Effect of noise on the vestibular system - Vestibular evoked potential studies in rats

Studies have shown that in order for sound to affect the vestibular end organs in the inner ear, very high intensities are required. Furthermore, in patients with noise induced hearing loss, vestibular signs, if present, are subclinical. In order to study possible auditory-vestibular interactions in a more controlled fashion, using physiological sound intensities, the present study used short latency vestibular evoked potentials (VsEPs) to impulses of angular (15,000 degrees /sec(2), risetime 1.5 msec) and linear (3-5 g, risetime 1.5 msec) acceleration were used to study the possible effects of sound on peripheral vestibular function in rats. Four different paradigms were used: a - an intense (135 dB pe SPL) click stimulus was presented 5 msec before the linear acceleration impulse and the VsEP to 128 stimuli were recorded with and without this click stimulus. There was no effect of the preceding intense click on the first wave (reflecting end organ activity) of the linear VsEP. b - 113 dB SPL white noise "masking" was presented while the VsEPs were elicited. A 10-20% reduction in the amplitude of the first VsEP wave was seen during the noise exposure, but 5 minutes after this exposure, there was almost complete recovery to pre-exposure amplitude. c - 113 dB SPL noise was presented for one hour and VsEPs were recorded within 15 minutes of cessation of the noise. The auditory nerve-brainstem-evoked response showed a temporary threshold shift while there was no effect on the VsEP. d - 113 dB SPL white noise was presented for 12 hours per day for 21 consecutive days. Auditory nerve-brainstem-evoked responses and vestibular (VsEPs) function were studied one week after the conclusion of the noise exposure. Auditory function was severely permanently depressed (40 dB threshold elevation and clear histological damage) while the amplitude of wave 1 of the VsEP was not affected. It seems therefore that even though intense noise clearly affects the cochlea and may have a "masking" effect on the vestibular end organs, the intensities used in this study (113 dB SPL) are not able to produce a long-term noise induced vestibular disorder in the initially normal ear. These differences between the response of the cochlear and vestibular end organs to noise may be due to dissimilarities in their acoustic impedances and/or their electrical resting potential.