be expected that masking must be with ABR evaluation on unilateral hearing loss to obtain authentical results. In ABR, interaural attenuation to clicks (65 dB) was greater the examined in pure-tone audiometry. Thence it follows that less intensity of masking was needed to eliminate the contralateral response.

Introduction

Auditory brainstem evoked response (ABR) has collaborated significantly with clinical audiology. It is an objective test with great sensitivity, non-invasive and easy to conduct 1. Its application allows assessment of auditory sensitivity of the newborn; children or adults in whom the auditory test is not reliable, or even in cases in which it is impossible to conduct other tests; diagnosis of auditory nerve or brainstem damage; monitoring of posterior fossa surgeries; monitoring of Intensive Care Unit coma or sedated patients, as well as the assessment of prognosis of patients, and to support the diagnosis of brain death 2, 3.

ABR refers to auditory evoked potential of short latency or early auditory evoked potentials, whose reposes take place within 0 to 10 milliseconds (ms) after the presentations of the acoustic stimuli.
This test consists of a series of seven positive waves, generated by acoustic signals that are collected by surface electrodes placed on the skin. The waves represent the bioelectrical activity of one or more structures of the auditory pathways.

Generally speaking, waves I, III and V are the most used ones, being that the others are not always present in the assessment of normal subjects. Normative studies conducted with absolute and interpeak latencies of those three waves concluded that such values bring several information not only to audibility thresholds, but also to auditory pathway damage neurodiagnosis 1, 2, 3.

In the assessment of patients that present asymmetric unilateral or bilateral hearing loss, some precautions are required. In behavioral audiometry, if there is a substantial difference in threshold from one ear to the other, we should pay attention to the intensity of the acoustic stimuli used when testing the impaired ear. A loud sound, if presented to the worst ear, can stimulate the opposite ear even if it reaches the threshold of the tested ear 4, 5. In such situation, the sound may stimulate the contralateral ear, despite the loss of energy, which is named interaural attenuation 5, 6. In the audiogram, a tracing similar to the audiometric curve of the other side is presented, but in a higher level (shadow curve)4.

These are the situations in which we use clinical masking. It is a commonly employed procedure in clinical audiology to detect true pure tone thresholds. Masking is noise introduced to the non-tested ear in order to artificially increase the auditory thresholds of the ear without interfering or influencing the audiometric results of the tested ear 4, 5, 6.

Differently from behavioral audiometry, the need for masking in ABR is still a controversial issue 7.
The study carried out by Finitzo-Hieber, Hecox & Cone (1979) did not find responses at 110 dB SPL (Sound Pressure Level) to clicks when assessing the impaired ear of subjects with unilateral hearing loss using ABR. Based on such findings, they concluded that contralateral masking applied to the normal ear would not be necessary to assess hearing through ABR and they also added that the realization that masking is not necessary would lead to shorter and quicker tests. In other studies conducted by Galambos & Hecox (1978) and Owen & Burkard (1991) they observed that contralateral masking did not influence substantially the ABR assessment.

Conversely, Chiappa, Gladstone & Young (1979), Ozdamar & Stein (1981), Humes & Ochs (1982), Smyth (1985) concluded in their studies that the use of masking applied to the non-tested ear would be recommendable to obtain reliable electrophysiological responses.

Considering that, the purpose of the present study was to check the need to use contralateral masking, employed in the normal ear, and to conduct ABR in subjects who had unilateral profound sensorineural hearing loss.

Material And Method

The present study was carried out in the Laboratory of Hearing Research in Auditory Evoked Potentials of the Teaching and Research Center, Department of Physical Therapy, Speech and Language Pathology and Audiology and Occupational Therapy, Medical School, University of São Paulo, between October and December 2002, and approved by the Ethics Committee of the same Department - 239/02.

To conduct the present study we included 22 subjects with unilateral profound sensorineural hearing loss being 10 female subjects ranging in age from 9 to 44 years (mean age = 23 years) and 12 male subjects, ages ranging from 15 to 39 years (mean age = 24 years).

