Slow Auditory Evoked Potentials (1 Interligne

R. BONIVER

VERVIERS, BELGIUM

Key words. Slow vertex response; conventional audiometry; forensic audiometry.

Abstract. Cortical electric response audiometry (slow vertex responses) in forensic audiology. The interest of slow vertex response audiometry (cortical evoked response audiometry), mainly in the diagnosis of pseudohypacousis is reported.The procedure is of interest to forensic audiometry.

In some cases, during forensic examination, the definition of the audiometric threshold is difficult because the patient tries to exaggerate his deafness.

Although there are many tests to demonstrate the existence of non-organic hearing loss (NOHL) none of them allows us to obtain the auditory threshold exactly. Among them:

- clinical observation of the patient,- several results of the audiometric threshold,- vocal audiometry,- Stenger's test,- Bekesy's test,- stapedius reflex threshold studies,- DAF test- ABEP (Auditory Brainstem Evoked Potentials (BISHARA et al.)), etc…

In 1982, we published a preliminary study on the interest in the study of cortical evoked responses audiometry (CERA) in the field of expertise.

HYDE et al. (6) published results confirming the interest in the study of slow vertex response in non-organic hearing loss.In this paper, the conclusions of twenty years experience are presented.

SLOW AUDITORY EVOKED POTENTIALS:

THE END OF MALINGERING IN AUDIOLOGY.

1. Introduction

During these thirteen years, 1200 tests were performed on a number of subjects suspected of non-organic hearing loss and several cochlear pathologies.

Most of the claimants were coal miners, asking a revalidation for their professional hypoacousy.

1.1. Test environment

Subjects were tested in a comfortable relaxed position, in a sound-proof room separated and isolated from the physician and apparatus.

1.2. Stimulus

- Apparatus: Madsen 22-50- Tone Burst : - rise decay time 5 msec.,

- duration time 40 msec.- Frequencies : 1 kHz, 2 kHz, 3 kHz- One stimulus every two seconds- n = 30- earphones

Masking the non-tested ear was required at high intensities (narrow band masking). The stimulus calibration was controlled the first time by an artificial ear Bruel Kjaer 4152 (3-4) and subsequently in comparison to normal ears. The time of analysis was 500 msec. The band width was 0.25-15 Hz.

1.3. Testing procedure

Sitting in a comfortable position and given something to read or a videofilm to watch, the patients were told not to move.

1.4. Analysis of the curve (fig. 1)

In our experience, the waves N1 and P2 were the most reproductible until the threshold, mainly N1.

2. Material and method

3. Results

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The measurements always began at 90 or 100 dBs. HL to obtain a good curve; then, the level was reduced trial by trial in 20 dB steps until the threshold was crossed by a clearly negative trial, i.e. the absence of a visible slow potential, and controlled around this level in five dBs steps.

The main problem was the exact interpretation of the threshold intensity level. To find this threshold with the best precision, it is absolutely necessary to have a good control of the vigilance status because when the patient is asleep or inattentive, the variability of the threshold is more than 15 dB per frequency.

The best control of the vigilance was obtained through reading or video stimulation.

The threshold was reached when N1 was not reproduced by two stimulations at the same level.

1.5. Normal subjects

When the alertness of the subject was weak, and attention not sustained, the variability of the threshold was greater.

In ten people, an experiment was conducted in darkness, without any stimulation and at 1000 Hz: the threshold of conventional audiometry and CERA varied between 10-20 dB which is higher than in comparison to the threshold in a normal situation.

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This explains why some drugs like neuroleptics, sedatives and so on, can modify the sensivity of the test by their action on vigilance.

1.6. Non simulating subjects

By averaging hearing loss in the 1000-2000-3000 Hz range, we compared the results of conventional (psychoacoustic) audiometry and CERA in 30 non-simulating subjects, who had developed hearing loss due to noise exposure.

A statistical analysis was conducted to compare the average of the threshold on these 3 frequencies measured by the two techniques (60 measurements, fig. 2-3).

We studied the T-student test for paired cases.

The average of differences between conventional audiometry and CERA threshold was 1.8 dB (S.D. = 7.5 dB).

This was not significant (p = 0.069) if we consider that threshold CERA may be better or worse than conventional audiometric threshold.

In figures 2 and 3 we report the comparison between the average of threshold at 1000, 2000, 3000 kHz obtained by conventional and CERA audiometries in 30 people with non organic hearing loss.

In most cases we see directly that the difference between the threshold is maximum (about 15 dB) except for case N° 11 (from the two ears) and 14 and 29 (from one ear).After analysis, it appeared that these exceptions concern people in whom the state of vigilance was difficult to maintain.

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1.7. Non organic hearing loss

In reference to the group of nonsimulating subjects, in our group of 87 cases of "simulators", the auditory threshold obtained by converntional and CERA audiometry was of great variability (fig. 4-5).

For example, at 1000 Hz the average difference in right ear between conventional and CERA threshold was 42.1 dB. (Standard Deviation = 16.25.).

Before conducting a CERA on this group of subjets, we ascertained that no sedative drugs had been taken before the test and that there was no recent acoustic trauma.

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CERA is a reproducible and good technique for the determination of the auditory threshold.

When the conditions of examination described in this report are achieved, i.e. a good level of vigilance, the precision of the CERA audiometric threshold is the same as in conventional audiometry. The fact that the state of vigilance was apparently not considered probably explains the difference with the results of Albera et alii (1-2) who found larger variability in the threshold.

We insist on the importance of keeping alertness of the subject during the test period. CERA becomes, in our experience, the best examination for detecting non-organic hearing loss but requires considerable experience to maintain excellent examination conditions. The cost of the apparatus and the length of the examination, however, are important factors in determining its frequency of use.

R. BONIVERRue de Bruxelles, 21B-4800 Verviers, BelgiumEmail : [email protected].

Presented at the Twenty-Seven International Congress of the Neurootological and Equilibriometric Society, Bad Kissingen, Germany, March 23-26, 2000.

1. ALBERA R., ROBERTO C., MAGNANO M., LACILLA M., MORRA B., CORTESINA G. Identification of the waveform of cortical auditory evoked potentials. Acta Otorhinolaryngol. Ital., 11: 543-549, 1991.

2. ALBERA R., CANALE G., MAGNANO M., LACILLA M., MORRA B., RUGIU M.G., CORTESINA G. Relations between pure tone audiometry and cortical evoked auditory potentials. Acta Otorhinolaryngol. Ital., 11: 551-562, 1991.

3. BISHARA L., BEN DAVID J., PODOSHIN L., FRADIS M., TESZLER C.G., PRATT H., SPACK T. FEIGLIN H., HAFNER H., HERLINGER N. Correlations Between Audiogram an Objective Hearing Tests in Sensorineural Hearing Loss. International Tinnitus Journal, 5: No2: 107-112, 1999.

4. BONIVER R. Intérêt de l'étude des potentiels évoqués corticaux en expertise. Acta Otorhinolaryngol. belg., 36: 377-381, 1982.

5. BONIVER R. Intérêt de l'étude des potentiels évoqués de latence moyenne. Acta Otorhinolaryngol. belg., 36: 997-1004, 1982.

6. DAVIS H., ZERLIN S. Acoustic Relations of the Human Vertex Potential. Journal of the Acoustical Society of America, 39: 109-116, 1966

Conclusion

Bibliography

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7. HYDE M., ALBERTI P., NORIAKI M., YAOLI L. Auditory Evoked Potentials in Audiometries. Assessment of Compensation and Medicolegal Patients. Ann. Otol. Rhinol. Laryng., 95: 514-519.

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