Distortion product of otoacoustic emissions as a sensitive indicator of hearing loss in pilots.

pilots. Aviat Space Environ Med 2004; 75:46-8. Background: When the ear is presented with two simultaneous pure tones, the cochlea produces intermodulation products called distortion product otoacoustic emissions (DP-OAEs). When cochlear function is affected by noise or ototoxic drugs, DP-OAEs decrease in amplitude or disappear. The aim of this study was to investigate whether DP-OAEs are a sensitive indicator of hearing loss in pilots, for whom flying time presumably correlates with exposure to steady noise. Methods: Subjects were 105 pilots, ages 25-44 yr, with flying times of 500-3500 hours. Pilots were divided into three groups according to their flying time and age-matched against control subjects (non-pilots) with normal hearing with no history of occupational exposure to noise. All subjects were tested by pure-tone audiometry, tympanometry, and measurement of DP-OAEs. Results: DP-OAEs were significantly different among the three pilot groups, decreasing gradually at 8 frequencies with flight time. For high frequency tones (F 2 7966 and 5623 Hz), age and noise were cofactors. However, 11% of the pilots in this study showed resistance to noise-induced hearing loss. Discussion: DP-OAEs are a sensitive measure for monitoring early noise-induced hearing loss in pilots. Accumulating flight hours, presumed to involve steady noise exposure, initially affected high-frequency hearing, then progressed to affect middle and low frequency hearing with increased flight time. There is individual variability in susceptibility and resistance to noise-induced hearing loss in humans.