Noise-induced Hearing Damage Research Articles

The influence of the cochlear efferent system on chronic acoustic trauma

The role of the olivocochlear bundle (OCB) in modulating noise-induced permanent injury to the auditory periphery was studied by completely sectioning the OCB fibers in chinchillas and exposing the animals while awake to a broad-band noise at 105 dB SPL for 6 h. Outer hair cell (OHC) function was assessed by measuring 2f1–f2 distortion product otoacoustic emissions (DPOAE) at frequencies from 1.2 to 9.6 kHz and cochlear microphonics (CM) at frequencies from 1 to 8 kHz. As a result of de-efferentation, the CM was decreased but the DPOAEs were unchanged in de-efferented ears as compared with efferented control and sham-operated ears. Following noise exposure, the ears that were de-efferented showed significantly more depression of DPOAE input/output functions and greater decrement of CM amplitude. The differences between de-efferented and efferent-innervated ears were evident across all the frequencies. The cochlear lesions of the OHCs reflected by traditional cytocochleograms, however, were minimal in both efferented and de-efferented ears. The results indicate that cochlear de-efferentation decreases the CM in chinchilla and increases the ear's susceptibility to noise-induced permanent hearing damage. More importantly, de-efferentation increases susceptibility at low frequencies as well as high frequencies.

Sound exposures and hearing thresholds of symphony orchestra musicians

To assess the risk of noise-induced hearing loss among musicians in the Chicago Symphony Orchestra, personal dosimeters set to the 3-dB exchange rate were used to obtain 68 noise exposure measurements during rehearsals and concerts. The musicians' Leq values ranged from 79-99 dB A-weighted sound pressure level [dB(A)], with a mean of 89.9 dB(A). Based on 15 h of on-the-job exposure per week, the corresponding 8-h daily Leq (excluding off-the-job practice and playing) ranged from 75-95 dB(A) with a mean of 85.5 dB(A). Mean hearing threshold levels (HTLs) for 59 musicians were better than those for an unscreened nonindustral noise-exposed population (NINEP), and only slightly worse than the 0.50 fractile data for the ISO 7029 (1984) screened presbycusis population. However, 52.5% of individual musicians showed notched audiograms consistent with noise-induced hearing damage. Violinists and violists showed significantly poorer thresholds at 3-6 kHz in the left ear than in the right ear, consistent with the left ear's greater exposure from their instruments. After HTLs were corrected for age and sex, HTLs were found to be significantly better for both ears of musicians playing bass, cello, harp, or piano and for the right ears of violinists and violists than for their left ears or for both ears of other musicians. For 32 musicians for whom both HTLs and Leq were obtained, HTLs at 3-6 kHz were found to be correlated with the Leq measured.

Vestibular implications of noise-induced hearing loss.

An extensive vestibular examination was carried out in a group of 29 noise-exposed technicians. A spontaneous nystagmus was found in 18 persons, and 24 had a positional nystagmus exceeding a velocity of the slow phase of 5 degrees/s in three or more positions. In 17 subjects a cervical nystagmus could be provoked, while a nystagmus preponderance of more than 20% in the rotation test was found in seven persons. A difference in excitability between the labyrinths of more than 20% was shown by seven subjects. None of the subjects showed pathology in the tests for central vestibular disorders. The technicians were divided into four groups, according to the severity of their hearing loss. No correlation was found between the grade of the hearing loss and the vestibular function disturbance. This can be explained in terms of the adaptive properties of the vestibular system. All subjects showed pathology in one or more of the vestibular tests. The medico-legal aspects of vestibular involvement in noise-induced hearing loss can be of some importance. Hearing loss itself does not affect work capability directly; however, a vestibular disorder might well do so. In consequence, noise-exposed individuals could be disabled because of vertigo or balance disorder--an important and perhaps neglected aspect of noise-induced hearing damage.

Possibilities of improving the diagnosis of noise-induced hearing damage by means of directional audiometry, the dichotic speech discrimination test, and the EEG1.

Several audiometric tests were made of 85 noise workers exposed to an intensive impulse noise during pressing and punching. These tests aimed at finding out noise-induced central hearing damages (directional audiometry, dichotic speech discrimination test according to Feldmann and checking of the induction of the impedance jump). In addition, a clinical EEG was taken from each subject. Having obtained the necessary norms with series of tests in subjects with normal hearing abilities for their respective ages, the following results have been obtained: (1) directional hearing has hardly been affected by noise-induced hearing damage. It behaves like age changes in the group compared with it; (2) the dichotic speech discrimination test shows increasingly pathological values with progressive noise-induced hearing damage; this was already obvious in the age group 41-50. The impedance jump is less and less produced with progressive noise-induced hearing damage and finally disappears at a noise intensity of up to 110 dB. Because of these findings a progressive degeneration of the spiral ganglion and hearing nerve in noise-induced hearing damage is discussed as a cause of a deteriorated speech discrimination and the absence of the impedance jump. The EEG findings show changes in the alpha-rhythm in about 50% of the cases at an exposure time of 10 years onwards which may be possibly due to damage to the reticular formation, once increasing noise-induced hearing damage no longer exists.

An Exprimental Study on NITTS Growth Curve and NIPTS Occurence

Prevention of noise induced hearing damage is assuming great importance and this study was performed to uncover the relations between NITTS and NIPTS.NITTSs were examined after exposing cats to wide and narrow bands of noise for increasing durations. NITTS audiogram showed that hearing loss was greatest at the highest frequency of bands of noise at the shorter exposure, however, there was a progressive high frequency loss at the longer exposure.The growth curves of NITTSs were almost linear with logarithmic exposure time. However, the curves at 16kHz showed an accelerated rate for the narrow band noise. The growth rate was smaller at 1kHz than the ones at 2, 4, 8, and 16kHz.On the occurrence of NIPTS, the NITTS curve had a distinct acceleration at growth rate following a saturation or a slight acceleration. Curiously enough, NIPTS did not necessarily appear at the frequencies of the distinct acceleration.

A study of noise induced hearing damage risk for operators of farm and construction equipment : Society of Automotive Engineers, Inc.Abstr. in US Government Research and Development Reports (Report No PB-188633)

A study of noise induced hearing damage risk for operators of farm and construction equipment : Society of Automotive Engineers, Inc.Abstr. in US Government Research and Development Reports (Report No PB-188633)