Right-left correlation in guinea pig ears after noise exposure

Influence of noise level and seniority in the workplace on the SAL, ELI and percentage of hearing loss indices in the diagnosis and prevention of hearing loss in the working population

INTRODUCTION

This research relates the most important work-related factors affecting the development of hearing loss to the main methods used as medical assessment criteria in the diagnosis of occupational deafness. These criteria are the Speech Average Loss Index (SAL), the Early Loss Index (ELI) and the Percentage of Hearing Loss, and are applied to data obtained from audiograms performed on workers in occupational medical examinations.

METHOD

Depending on the assessment method selected, these often return different results in grading an individual's hearing status and predicting how it will evolve. To address this problem, medical examinations (including audiograms) were carried out on a heterogeneous sample of 1,418 workers in Spain, from which demographic or personal data (gender, age, etc.), occupational data (noise level to which each individual is exposed, etc.) and other non-work-related factors (exposure to noise outside work, family history, etc.) were also gathered. Using Bayesian Networks, the conditional probability of an individual developing hearing loss was obtained taking into account all these factors and, specifically, noise level and length of service in the workplace. Sensitivity analyses were also carried out using the three scales (SAL, ELI and Percentage Hearing Loss Index), proving their suitability as tools the diagnosis and prediction of deafness. These networks were validated under the Receiver Operating Characteristic curve (ROC) criterion and in particular by the Area Under the Curve (AUC).

RESULTS

The results show that all three methods are deficient in so far as detecting preventive hearing problems related to noise in most workplaces.

CONCLUSIONS

The most restrictive methods for detecting possible cases of deafness are the SAL index and the Percentage Loss Index. The ELI index is the least restrictive of the three methods, but it is not able to discriminate the causes of hearing problems in an individual caused by exposure to noise, either by its intensity level or by the time of exposure to noise. Practical Applications: The use of the three methods in the field of occupational risk prevention is extremely limited and it seems reasonable to think that there is a need for the construction of new scales to correct or improve the existing ones.

A novel RFP-RET transgenic mouse model with abundant eumelanin in the cochlea

We report on the cochlea of a novel metallothionein-I (MT)/RFP-RET transgenic mouse model with severe systemic melanosis. Electron microscopy revealed that these transgenic mice possess abundant quantities of melanin in the intermediate cells of the stria vascularis. High performance liquid chromatography analysis indicated that cochleae of these transgenic mice contained about twice as much eumelanin as cochleae of control C57BL/6 mice and that the amount of pheomelanin was approximately equal in these two strains. Auditory brainstem responses at 2, 4, 8, and 16 kHz were not significantly different between transgenic and control mice. This is the first report on a mouse model of overproduction of cochlear eumelanin, and our results suggest that this transgenic mouse is an excellent model for investigating the effects of overexpression of cochlear eumelanin. In addition, we provide evidence that eumelanin overproduction in the cochlea does not affect normal hearing.

Cochlear function in young and adult Fischer 344 rats

Fischer 344 (F344) rats are often used as an animal model for investigation of the mechanisms underlying age-related hearing loss. The aim of this study was to assess cochlear function in young (1-month-old) and adult (6-month-old) F344 rats using recording of otoacoustic emissions and auditory brainstem responses (ABRs). The results were compared with control groups of Long Evans (LE) rats of the same ages. The results demonstrate a significant increase in the hearing threshold in F344 rats in comparison with LE rats, expressed mainly at low frequencies (1-2 kHz). In F344 rats, transient evoked otoacoustic emissions were not measurable and distortion product otoacoustic emissions could be detected within a frequency range of 2.4-6.3 kHz. Tympanometric measurements did not reveal any differences in middle ear parameters between F344 and LE rats. The amplitudes of click-evoked ABRs were significantly lower in 6-month-old F344 rats than in LE rats, but other parameters of the ABRs were almost identical in both rat strains. The results demonstrate a significant deficit in low-frequency hearing and altered otoacoustic emissions in both young and adult F344 rats, suggesting a defect of the inner ear sensory epithelium at the apical part of the cochlea.

