Vibration-induced hearing loss: mechanical and physiological aspects.

Hypothesis: The sensorineural hearing loss (HL) after middle ear surgery has been explained by the noise generated by drilling, without considering the vibration generated by the burr.

Background: The role of temporal bone vibration in the etiology of the HL was evaluated.

Methods: An electromagnetic shaker was used to vibrate the bony external ear canal of guinea pigs at different frequencies ranging from 32 to 1,000 Hz and at intensities ranging from 4.2 to 18.8 m/s for 15 minutes. The hearing threshold was measured with auditory evoked responses. A total of 30 animals were tested.

Results: After vibration, 60% of the guinea pigs developed a threshold shift (TS) exceeding 10 dB at two frequencies, with average TS of 8.8 dB across all frequencies and animals. The exposure to vibration at higher frequencies (range, 500-1,000 Hz) produced stronger TS than did the exposure to lower frequencies (range, 32-250 Hz). The vibration-induced TS showed prominent recovery so that after 7 days, TS was 2.4 dB on average and 27 of 30 animals had recovered. After 14 days, the TS was 1.3 dB. The vibration excitation measurements showed that at lower frequencies, the vibration transmission into the skull was significantly greater than at higher frequencies, at which the transmission was heavily attenuated. There were no acoustic resonances detected in the skull. The frequency of vibration and the hearing frequency in auditory brainstem response were significant determinants in the model explaining the vulnerability of vibration on hearing. Hearing loss primarily occurred at higher frequencies. The HL was mostly reversible, consistent with the results observed after human temporal bone surgery.

Conclusion: We conclude that in the guinea pig model, the temporal bone vibration at higher frequencies produced a more severe HL than did the vibration at lower frequencies, although the vibration at higher frequencies caused less efficient transmission from the vibrating probe to the temporal bone. The guinea pig model may be useful in the development of surgical techniques and in the understanding of temporal bone pathology.