Absence of otoacoustic emissions in subjects with normal audiometric thresholds implies exposure to noise

Otoacoustic emissions and contra-lateral sound activated efferent suppression of emissions were examined to determine whether they provide any early indication of auditory damage from exposure to noise. Three groups were studied: noise exposed workers (n=50, mean age 42 years), patients with Meniere's disease (n=24, mean age 48 years) and normal subjects (n=24, mean age 41 years). All subjects underwent routine pure tone audiometry, tympanometry and otoacoustic emission testing. As a number of studies have shown that with hearing threshold better than 30 dB HL, emissions are almost always present and are generally absent with hearing loss greater than 30 dB HL, subjects in this study were sub grouped into these two categories in order to examine the incidence of emissions. Absence of emissions in subjects with mean hearing thresholds better than 30 dB HL varied from 0% in normal controls, 8% in patients with Meniere's disease and a significantly high 56% in noise exposed workers despite similar mean hearing thresholds for all groups. The mean transient emission levels for the noise exposed workers was significantly lower than the controls and Meniere's groups. This study clearly indicates that in the noise-exposed group there is sub clinical and sub audiometric damage to the outer hair cells responsible for generation of otoacoustic emissions. Of those with normal otoacoustic emissions, the efferent suppression was absent in 60% of noise exposed workers but in only 3.8% of control subjects implying that the efferent control may also be affected in a significant proportion despite normal hearing thresholds and emissions.