The effects of linear acceleration on distortion product otoacoustic emissions in human ears.

Background: The effect of high levels of linear acceleration (G) on the cochlea has never been studied prospectively.

Hypothesis: Linear acceleration at high levels has no effect on the human cochlea as demonstrated by a comparison of pre- and post-exposure measurements.

Methods: There were 22 healthy volunteers who underwent exposure to up to 9 G in a military aviation training centrifuge. Prior to exposure they were screened for cochleovestibular disorders and underwent tympanometry, audiometry and distortion product otoacoustic emissions testing (DPOAE). Immediately after exposure, they underwent serial testing of each of these parameters until they returned to baseline.

Results: There was no significant change in tympanometry in any subject. Audiometry revealed a temporary threshold shift of 30 db at 6 kHz in one ear of a single subject. This was accompanied by a complete loss of DPOAE at the same frequency. DPOAE did not return to baseline at 2 weeks even though the audiogram had reverted to baseline by 8 d. Four other ears displayed significant losses of emissions at single frequencies without an accompanying change on the audiogram.

Conclusions: This study demonstrates that accelerative stress may cause transient injury to the cochlea. The mechanism of injury due to acceleration is probably ischemia, although a purely mechanical effect on the outer hair cells cannot be precluded. These data also reinforce a growing body of evidence that demonstrates the greater sensitivity of DPOAE over psychoacoustic testing in detecting early or subclinical cochlear damage.