[Effect of inner ear hearing loss on delayed otoacoustic emissions (TEOAE) and distortion products (DPOAE)].

BACKGROUND

The assessment of inner ear hearing loss by means of evoked otoacoustic emissions (EOAE) is already established in practice. Nevertheless, empirical data on the relations between emission and audiologic parameters are required.

PATIENTS AND METHODS

EOAEs werde measured and analysed in 240 ears with sensorineural hearing loss (excluding cases with conductive and retrocochlear disorders) of 120 patients using ILO88/92 equipment with standard test conditions. Audiologic examination consisted of pure tone audiogram, tympanometry and auditory brainstem responses. The results of TEOAE (Transitory Evoked Otoacoustic Emissions), measured with clicks at 80 dB SPL, and DPOAE (Distortion Product Otoacoustic Emissions), measured with frequencies ranging from f2 = 1 kHz to 4 kHz at 70 dB SPL (L2 = L1, f2 = 1.2 f1), were compared to each other and to the hearing thresholds determined by subjective pure tone audiometry.

RESULTS

A significant negative correlation is found between the amplitude of TEOAEs and the average hearing loss in the range of 0.5 to 4 kHz. Similarly, the amplitude of DPOAEs is significantly correlated to the hearing threshold encountered at the higher of the two stimulus frequencies. Nevertheless, the prediction of hearing loss from the response amplitude is not possible because 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 subjective thresholds in the range 0.5 to 4 kHz) no TEOAEs can be registered. For DPOAEs, the responses disappear if the hearing loss at the higher stimulus frequency exceeds 47 +/- 3 dB (limits for 50% OAE incidence are given in both cases). In conclusion, the hearing thresholds of ears 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. Therefore, the combination of TEOAE and DPOAE recording with the purpose of hearing threshold assessment is especially useful in young patients. Further evaluation shows that male and female patients do not differ in the hearing loss dependent prevalence of otoacoustic emissions. On he other hand, a systematic but not significant influence of the audiogram shape on the parameters of the discrimination function can be observed: steep audiograms are described with a discrimination function, which drops to zero at higher levels of hearing loss and with a larger slope than shallow audiograms. The comparison of right and left ear emissions of the same patient shows that large amplitude differences occur even in cases of comparable subjective thresholds; missing emissions in one ear are observed if and only if the interaural threshold difference exceeds 30 dB.