Distortion-product otoacoustic emission spectra and high-resolution audiometry in noise-induced hearing loss

dDistortion product otoacoustic emissions (DPOAE) elicited by 60dB SPL pure tones at A and f2 were collected at 2f1-f2, in 1/10th octave steps, in a sample of 36 ears from 27 patients with noise-induced hearing loss (NIHL). They were analyzed in the frequency domain against the outcome of high-resolution pure-tone audiometry, performed with the help of a Bekesy sweep-frequency automatic audiometer. The characteristics of DPOAE level plots as a function of frequency (the so-called DP-grams), relative to the DPOAE levels of a control age-matched group, were compared to their alleged counterparts on the audiograms, i.e., the lower and upper frequency boundaries of the interval with hearing loss. Ears with NIHL split into two subgroups, one (n = 25) with a notch in the DP-gram such that its lower boundary matched the lower limit of the audiometric notch (linear regression with a slope of 0.91, r(2) = 0.644, p < 0.001). Likewise, when it existed, its upper boundary matched its upper counterpart on the audiogram (linear regression with a slope of 0.96, r(2) = 0.89, P < 0.001). In this respect, DP-grams performed better than transient-evoked OAE spectra, which exhibited poor correlations with audiogram patterns. The second subgroup (n = 11) exhibited normal DPOAEs at all frequencies despite audionnetric losses similar to those of the first subgroup. In all cases, DPOAE levels were poor predictors of the degree of hearing losses. It is hypothesized that NIHL in the second subgroup involves inner hair cells or auditory neurons, instead of outer hair cells in the first subgroup. Provided NIHL affected outer hair cells, DP-grams provided a comparatively accurate predictor of the spectral extent of hearing loss. (c) 2005 Elsevier B.V. All rights reserved.