Cochlear microphonic potentials: a new recording technique

[Auditory fatigue]

INTRODUCTION AND OBJECTIVES

Given the relevance of possible hearing losses due to sound overloads and the short list of references of objective procedures for their study, we provide a technique that gives precise data about the audiometric profile and recruitment factor. Our objectives were to determine peripheral fatigue, through the cochlear microphonic response to sound pressure overload stimuli, as well as to measure recovery time, establishing parameters for differentiation with regard to current psychoacoustic and clinical studies.

MATERIAL AND METHOD

We used specific instruments for the study of cochlear microphonic response, plus a function generator that provided us with stimuli of different intensities and harmonic components. In Wistar rats, we first measured the normal microphonic response and then the effect of auditory fatigue on it.

RESULTS

Using a 60dB pure tone acoustic stimulation, we obtained a microphonic response at 20dB. We then caused fatigue with 100dB of the same frequency, reaching a loss of approximately 11dB after 15minutes; after that, the deterioration slowed and did not exceed 15dB. By means of complex random tone maskers or white noise, no fatigue was caused to the sensory receptors, not even at levels of 100dB and over an hour of overstimulation.

CONCLUSIONS

No fatigue was observed in terms of sensory receptors. Deterioration of peripheral perception through intense overstimulation may be due to biochemical changes of desensitisation due to exhaustion. Auditory fatigue in subjective clinical trials presumably affects supracochlear sections. The auditory fatigue tests found are not in line with those obtained subjectively in clinical and psychoacoustic trials.

[Cochlear microphonic latency]

INTRODUCTION AND OBJECTIVE

By using appropriate instrumentation, we have found that cochlear microphonics (CM) advance or delay their appearance, depending on the sound pressure that generates them. This time variation is on the order of microseconds. We have not found any reference to this behaviour, which is why we make the finding known.

MATERIAL AND METHOD

We used the standard instrumentation specified for the study of CM. The method was based on the phase shift function of the CM according to the intensity of the stimulus.

RESULTS

Latency was observed in CM, and we determined that latency time diminishes as the intensity of the stimulus increases.

CONCLUSIONS

From the sound stimulus to the bioelectric potential transduction, there is a time period of microseconds, the shorter the more powerful the stimulus. This suggests that electromechanical transduction is not a simple mechanical process.

Electrophysiological monitoring of hearing function during cochlear perilymphatic perfusions

CONCLUSION

The cochlear perilymphatic perfusion produces, by itself, significant effects in the cochlear physiology that could be associated with the surgical procedure. These effects need to be well characterized to allow a reliable quantification of the effects of the experimental agent being tested.

OBJECTIVES

The study focused on the accurate description of the electrophysiological effects on the cochlear potential recordings of perilymphatic perfusions.

METHODS

Two successive cochlear perilymphatic perfusions were carried out. The first used artificial perilymph. The second used artificial perilymph alone or a kainic acid (KA) solution in artificial perilymph. The compound action potential of the auditory nerve (CAP-AN) was recorded: (1) before the first perfusion, (2) after the first perfusion and (3) after the second perfusion, and compared between groups.

RESULTS

The first intracochlear perfusion with artificial perilymph produced significant effects in the CAP-AN that could be related to the surgical procedure. These effects were analysed separately from the effects produced by the KA. In particular, the KA administered intracochlearly produced a significant increase in the latency and a decrease in the amplitude of the CAP-AN N1 wave compared with the controls that were perfused twice with artificial perilymph.

Cochlear microphonic audiometry: a new hearing test for objective diagnosis of deafness

CONCLUSIONS

Objective audiometric tests could constitute a valuable tool for detection of deafness. This could be especially useful in children (universal newborn hearing screening) and non-collaborative patients, who are especially difficult candidates for classic audiometry. The cochlear microphonic audiometry (CMA) technique offers the possibility of obtaining objective audiometric profiles, highly correlated with those obtained by pure tone audiometry (PTA). Therefore, CMA could be used as an alternative test to obtain the audiometric profile of these patients.

OBJECTIVE

The main purpose of the present study was to demonstrate that CMA provides objective audiometric profiles by avoiding active participation by the patient. Subjects and methods. CMA specific equipment, improved for non-invasive recording of cochlear microphonic potentials, was used. This tool plots the recordings obtained as the classic audiogram. Verification of the method was carried out in adult patients by comparing the PTA with the CMA audiometric profiles obtained for each patient.

RESULTS

Our findings showed that audiometric profiles obtained from CMA are highly correlated, without statistical differences, to those obtained with PTA. More than 81% of patients explored (91.67% at 250 Hz) exhibited differences below 10 dB(HL) between tests at all exploration frequencies, while a low number of cases showed differences over 20 dB(HL).

[A procedure to obtain the recruitment using cochlear microphonic potentials]

OBJECTIVE

To present an objective procedure for recruitment measure. Nowadays all recruitment methods are based on the patients' subjective responses. This procedure is based on the ratio between an increase in the cochlear microphonic potential and the increase in stimuli intensity. The system allows recruitment to be measured down to one tenth of a decibel; the precision necessary to differentiate the auditory capacity of patients.

METHOD

We used a specific technique to detect cochlear microphonic potentials. We have compared the results obtained with this new technique with those determined by the Luscher method. We present 10 adults with no statistic AIM. We first tested the physical accuracy followed by measures with laboratory animals and humans.

RESULTS

We obtained a maximum deviation of 0.3 dB with respect to the patients' subjective appreciation. During the repetition of the objective measure with the same patients, we obtained identical results, whereas significant differences were observed with the repetitions in the subjective study.

CONCLUSIONS

The possibility of an objective measurement for recruitment opens up new perspectives for early diagnosis. This technique may be of assistance in the decision to place an implant or how to adapt a hearing aid. Through automatic volume control as well as the correct adjustment of the selective gain in frequency, we can also obtain the threshold audiometric profile simultaneously.