DISTORTION-PRODUCT OTOACOUSTIC EMISSIONS IN NORMAL-HEARING INDIVIDUALS AND PATIENTS WITH SENSORINEURAL HEARING LOSSES

Based on a variety of experimental and clinical studies the diagnostic significance of transient evoked otoacoustic emissions (TEOAEs) is well established. This is also due to the availability of commercial hardware and software which is optimized for the recording of click-evoked OAEs. In cases with distortion product otoacoustic emissions (DPOAEs) the significance for clinical purposes is not yet clear. This study presents results obtained in normal hearing subjects and patients with sensorineural hearing losses (214 ears). The study focuses on three problems: 1. Incidence of DPOAEs in different groups of hearing loss. 2. Level of DPOAEs at various frequencies in each group of hearing loss. 3. Frequency-specific relationships between hearing loss and DPOAEs. Recording of DPOAEs was performed with the ILO92 system and standard configuration (primary tone level, 70 dB SPL). In normal hearing ears DPOAEs were observed at all frequencies above 1 kHz. In cases with moderate hearing loss the detection rate decreased to about 80%. With further increases in hearing loss the detection rate and level of DPOAEs decreased. At frequencies below 1500 Hz even in normal hearing subjects, the level of DPOAEs only slightly exceeded the noise level. The upper limit of hearing loss for which DPOAEs could be observed depended on frequency. In the low-frequency range DPOAEs were absent in ears with a sensorineural hearing loss greater than 35-45 dB; at frequencies between 4 and 6 kHz DPOAEs were observed even in ears with a hearing loss 65 dB. Using standard configuration of this commercially available system we conclude the following: 1. The correlation between DPOAEs (incidence and amplitude) and pure tone audiogram was best in the frequency range between 1500 and 4000 Hz. 2, The lack of DPOAEs at frequencies below 1 kHz does not indicate a hearing deficit. Therefore, the value of recording DPOAEs at these frequencies should be questioned, 3. The presence of DPOAEs at frequencies between 4 and 6 kHz does not exclude a significant hearing loss at these frequencies,