All subjects lived in São Paulo and periodically had audiological follow up in the Laboratory of Hearing Research in Auditory Evoked Potentials of the Teaching and Research Center, Department of Physical Therapy, Speech and Language Pathology and Audiology and Occupational Therapy, Medical School, University of São Paulo.

After collection of information, we started the audiological assessment, being that the following procedures were employed:

1. Inspection of the auditory external canal. The procedure was conducted in order to check cerumem build-up or presence of foreign body that could prevent sound passage and consequently affect the subsequent audiological tests;
2. Pure tone audiometry and speech audiometry (SRT - Speech Reception threshold, IPRF - speech recognition index, and SDT - speech detection threshold). The test was conduced using the audiometers GSI 10 and GSI 16 - brand Grason Stadler, according to the criteria proposed by Mangabeira Albernaz et al. (1981);
3. Acoustic immittance measures including tympanometry, static compliance measures and acoustic reflex, both ipsilateral and contralateral. Acoustic immittance measures were conducted using the Immittanciometer GSI 33 - brand Grason Stadler. To interpret the results concerning the audiometric curves and acoustic reflexes we used, respectively, the classifications proposed by Jerger (1970) and Lopes F.º (1972).
4. Auditory brainstem response (ABR) both with and without contralateral masking.

The ABR was conducted with device model Traveler Express, brand Biologic (version 5.70 model 317). The accessories connected to the equipment were: supraaural phones TDH-39 and platinum electrodes. The stimulus was a click with rarefied polarity, duration of 0.1 millisecond, presentation speed of 19.0 clicks/s. We used the window of 0 to 10.24 ms and low and high pass filters, respectively, of 100 Hz and 3000 Hz. A total of 2,000 stimuli were tested twice to analyze reproducibility of tracings for each tested intensity. The type of masking used in the normal ear was White Noise, which proved to be effective in the test. The masking intensity was defined according to the electrophysiological threshold of each assessed subjects.

The subjects were placed on a reclining chair. We cleaned the skin with abrasive paste, applied electrolytic paste to improve electrode contacts and they were fixed using duck tape. We placed three electrodes to allow ipsilateral shunts: active electrode (vertex), reference electrode (tested mastoid) and ground electrode (non-tested mastoid). The impedance of the electrodes was measured so as to be below 5 K© and the value of the difference between one electrode and the other was maximum 2 k©.
The subject was instructed to be as relaxed as possible, avoiding sudden movements, especially of the head and neck muscles, which could interfere significantly in tracing collection.

After that, we conducted the test of the normal ear at 90 dB HL to assess the integrity of the auditory pathways and next, we studied the electrophysiological threshold of the same ear. Upon assessing the impaired ear, we tested the electrophysiological thresholds starting ABR at maximum intensity of acoustic output (90 dB NA), because in some cases at 80 dB HL no electrophysiological responses could be evidenced. After detecting the electrophysiological thresholds of both ears without masking, the value of intraaural attenuation to clicks was calculated for each subject and ABR was again tested in the presence of contralateral masking.

To calculate the initial value of masking applied to the normal ear, for each individual, the following calculations were made: 1. The value of the interaural attenuation to clicks was subtracted from the electrophysiological thresholds of the impaired ear; 2. The sum was added by 10 dB intensity; 3. The result of the former was the minimum intensity of the masking employed. For example, if the electrophysiological threshold of the normal ear is 15 dB HL and that of the impaired ear is 80 dB HL (without contralateral masking) interaural attenuation would be 65 dB and the initial value of masking should be 25 dB WN (80 dB - 65 dB + 10 dB HL).

