Effects of shift work and intermittent noise exposure on hearing: mechanisms and prophylactic potential.

It is well established that intermittent noise exposure characteristically produces less hearing loss than equal energy/intensity continuous noise in animal models. Ongoing different shift work regimes open for direct studies on hearing effects of intermittent noise exposure in man without ethical concern. Amazingly, few such studies are reported. In one recent study in the present volume, noise-exposed employees working 12 hours a day for two consecutive days followed by two days off, the cycle then repeated, had significantly lower permanent hearing loss than employees working nine-hour shifts from 8 am to 5 pm Monday to Friday. This commentary refers to the few studies reported, gives a short overview of the mechanisms behind noise-induced hearing loss and the protective effect of intermittent exposure, and concludes that direct studies in man on the effects of different shift work regimes on occupational hearing loss under specified noise conditions represent a prophylactic potential that calls for increased research activity. Such studies might pave the way for direct use of more optimal intermittent noise exposure regimes in future design of the noise exposure workday/-week and make future hearing conservation programs more effective.

Hearing loss induced by occupational and impulse noise: results on threshold shifts by frequencies, age and gender from the Nord-Trøndelag Hearing Loss Study.

The aim of the study was to compare the frequency-specific effects of noise on hearing acuity across the range 250-8000 Hz and the extent to which the patterns of frequency-specific threshold shifts differ between occupational noise and impulse noise. Pure-tone audiometry was administered to an adult general population sample with 51 975 subjects who also provided questionnaire information about noise exposure and other risk factors. Threshold shifts induced by life-long occupational noise and impulse noise (mostly shooting) were estimated separately in six age and sex groups for eight frequencies. Reported noise exposure, as well as observed threshold shifts, were moderate among women. Threshold shifts averaged over both ears among subjects in the higher 2% of exposure to occupational noise, reached 13 dB (3000 Hz, age 65 years + ) among men and were generally largest at 3000-4000 Hz. The shifts induced by impulse noise reached approximately 8 dB among men 45-64 years and men 65 years+. The effects of impulse noise were strongest at 3000-8000 Hz and varied little within this frequency range.

Screened and unscreened hearing threshold levels for the adult population: results from the Nord-Trøndelag Hearing Loss Study.

This paper presents normative data of hearing threshold levels of a population screened with various criteria, as compared to unscreened population data. Computer-controlled pure-tone audiometry was administered to the adult population in Nord-Trøndelag County, Norway, during 1995-1997. The 51975 participants also provided questionnaire information about occupational and leisure noise exposure, previous ear infections, and head injury. While screening had little effect on the median hearing threshold levels of young adults, there was a substantial effect when screening men above 40 years of age for a history of noise exposure. Screening for known ear-related disorders and diseases resulted in small effects on the mean hearing threshold levels The median hearing thresholds of both the screened and the unscreened sample exceeded the age and sex specific thresholds specified by the ISO 7029.

Otoacoustic emissions in the general adult population of Nord-Trøndelag, Norway: III. Relationships with pure-tone hearing thresholds.

This study aims to describe the association between otoacoustic emissions (OAEs) and pure-tone hearing thresholds (PTTs) in an unscreened adult population (N =6415), to determine the efficiency by which TEOAEs and DPOAEs can identify ears with elevated PTTs, and to investigate whether a combination of DPOAE and TEOAE responses improves this performance. Associations were examined by linear regression analysis and ANOVA. Test performance was assessed by receiver operator characteristic (ROC) curves. The relation between OAEs and PTTs appeared curvilinear with a moderate degree of non-linearity. Combining DPOAEs and TEOAEs improved performance. Test performance depended on the cut-off thresholds defining elevated PTTs with optimal values between 25 and 45 dB HL, depending on frequency and type of OAE measure. The unique constitution of the present large sample, which reflects the general adult population, makes these results applicable to population-based studies and screening programs.

Hearing loss associated with ear infections in Nord-Trøndelag, Norway.

