Malini Subramaniam

Individual susceptibility to noise-induced hearing loss: an old topic revisited.

Abstract The wide range in susceptibility to noiseinduced hearing loss has intrigued researchers and hearing conservationists alike. Some of these differences in variability have been attributed to various intrinsic factors such as eye color, gender, age, etc. However, a review of controlled research shows that the influence of these intrinsic variables is relatively small and cannot explain the wide range of hearing loss observed in demographic studies. Fureherrnore, uncontrolled variables or unrecognized drug and noise interaction may obscure the relation between noise exposure and hearing loss. With the growing understanding of the physiology of the auditory system, new possibilities are emerging that may explain the range of susceptibility. A review of the role of acoustic reflex effectiveness, cochlear efferent function, and history of noise exposure provide a perspective for future strategies in predicting susceptibility to noiseinduced hearing loss.

IMPACT NOISE : THE IMPORTANCE OF LEVEL, DURATION, AND REPETITION RATE

The applicability of the equal energy hypothesis (EEH) to impact noise exposures was studied using chinchillas. Hearing thresholds were estimated by recording the evoked potentials from a chronic electrode implanted in the inferior colliculus. The animals were exposed to broadband impacts of 200-ms duration. The study was carried out in two parts. In experiment I, six exposure levels (107, 113, 119, 125, 131, and 137 dB SPL) and three repetition rates (4/s, 1/s and 1/4s) were employed. In the second experiment, the total duration of the exposure as well as the total energy were kept constant by trading level and rate. Results indicate that hearing loss resulting from exposure to impact noise does not conform to the predictions of the EEH. The permanent threshold shift as well as the hair cell loss are more or less equal across the lower peak exposure levels. However, both the hearing loss and the hair cell damage increase for exposures with higher peak levels. Furthermore, hearing loss and cochlear damage are dependent upon the rate of exposure. Thus the amount of hearing loss and hair cell damage appears to depend on the interaction of several factors including peak level, rate, and the susceptibility of the animal.

Changes in distortion product otoacoustic emissions and outer hair cells following interrupted noise exposures

Changes in distortion product otoacoustic emissions (DPOAEs) were examined during and after interrupted noise exposures and compared to the condition of the outer hair cells (OHCs) and inner hair cells (IHCs) as assessed by scanning electron microscopy (SEM). Binaural, adult chinchillas were exposed to a 95 dB SPL, octave band noise centered at 0.5 kHz for 15 days using a 3 h on/9 h off schedule. DPOAEs were measured before, during and after the exposures. DPOAE amplitudes decreased significantly during the first few days of the interrupted noise exposures and then began to recover. At most frequencies, the emission amplitudes recovered completely to pre-exposure baseline values by five days after the last exposure. The results of the present study indicate that the changes in DPOAE amplitude paralleled the recovery in the amplitude and threshold of the compound action potentials as reported previously (Boettcher et al., 1992). Although the DPOAEs completely recovered, considerable OHC loss and stereocilia disarray was evident even four weeks after exposure.

Effects of noise exposure, race, and years of service on hearing in U.S. Army soldiers

Over the last two decades the U.S. Army has instituted a comprehensive hearing conservation program (HCP) to reduce the prevalence of hearing loss in soldiers and civilian employees. As a component of this program, hearing threshold levels (HTLs) are stored in a central computerized Army-wide hearing conservation data registry. The aim of this study was to analyze the hearing threshold data to compare the hearing loss among soldiers representing different (1) race groups, (2) noise exposure groups, and (3) durations of military service. HTLs were age-corrected using data base B values from IS0 1999 (1990). As may be expected, soldiers exposed to high noise levels had significantly poorer hearing than the group of soldiers with limited noise exposure. On the average, results indicated a significant difference in HTLs among the race groups with black soldiers having the most sensitive hearing and white soldiers having the poorest. Also, subjects with greater durations of military service had the least sensitive hearing. Finally, race and years of service factors were found to interact in their effect on HTL. The findings are discussed in terms of implications of race differences, normative data, and effectiveness of the U.S. Army HCP.

Effect of high-frequency interrupted noise exposures on evoked-potential thresholds, distortion-product otoacoustic emissions, and outer hair cell loss.