The electrophysiological threshold of the impaired ear was again simultaneously tested with application of contralateral masking in the intensity previously explained. If the response was maintained, masking intensity was increased by 10 dB and the test was repeated. However, in the absence of response, the intensity for the acoustic stimuli (clicks) was increased 10 dB and the calculations were adjusted. Thus, the test was conducted up to the maximum intensity of the acoustic stimuli (click) in the impaired dear (90 dB HL).

In order to check the need to apply masking during the assessment of hearing using ABR, we conducted an analysis of the results obtained through the following comparisons and statistical analysis: 1. Comparison of the electrophysiological responses of the impaired ear obtained with and without contralateral masking; 2. Analysis of interaural attenuation for clicks; 3. Analysis of the quantity of masking necessary for the electrophysiological contralateral response (of the normal ear) not to interfere in the impaired ear; 4. Absolute latency and amplitude analysis of wave V at 90 dB HL for clicks of the impaired ear in the absence of masking.

Table 1. Audibility thresholds (dB HL) for air conduction in pure tone audiometry of the 22 studied cases including frequencies 250 Hz to 8000 Hz.

* AR: Absent response in the maximum intensity of the device.


Table 2. Electrophysiological thresholds (dB HL) for air conduction in brainstem evoked response, with and without contralateral masking, minimum intensities of masking necessary to be applied to the normal ear to test the impaired ear at 90 dB HL and interaural attenuation for clicks obtained in the 22 studied cases.

** AR: Absent response in the maximum intensity of the device.


Table 3. Mean values of absolute latency and amplitude of wave V obtained with 90 dB HL of acoustic stimulus (click) in the impaired ear in the absence of contralateral masking.

Confidence Interval (IC) 95%


Table 4. Percentage distribution of the values of interaural attenuation to clicks detected in the study.

*P<0,001



Figure 1. Brainstem evoked potential with and without contralateral masking (Case 10).
A: Electrophysiological threshold of the normal ear without masking.
B: Electrophysiological threshold of the impaired ear without masking.
C: Absence of electrophysiological responses in the impaired ear in the presence of contralateral masking.

Results and Discussion

In pure tone audiometry, we confirmed the unilateral profound sensorineural hearing loss in 100% of the subjects, being 68% on the right and 32% on the left (Table 1).

As to acoustic immittance measures, in all cases, we had tympanometry curves type A in both ears. However, in 32% of the studied population we detected absence of contralateral acoustic reflex in the normal ear in all studied frequencies (Cases 4, 7, 12, 16, 17, 20 and 21). This affection of acoustic reflex was irrelevant, since the data did not interfere in the purpose of the study.

In the ABR, we assessed normal ears in 22 cases of the study and had 100% of electrophysiological thresholds within normal range (below or equal to 25 dB HL - Table 2), being that 95.5% of the ears had absolute latencies of waves I, III and V and interpeak latencies I-III, III-V and I-V within the normal range. In one case (Case 21), 4.5% remaining, we observed delay in absolute latencies of waves III and V and consequent increase in interpeak latencies I-III and I-V, being values suggestive of the presence of abnormalities in the auditory pathways of the upper brainstem. This increase in absolute latency of waves III and V did not interfere in the purpose of the present study, because in the subsequent assessments the results were similar to those detected in other cases.

ABR on the impaired ears without contralateral masking resulted in wave V delayed in absolute latency in all cases and in the following electrophysiological thresholds: 70 dB HL (4.5%), 80 dB HL (18%), 85 dB HL (54.5%) and 90 dB HL (23%) (Table 2). In the 90 dB HL intensity (maximum output of the device) all subjects had presence of Wave V with absolute mean latency of 7.27 milliseconds and mean amplitude of 0.32 (µV) (Table 3).

The occurrence of interaural attenuation for clicks of 65 dB was higher than for 70 dB and 75 dB (Table 2), being that there was statistically significant difference between attenuation of 65 dB and attenuation of 70 dB and 75 dB (Table 4). The value of minimum interaural attenuation for the 65 dB clicks found in the study was similar to that of the study by Chiappa, Gladstone & Young (1979) and Ozdamar & Stein (1981). However, in other studies, interaural attenuation was found in 50 dB (Smyth, 1985) and 70-75 dB (Humes & Ochs, 1982). Based on different values of interaural attenuation for clicks found in this study and in the studied literature, we detected the importance of having attenuation values for each device used in medical facilities.