Noise-induced hearing loss: the effect of melanin in the stria vascularis

Conflicting investigations regarding the potential protective effect of melanin against noise-induced sensorineural hearing loss have suggested that eumelanin and pheomelanin may have differing effects within the stria vascularis. Three strains of C57BL/6J mice, (+/+, a/a) wild-types (dark coats/black eyes), (c2j/c2j, a/a), albinos (white coats/pink eyes), and (+/+, Ay/Ay) yellow mice (yellow coats/black eyes), were subjected to five consecutive days of broad band noise exposure at 112 dB(A) SPL for 3 h/day. Cochlear function was evaluated with auditory brainstem response audiometry to pure tones immediately pre-exposure, 5-6 h postexposure, and 14 days post-exposure. No significant difference in the degree of sensorineural hearing loss induced in the three strains of mice was identified. The eumelanin and pheomelanin content of each stria vascularis and amount of protein per stria for both mouse and guinea pig (2/NCR) were determined via high performance liquid chromatography. No pheomelanin was found in the stria of yellow mice, suggesting that coat color is not an accurate predictor of strial melanin content. The melanin content per mg of strial protein was higher in mice than in guinea pigs. A species-specific difference in melanin content does not explain the absence of a protective effect in mice.

Extended high-frequency thresholds in noise-induced hearing loss

Air-conduction and bone-conduction thresholds were measured in the conventional audiometric frequency ranges, and air-conduction alone in the extended high-frequency range of 9-18 kHz in 167 males with a history of occupational noise exposure. The subjects were grouped according to age. Hearing loss in the conventional frequency range was classified in different grades. Threshold elevation in the extended high-frequency range was present in all age groups and grades of conventional frequency hearing loss. An age effect in the extended high-frequency range was present only in the lowest grades of conventional frequency noise-induced hearing loss.

Quantitative assessment of human cochlear function by evoked otoacoustic emissions

The amplitudes of evoked otoacoustic emissions (EOE) and their detection threshold were measured in 44 normal young adults and 118 patients with two categories of cochlear dysfunction, acoustic trauma and presbycusis. A different method was used for each category: detection of click EOE or of stimulus frequency emissions. A partial correlation and multivariate analysis was performed for both groups of results to investigate the relations between EOE threshold one pure tone audiometric thresholds (250 to 8000 Hz). Only one significant correlation was found, linearly relating EOE threshold and hearing threshold at 2 kHz (P less than 0.001), independently of the origin of cochlear dysfunction. It suggests that EOE threshold is not frequency-specific since the frequency of EOE at threshold was nearly always close to 1 kHz. A simple model is proposed, based on the assumption that EOE amplitudes and threshold are proportional to the total number of residual active sites in the organ of Corti, i.e. to the total length of active basilar membrane. It is shown that this model accounts for the results disclosed by the statistical analysis and fits the experimental data. It can be used for quantitatively predicting the residual cochlear activity of a patient. However, the EOE threshold is only sensitive to already important cochlear alterations and this parameter does not seem to allow a follow-up of early stages of cochlear dysfunction.

Left-right asymmetry in the human response to experimental noise exposure. I. Interaural correlation of the temporary threshold shift at 4 kHz frequency

Most epidemiological surveys concerning populations exposed to occupational noise or random populations have shown that the left ear is slightly but significantly poorer than the right ear, especially at frequencies most susceptible to noise damage. To experimentally study the possible left-right asymmetry in response to noise exposure, 28 non-shooting healthy young adults were exposed binaurally to symmetrical broad-band noise for a maximum of 8 h. 4 kHz hearing thresholds of each individual were monitored alternately in the left and right ears during short interruptions in the exposure. Considerable efforts were made to exclude any external factors that might influence the left-right asymmetry. A significant positive correlation between the temporary threshold shifts (TTS) in the left and in the right ear was found. The average TTS was higher in the left than in the right ear, the difference between ears being statistically indicative.

Noise-induced threshold elevation as a function of peak sound pressure level

Thirty-three groups of guinea pigs, consisting of five animals in each group, were exposed to a simulated impact noise with peak levels ranging between 119.5 and 134.5 dB SPL. By varying the repetition rate, different equivalent levels could be set at each peak level. The equivalent levels ranged from 96 to 117 dB SPL, and the exposure duration was 1.5 to 24 hours. The compound action potential thresholds were measured in 1/3-octave steps between 1 and 20 kHz, one month after the exposure. Higher peak levels resulted in a peak-shaped threshold elevation with a maximum around 8 kHz. For constant peak levels, the equal energy theory was supported. For exposures of equal energy but different peak levels, significantly higher threshold elevations resulted after exposure to higher peak levels.