This study examines the long-term stability of otoacoustic emissions (OAEs), and we selected click-evoked otoacoustic emissions (CEOAEs) and distortion product otoacoustic emissions (DPOAEs) from several types of OAEs. We then compared these methods for the long-term monitoring of inner ear functions. Over a period of 8 weeks, we measured CEOAEs and DPOAEs in guinea pigs (n = 8) every week and analyzed the changes in the total echo power of CEOAE, the echo power of frequency bands of CEOAE and the DPOAE level (f2/f1 = 1.2). We considered as 'stable' parameters those that showed small changes or of which the p value was close to 1.0. Good p values of CEOAEs were obtained over the 2- to 3-kHz range; better p values of DPOAEs were obtained over the 2- to 8-kHz range. Thus, DPOAEs afforded the extended frequency information achievable with CEOAEs. According to our results, DPOAEs were superior to CEOAEs as parameters of long-term stability because they showed fewer changes in level at each frequency.
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