Upon reassessing the impaired ears using ABR and applying contralateral masking, we observed that previously visualized waves V were absent in the maximum intensity of acoustic stimuli (90 dB HL) (Table 2), being that the intensity of necessary masking was 10 dB higher than the interaural attenuation in all cases. Similarly, we concluded that electrophysiological thresholds primarily captured without the use of masking were responses coming from the normal ear. Based on such results, we could confirm that the use of masking during ABR assessment is a necessary procedure to examine subjects with hearing loss asymmetry (equal or greater than 65 dB) and to collect reliable results. Such findings showed agreement with the studied literature 11-14. Other authors, confirming our data, stated that masking was necessary whenever we used an acoustic stimulus greater than 70 dB HL and when wave I was absence in the presence of wave V with delay in absolute latency 1. Conversely, the results obtained in the present study are not in agreement with the findings by Galambos & Hecox (1978), Finitzo-Hieber, Hecox & Cone (1979) and Owen & Burkard (1991), who concluded that contralateral masking did not substantially influence the ABR assessment.

Figure 1 represents the electrophysiological responses obtained with and without masking in one of the studied cases (Case 10). In Figures 1-A and 1-B, we can observe, respectively, electrophysiological thresholds of the normal ear at 20 dB HL and of the impaired ear at 85 dB HL in the absence of masking. The interaural attenuation for such case was 65 dB (85 - 20 dB). Please note that pure tone audiometry showed absence of responses in the impaired ear in the maximum output of the device (105 dB HL to frequencies of 250 to 8000 Hz and 120 dB HL to frequencies of 500 Hz to 6000 Hz - Table 1). Upon retesting the affected ear using ABR, the electrophysiological threshold initially detected (85 dB HL) disappeared when we introduced contralateral masking at 30dB WN (85 dB - 65 dB + 10 dB) (Figure 1-C). Finally, as shown in Figure 1-C, the intensity of the acoustic stimuli (clicks) was increased to 90 dB HL(maximum device intensity) and we concomitantly applied contralateral masking to the normal ear at 35 dB WN (90 dB - 65 dB + 10 dB). The response previously detected was also absent. This illustration clearly shows the importance of using masking to exclude the response of the contralateral pathway and obtain a reliable result of the assessment using ABR in subjects with unilateral profound hearing loss.
The minimum and maximum intensities needed to mask the ABR responses in the present study considering the total number of cases and interaural attenuation for individual clicks were respectively 25 dB WN (white noise) and 35 dB WN (Table 2). According to the studied literature, the intensity of masking should be 40 to 45 dB WN in the non-tested ear to avoid contralateral electrophysiological response 1. In view of such results we suggest that the amount of masking to be applied should be determined according to the used equipment.

Conclusions

Based on the critical analysis of the results of the hearing assessment using ABR in subjects with unilateral sensorineural hearing loss, we concluded that:

1. Contralateral masking applied to the normal ear is extremely important to conduct ABR in subjects with unilateral profound sensorineural hearing loss so as to have reliable results.
2. Interaural attenuation for clicks was greater (65 dB) than what was observed in the pure tone audiometry, requiring lower masking intensity to exclude the response of the contralateral auditory pathway.
3. The values of interaural attenuation and quantity of masking necessary to conduct ABR in subjects with unilateral or asymmetrical hearing loss should be defined in each facility.