The effect of high-frequency interrupted noise exposures on evoked potential (EP) thresholds, distortion-product otoacoustic emissions (DPOAEs), and status of the outer hair cells was studied with the aim of understanding the correspondence among the three measures. Animal subjects were exposed to an octave band noise centered at 4 kHz at 85 dB SPL for 6 hr/day for 10 days. EP and DPOAE recordings were made before the exposure and on days 1, 2, 4, 6, 8, and 10 of exposure. A final set of measurements were made 5 days after the last exposure, following which the animals were sacrificed and their cochleas were examined using scanning electron microscopy. Both EPs and DPOAEs showed a worsening of auditory function after the first exposure and then showed a progressive recovery toward baseline. However, there was no consistent relationship between changes in EP thresholds and changes in DPOAEs nor were there any systematic changes in outer hair cells that corresponded with the changes in DPOAEs. Furthermore, EP thresholds often revealed considerable deficits in function while DPOAEs were normal.

Advances in Our Understanding of Noise-Induced Hearing Loss

Abstract Noise-induced hearing loss (NIHL) is one of the major occupational hazards. Several standards are currently available that stipulate the safety limits for noise exposures. While these standards are generally effective, this review highlights results from recent laboratory studies that show that these standards share certain limitations. For instance, they do not cover impulse/impact noise exposures satisfactorily. Likewise, they do not have provisions to account for interaction of noise with other agents, such as chemotherapeutic agents or industrial solvents. In addition, results from studies on susceptibility to NIHL are presented which suggest that an individuals susceptibility to NIHL can be moderated by prior noise exposures. Areas of potential interest for future research on NIHL are also identified.

Effect of ‘‘conditioning’’ on hearing loss from military noise exposures

It has been shown with chinchillas as subjects that prophylactic (‘‘conditioning’’) exposures to an interrupted OBN centered at 0.5 kHz for 10 days provide protection against permanent threshold shift (PTS) from high level impulse noise in chinchillas [L. W. Henselman et al., Hear. Res. 78, 1–10 (1994)]. The present investigation studied the effect of ‘‘conditioning’’ with helicopter noise on PTS resulting from high level impulse noise. Thirty‐eight chinchillas were randomly assigned to one of four experimental groups (helicopter noise exposure prior to impulse noise exposure) or a control group (impulse noise exposure only). It was shown that (a) interrupted exposures over a 10‐day period to helicopter noise presented at 112 dB SPL for 1.5 h/day caused TTSs to decrease as exposure days continued at the test frequencies of 0.5 to 8 kHz, and (b) after 4 weeks of recovery, subjects were protected from PTS after ‘‘conditioning’’ with helicopter noise (except the group with prolonged ‘‘conditioning’’). Histol...

Correlation of distortion product otoacoustic emissions (DPOAE) with hearing loss produced in a ‘‘toughening’’ exposure

The relation between distortion product otoacoustic emissions (DPOAE) and hearing loss was studied using a ‘‘toughening’’ exposure. The subjects were monaural chinchillas [N=5]; DPOAE were measured using a Virtual 330 and hearing was estimated using evoked potentials recorded from a chronic electrode in the colliculus. Hearing thresholds and DPOAE were measured before any exposure began, then after each 6‐h exposure of 0.5 kHz OB at 95 dB SPL for 10 days and, finally, 5 days after the last exposure. Over the 10 days of exposure, the daily threshold shifts became progressively less. Initially, the DPOAE were depressed below the noise of the DPOAE system, but by days 8 to 10, the subjects had recovered to pre‐exposure levels even though there were still significant shifts in hearing thresholds. Five days post‐exposure, hearing levels were normal and DPOAE were normal or enhanced. Results will be discussed in terms of ‘‘toughening’’ induced by noise exposure. [Work supported by NIH 1 R01 DC01237‐01A11.]

Frequency differences in the development of protection against NIHL by low‐level “toughening” exposures

The aim of this study was to determine if low‐level intermittent exposures to a 0.5‐kHz OBN has a protective effect for future high‐level exposures (1) at the same frequency or (2) at a higher frequency (4‐kHz OBN). Four groups of monaural chinchillas were used as subjects. Thresholds were determined using evoked potentials recorded from a chronic electrode implanted in the inferior colliculus. In the first experiment, the experimental group was exposed to a low‐level noise (an OBN centered at 0.5 kHz at 95 dB for 6 h/day for 10 days). After a 5‐day recovery period when the thresholds returned to baseline, the animals were re‐exposed to the same noise at 106 dB for 48 h. A control group was exposed only to the higher‐level noise. The second experiment has an identical exposure schedule, except that the traumatic exposure involved a 4‐kHz OBN presented at 100 dB. The 10‐day “toughening” exposure reduced the amount of PTS in the low‐frequency experimental group by 20–30 dB over the range of 0.5–4 kHz. Conve...