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Abstract
Otoacoustic emissions (OAEs) are sounds generated by a healthy cochlea, as a by-product of the outer hair cell (OHC) activity, and therefore they can provide valuable information on OHC function. Hence, OAE recording techniques can be used for assessment of noise damage, as the most extensive noise-induced changes occur in the cochlea, involving the OHCs, which seem to be particularly vulnerable to noise. OAE recording techniques are simple, non-invasive, and they provide objective information and they are available in routine clinical practice. OAEs are characterized by a high sensitivity in detecting subtle changes of the OHCs and this property makes them very valuable in identifying early noise induced change, even before any notable shift in the audiometric thresholds. Although there is a high inter-individual variability, OAEs display remarkable intra-individual stability and, therefore, they are useful in intra-individual monitoring of subtle changes in cochlear integrity following noise exposure. In addition to the applications for assessment of structural damage, OAEs may also provide an insight into functional aspects of noise damage, which are less described in the literature. This includes the use of OAEs in objective evaluation of noise-induced tinnitus and hyperacusis, which often occur following noise exposure. Although the expectation that OAEs can provide objective evidence for tinnitus and hyperacusis is unrealistic, their recording may be a step towards evidence-based evaluation of tinnitus and hyperacusis, particularly in conjunction with other auditory tests. Additionally, OAEs can be applied in the evaluation of the medial olivocochlear (MOC) system that controls the OHCs through a direct neural input from the medial superior olivary complex. The relevance of the MOC system to noise damage is that the MOC system may be implicated in physiological protection mechanisms against noise and, therefore, the efficacy of the MOC can be considered a potential factor of inter-individual susceptibility to the damaging effect of noise. A dysfunction of the MOC system may also be of relevance in developing tinnitus.