THE INFLUENCE OF INNER-EAR HEARING-LOSS O N TRANSITORY EVOKED AND DISTORTION-PRODUCT OTOACOUSTIC EMISSIONS

Background: The assessment of inner ear hearing loss by means of evoke d otoacoustic emissions (EOAE) is already established in practice. Nev ertheless, empirical data on the relations between emission and audiol ogic parameters are required. Patients and Methods: EOAEs werde measur ed and analysed in 240 ears with sensorineural hearing loss (excluding cases with conductive and retrocochlear disorders) of 120 patients us ing ILO88/92 equipment with standard test conditions. Audiologic exami nation consisted of pure tone audiogram, tympanometry and auditory bra instem responses. The results of TEOAE (Transitory Evoked Otoacoustic Emissions), measured with clicks at 80 dB SPL, and DPOAE (Distortion P roduct Otoacoustic Emissions), measured with frequencies ranging from f(2) = 1 kHz to 4 kHz at 70 dB SPL (L(2) = L(1), f(2) = 1.2 . f(1)), w ere compared to each other and to the hearing thresholds determined by subjective pure tone audiometry. Results: A significant negative corr elation is found between the amplitude of TEOAEs and the average heari ng loss in the range of 0.5 to 4 kHz. Similarly, the amplitude of DPOA Es is significantly correlated to the hearing threshold encountered at the higher of the two stimulus frequencies. Nevertheless, the predict ion of hearing loss from the response amplitude is not possible becaus e of its large amplitude variations between individuals. The analysis of the relation between incidence of EOAEs and hearing loss shows that in ears exceeding a hearing loss of 34+/-4 dB (minimal value of subje ctive thresholds in the range 0.5 to 4 kHz) no TEOAEs can be registere d. For DPOAEs, the responses disappear if the hearing loss at the high er stimulus frequency exceeds 47 +/- 3 dB (limits for 50% OAE incidenc e are given in both cases). In conclusion, the hearing thresholds of e ars exhibiting DPOAEs but no TEOAEs are located between approximately 30 and 50 dB HL. Conclusions: The sharpness of the transition between clear responses and absent responses decreases with increasing age. Th rerefore, the combination of TEOAE and DPOAE recording with the purpos e of hearing threshold assessment is especially useful in young patien ts. Further evaluation shows that male and female patients do not diff er in the hearing loss dependent prevalence of otoacoustic emissions. On he other hand, a systematic but not significant influence of the au diogram shape on the parameters of the discrimination function can be observed: steep audiograms are described with a discrimination functio n, which drops to zero at higher levels of hearing loss and with a lar ger slope than shallow audiograms. The comparison of right and left ea r emissions of the same patient shows that large amplitude differences occur even in cases of comparable subjective thresholds; missing emis sions in one ear are observed if and only if the interaural threshold difference exceeds 30 dB.