Evaluating noise induced hearing loss with distortion product otoacoustic emissions.

This study assessed the clinical efficacy of screening for noise induced hearing loss (NIHL) with distortion product otoacoustic emissions (DPOAE). DPOAEs were recorded from 76 military personnel (137 ears) aged between 17 and 41 years in response to equilevel 70 dB SPL primary stimulating tones. The 2f1-f2 DPOAE levels were correlated with audiometric thresholds at frequencies close to f2. Ears with normal audiograms, but with a history of military noise exposure, had DPOAEs that were significantly decreased in amplitude as compared to the ears of normal hearing non-exposed to noise subjects. These ears also had an increased absence of DPOAEs as compared with the ears of the normal hearing non-exposed to noise subjects. Although, in general, the DPOAE amplitudes and spectral frequency ranges reflected the audiometric NIHL configurations, in a number of cases DPOAEs were present for hearing losses up to 75 dB HL. As a consequence, DPOAEs correlated moderately and negatively with the audiometric thresholds. Applying test criteria designed to logically reflect NIHL, DPOAE sensitivity and specificity levels ranged between 0.51-0.90 and 0.63-0.25, respectively. These findings indicate that DPOAEs, recorded and analysed as described, are not sufficiently sensitive to serve as a single test to identify NIHL.

Detailed analysis of auditory brainstem responses in patients with noise-induced tinnitus.

The role of the auditory brainstem in tinnitus is questionable. This study aimed comprehensively to assess auditory brainstem responses (ABRs) in patients suffering from noise-induced tinnitus (NIT). ABRs were recorded from 13 chronic NIT patients (21 ears) and 11 (21 ears) age and hearing matched control subjects without tinnitus. ABRs were recorded with scalp electrodes placed ipsilateral and contralateral to the stimulated ear, and in three orthonormal differential configurations. The ABRs were analyzed as a function of time, frequency and voltage space. A significantly enhanced ipsilaterally recorded, time domain wave III amplitude was observed for the tinnitus patients. This finding was not confirmed by any of the other ABR measures, which were indistinguishable between subject groups. Although this may be a spurious result, it nonetheless may point to an alteration in the functioning of the putative wave III auditory brainstem generator, which deserves further study.

Noise-induced otoacoustic emission loss with or without hearing loss.

The association between audiometric hearing thresholds and click-evoked otoacoustic emission (CEOAE) spectral properties was examined in 129 adult subjects with and without a noise-induced hearing loss (NIHL). Subjects were grouped according to their "beginning of hearing loss frequency" and their exposure to hazardous noise. Emissions were recorded with an ILO88 Otodynamic Analyzer (Version 2.9) used in the default mode. CEOAE levels decreased as the hearing threshold increased at each of the test frequencies (1,2,3, and 4 kHz). At frequencies where hearing thresholds were worse than 20 dB HL, CEOAEs could not be recorded. Thus as the "beginning of hearing loss frequency" decreased, the frequency range of the emissions became narrower. The hearing threshold for which emissions were not recorded varied significantly between subjects, such that even at frequencies where the hearing threshold was 0 dB HL emissions were not always observed. Noise-exposed, normal-hearing subjects had reduced overall CEOAE power with a narrow frequency range as compared with normal-hearing, nonexposed to noise subjects. For our test conditions, the presence of CEOAEs necessarily suggests hearing thresholds of 20 dB HL or less at the corresponding frequency. A lack of emissions does not necessarily indicate hearing thresholds beyond 20 dB HL.

Characteristics of click-evoked otoacoustic emissions in ears with normal hearing and with noise-induced hearing loss.

The clinical application of click-evoked otoacoustic emissions (EOAE) in the assessment of noise-induced hearing loss (NIHL) was examined in a group of 72 ears with NIHL and 61 ears with normal hearing (NH). The characteristics of the EOAE in ears with NIHL significantly differed from the NH, according to all EOAE parameters tested in the present study. The mean overall EOAE level was lower and the mean EOAE nonlinearity threshold was worse in the NIHL group. In 95% of the NH ears the EOAE spectrum range was wide, while in 91.5% of the NIHL ears the range was narrow. Moreover, in 94% of the ears with NIHL, the frequency at which the hearing loss began (BHL) was at or above the frequency of the last peak in the EOAE spectrum (FLP). Furthermore, combination of EOAE spectral measures correctly discriminate on average 93.5% of ears with NH from NIHL (sensitivity) and 92% of ears with NIHL from NH (specificity). In contrast, the nonlinearity threshold and the overall level of EOAE yielded lower specificity of less than 33%. It was therefore concluded that EOAE spectrum may serve as a useful and objective tool in screening adults with suspected noise-induced high frequency hearing loss.

Manifestations of intense noise stimulation on spontaneous otoacoustic emission and threshold microstructure: experiment and model.

Comparison between changes that occur simultaneously on spontaneous otoacoustic emissions (SOAEs) and on other cochlear origin phenomena can contribute to the understanding of cochlear micromechanical activity. The temporary changes that arise after short noise exposure are investigated in the following paper. The effects of noise exposure on the threshold microstructure near an SOAE and on the amplitude and frequency of the SOAE were measured. These experimental results indicate the following: (1) exposure to wideband noise for a short time causes a temporary reduction in the SOAE frequency and amplitude, and alters reversibly the threshold microstructure in the vicinity of the SOAE. The difference between the minimum and maximum in the threshold microstructure is reduced, and the frequency that yields the minimum threshold decreases; (2) the threshold at the SOAE frequency is most sensitive to noise exposure; (3) intense stimulation causes a relatively small increase, or even a decrease, in threshold at frequencies near the SOAE. The experimental results are interpreted in terms of a nonlinear transmission line model which includes nonlinear amplifiers. The effect of the noise exposure is modeled by reduction in the cochlear partition amplification term. Most of the experimental results are predicted by this model.