DISTORTION-PRODUCT OTOACOUSTIC EMISSIONS IN MIDDLE-AGED SUBJECTS WITHNORMAL VERSUS POTENTIALLY PRESBYACUSIC HIGH-FREQUENCY HEARING-LOSS

Distortion-product otoacoustic emissions (DPOAEs) are still undergoing evaluation for clinical use. Although the effects of ageing on otoaco ustic emissions have been studied quite extensively in the past, DPOAE response-growth or input-output (I-O) measures, which are well suited as an objective method for monitoring cochlear function at specific f requencies, have been less thoroughly examined. The aim of the present study was to assess the 2f1-f2 DPOAEs in a clinical setting in order to examine the response of 20 normally hearing middle-aged adults and to compare the results with those of 20 people of the same age with ea rs of sensorineural high-frequency hearing loss (HL). The experiment c onsisted of two stages. First, the DPOAE-gram was recorded in 1-4-octa ve steps at a stimulus level of 70 dB SPL over a frequency range of th e f2 primary tone which extended from 1.001 to 6.299 kHz. Secondly, in order to elicit DPOAE I-O functions, the two primary stimuli were pre sented at equilevel intensities ranging from 20 to 71 dB SPL. The stim ulus-level step size was 3 dB. The I-O functions were recorded at five separate DPOAE frequencies, with the f2 frequency most closely relate d to the clinical audiogram (f2=1.0, 1.5, 2.0, 4.0 and 6.0 kHz). Two c learly separated portions in the form of the I-O function for normally hearing ears were found. The first portion, in response to primary le vels of 60 dB SPL and below, showed a plateau (saturating) behaviour. If primary levels exceeded 60 dB SPL, I-O functions became more linear . The attenuation of the saturation portion of the I-O function in ear s with high-frequency HL across the frequency-test range is difficult to explain because elevated behavioural thresholds were observed only for frequencies >1.5 kHz. Thus, the more linear I-Os associated with t he hearing-loss frequencies may indicate deficiencies in the active pr operties of outer hair cells (OHCs), whereas those for I-Os <1.5 kHz, where hearing was normal, may indicate a beginning of damage to active OHC micromechanical processes prior to their clinical manifestation. DPOAE recordings from people with high-frequency HL, possibly age-rela ted, supplement recordings of TEOAEs and give complementary informatio n on degenerative changes in the outer hair-cells. DPOAE I-O functions may reveal discrete pathological alterations both in the active cochl ear signal processing and in the passive mechanisms of the cochlea pri or to their detection by clinical audiometric tests.