PP310 - Extended High-Frequency Bone Conduction Audiometry

Abstract:
This study examines the reliability of extended high-frequency bone conduction (BC) audiometry at 6 and 8 kHz using a clinically available audiometer. Thirty-three young-adult participants underwent air and bone conduction testing for 0.25-8 kHz. Tympanometry with 0.226 and 1 kHz tones, wideband acoustic immittance (0.226-8 kHz), and distortion product otoacoustic emissions (0.5-10 kHz) were also measured. Results showed that BC audiometry at 6 and 8 kHz produced thresholds that were on average within 10 dB of air conduction thresholds and that were consistent with wideband immittance assessments. Findings support the inclusion of extended high-frequency BC testing in comprehensive audiometry.

Summary:
Rationale
This project investigates the reliability of high-frequency bone conduction (BC) audiometry at 6 and 8 kHz. High frequency hearing loss, often resulting from noise exposure, ototoxic medications, and aging, is generally assumed to be sensorineural in nature beyond the upper limit of 4 kHz in standard BC audiometry protocols. Extending BC testing to 6 and 8 kHz has the potential of allowing differentiation of the type of hearing loss in this range.

Design
This study involved 33 young-adult participants with normal hearing thresholds. Testing included air conduction (AC) audiometry followed by BC audiometry from 0.250 to 8 kHz conducted in four conditions: forehead un-occluded, forehead occluded, mastoid un-occluded, and mastoid occluded with a commercially-available audiometer and bone oscillator. Additional measurements, including wideband acoustic immittance (WAI; 0.226-8 kHz ) and distortion product otoacoustic emissions (DPOAEs; 0.5-10 kHz), were used to assess middle-ear status and cochlear outer hair cell function, respectively. The order of testing conditions was randomized to reduce order effects and descriptive statistics were used to analyze AC and BC thresholds, air-bone gaps, and the influence of oscillator placement location and occlusion on these measurements.

Results
Average BC thresholds at 6 and 8 kHz were below 10 dB HL in the mastoid un-occluded condition, with air-bone gaps less than or equal to 10 dB. Across all other test conditions, mean air-bone gaps were less than 10 dB on average. There was no significant difference between average occluded BC and un-occluded BC thresholds at 6 and 8 kHz, but there was a significant difference at low frequencies as expected due to the well-documented occlusion effect. The average WAI values were within normal range except for frequencies lower than 500 Hz which were slightly above the normative range. The average DPOAE levels were 6 dB or more above the average noise floor levels. The absolute magnitude of DPOAEs trended downward for test frequencies above 4 kHz.

Conclusions
This study provides evidence in support of the clinical reliability of BC audiometry at 6 and 8 kHz with commercially-available audiometers. Occlusion nor bone vibrator placement significantly affected the BC thresholds at 6 and 8 kHz. By including these frequencies in routine BC audiometry, clinicians will be able to extend the range of differential diagnosis of high-frequency hearing loss. Extending bone conduction testing to 6 and 8 kHz and beyond could be a particularly useful approach to directly assess high-frequency hearing sensitivity for individuals with compromised middle ear function secondary to chemotherapy, radiation, and/or surgical treatment for head and neck cancer. Future research will involve participants with hearing loss to further validate BC audiometry and establish the output limits of BC oscillators.