References

1. Munhoz MSL, da Silva MLG, Caovilla HH, Frazza MM, Ganança MM, Câmera JLS. In: Munhoz MSL, Caovilla HH, da Silva MLG, Ganança MM. Audiologia Clínica; série Otoneurológica. Volume 2. São Paulo: Editora Atheneu; 2000. p.191- 230.
2. Lima MAMT. In: Frota S. Fundamentos em Fonoaudiologia: Audiologia. 1ª edição Rio de Janeiro: Guanabara Kogaan; 1998. p. 147-60.
3. Musiek FE, Borenstein SP, Hall III JW & Schwaber. In: Katz J. Tratado de Audiologia Clínica. 4ª edição. São Paulo: Manole; 1999. p. 349-71.
4. Redondo MC. In: Frota S. Fundamentos em Fonoaudiologia: Audiologia. 1ª edição. Rio de Janeiro: Guanabara Kogaan; 1998. p. 69-76.
5. Sanders JW & Hall III JW. In: Musiek FE & Rintelmann WF. Perspectivas Atuais em Avaliação Auditiva. 1ª edição brasileira. Barueri: Manole; 2001. p. 63-84.
6. Goldstein BA & Newman CW. In: Katz J. Tratado de Audiologia Clínica. 4ª edição. São Paulo: Manole; 1999. p. 109-31.
7. Durrant JD & Ferraro JA. In: Musiek FE & Rintelmann WF. Perspectivas Atuais em Avaliação Auditiva. 1ª edição brasileira. Barueri: Manole; 2001. p. 193-238.
8. Finitzo-Hieber T, Hecox K & Cone B. Brain stem auditory evoked potentials in patients with congenital atresia. Laryngoscope 1979;89:1151-8.
9. Galambos R & Hecox KE. Clinical applications of the auditory brainstem response. Otolaryngology Clinics of North American 1978;11:709-22.
10. Owen GA, & Burkard R. Ipsilateral contralateral and binaural masking effects on the human brain-stem auditory-evoked responses to click stimuli. Journal of Acoustical Society of America 1991;89:1760-7.
11. Chiappa KH, Gladstone KJ & Young RR. Studies of waveform variations in 50 normal human subjects. Arch Neurol 1979;36:81-7.
12. Ozdamar O & Stein L. Auditory brain stem response (ABR) in unilateral hearing loss. Laryngoscope 1981;41:565-74.
13. Humes LE & Ochs MG. Use of contralateral masking in the measurement of the auditory brainstem response. Journal of Speech and Hearing Research 1982;25:528-35.
14. Smyth V. On the effect of cross-hearing and clinical masking on the auditory brain-stem evoked response. Electroencephalography and Clinical Neurophysiology 1985;61:26-9.
15. Mangabeira Albernaz P, Mangabeira Albernaz PL, Mangabeira Albernaz LG e Mangabeira Albernaz FP. Otorrinolaringologia Prática. 10ª edição. São Paulo: Savier; 1981.
16. Jerger J. Clinical experience with impedance audiometry. Arch Otolaryngol 1970;92:311-24.
17. Lopes Fº OC. Contribuição ao estudo da impedância acústica. São Paulo. Tese de Doutorado - Faculdade de Medicina da Universidade de São Paulo, 1972.

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1 Speech Therapist and Audiologist, Improvement Studies in Speech and Language Pathology and Audiology under course, Medical School, University of São Paulo.
2 Speech Therapist and Audiologist. Ph.D. Professor of the School of Speech and Language Pathology and Audiology, Medical School, University of São Paulo.
Monograph submitted for the completion of the Improvement Studies in Clinical Speech Therapy, Department of Physical Therapy, Speech and Language Pathology and Audiology and Occupational Therapy, Medical School, University of São Paulo.
Address correspondence to: Melissa Mayumi Takahashi Toma - Rua Álvaro Luis Roberto de Assumpção, 79 apt. 61 Campo Belo 04618-020 São Paulo SP.
Tel (55 11) 5531-5084/ 9788-4528 - E-mail: melissa_toma@yahoo.com.br
Article submitted on March 01, 2003. Article accepted on May 15, 2003.

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