OBJECTIVE: We have assessed the effect of recurrent childhood ear infections on adult hearing. We also examined whether adult hearing is poorer with an early age of onset of ear infections compared with later onset. DESIGN: A population-based cohort of 50,398 subjects, 20 yr of age or older, were examined with air-conduction, pure-tone audiometry and reported if they had had recurrent ear infections (EI) in childhood (or subsequently) with the age of onset. RESULTS: There were poorer hearing thresholds associated with recurrent EI for all frequency ranges from 0.25 kHz to 8 kHz, regardless of age or gender. The effect increased with age from approximately 2 dB among younger subjects (20 to 44 yr old) to approximately 5 to 6 dB among older subjects (65 yr or older). Among younger subjects, the effect of EI was somewhat stronger for men, whereas among older subjects the effect was somewhat stronger for women. Early age of onset for EI was associated with poorer hearing thresholds than late onset. The mean loss was close to 9 to 10 dB for all frequency ranges among older subjects reporting onset of EI before 2 yr of age. The mean loss was only about 4 dB among older subjects reporting onset of EI after 7 yr of age. CONCLUSIONS: Reported EIs are associated with similar consequences in terms of reduced hearing across frequencies 0.25 to 8 kHz for male and female subjects. The observed effects are stronger among older than among younger subjects, perhaps because effects increase with age or possibly because EI affected the hearing more before 1940 than during subsequent decades. Reported early age of onset of EI increases the risk of a substantially reduced hearing level later in life.

Selective posterior cerebral artery amobarbital test: its role in presurgical memory assessment in temporal lobe epilepsy.

PURPOSE: To evaluate the efficacy and risk of complications of selective posterior cerebral artery (PCA) amobarbital anesthesia in memory assessment of patients with epilepsy under consideration for temporal lobe resection. METHODS: Thirty-two candidates for temporal lobectomy in whom conclusive memory assessment could not be obtained by the standard intracarotid amobarbital procedure were submitted to a selective PCA amobarbital test. A mean dose of 75 mg amobarbital was injected via microcatheter into the P2 segment of the PCA. Ten common objects were presented for naming and remembering while the anesthesia was judged efficient. After return to neurologic baseline, recall and recognition memory were assessed. RESULTS: In all of the 32 patients, angiography and PCA anesthesia were successfully accomplished without serious adverse events. All but one of the patients remained alert and cooperative for memory testing under the anesthesia, and 28 of these patients showed adequate memory capacity of the hemisphere contralateral to the side targeted for surgery. So far, 19 patients have proceeded to surgery, and no case of global amnesia or serious, material-specific memory impairment has resulted. Three patients failed the PCA test (fewer than 67% items correctly recognized) and were excluded from surgery, partly on the basis of the PCA test results, but also supported by an overall evaluation of all the diagnostic procedures used. CONCLUSIONS: The selective PCA amobarbital test appears justifiable when performed by interventional neuroradiologists and may significantly reduce the risk of erroneously excluding patients with epilepsy from temporal resection. Further corroboration of the safety of the procedure seems warranted.

Hearing loss induced by noise, ear infections, and head injuries: results from the Nord-Trøndelag Hearing Loss Study.

Pure-tone audiometry was administered to the adult population in Nord-Trøndelag County, Norway, 1995-97. The 51,975 participants also provided questionnaire information about occupational and leisure noise exposure, previous ear infections, and head injury. Values averaged over both ears were calculated for low (250 and 500 Hz), middle (1 and 2 kHz) and high frequencies (3, 4, 6 and 8 kHz). The aim was to estimate the magnitude of hearing loss associated with various types of exposure by age and sex. Noise, ear infections and head injury explained 1-6% of the variance in hearing loss (varying with age, sex, and frequency range), in addition to what could be explained by age alone (30-58%). Only moderate effects of noise could be detected among women. The upper tenth percentile regarding occupational noise among men older than 44 years had on average an 8-9-dB high-frequency loss, adjusted for other predictors. Exposure to impulse noise (hunting, sports shooting) caused a 7-8-dB high-frequency loss in the same group. No significant effects of frequent use of personal stereo players or regular attendance at discotheques or rock concerts could be demonstrated. There were clear effects of recurrent ear infections and head injury.