Occluding the round window causes no change in threshold,
but an increase in hearing loss following noise exposure
Ronen Perez*, Cahtia Adelman, Jean Yves Sichel*, Haim Sohmer§
Journal of Basisc & Clinical Physiology & Pharmacology, 2009,
Vol 20, No 3, 197-205
*Department of Otolarynglogy and Head & Neck Surgery, Shaare Zedek
Speech and Hearing Center, Hadassah University Hospital,
§Department of Physiology, Hebrew University - Hadassah Medical School,
Aim: To assess the effects of occluding the round window on the
degree of hearing loss following exposure to broad band noise.
Design: Following opening of the middle ear bulla in both ears
of ten sand rats, auditory nerve-brainstem evoked response (ABR) thresholds
were determined in each ear separately using an insert earphone. The round
window of one ear was then occluded with super-glue. The opposite
ear was sham-operated. ABR thresholds were again assessed immediately.
The animals were then exposed to 113 dB SPL broad band noise for 12 hours.
24 hours after the round window was occluded, which was 8-10 hours after
the end of the noise exposure, ABR thresholds were again determined in
each ear. In four control animals, the round window was blocked, but they
were not exposed to noise.
Results: Following the noise exposure, the mean ABR threshold
elevation in the round window blocked ears (54.5 ± 5.5 dB) was
significantly (p<0.004) greater than that in the sham-operated ears
(40.5 ± 8.6 dB). In the four control ears, there was no change
in ABR threshold 24 hours after the round window was occluded.
Conclusion: Occluding the round window was not accompanied by
a threshold elevation, but following noise exposure, the noise
induced hearing loss was increased, probably by reducing the efficacy
of an inherent protective mechanical mechanism. (Bold text emphasis
by Martin Braun)
Immobilization of the round window reduces, or even abolishes, a local
motion of the basilar membrane (BM) toward the scala timpani upon
sound exposure. The reason is that a volume shift in the scala timpani
is impeded, or even made impossible, if the round window membrane can
no longer bulge outwards. Therefore, a sound induced BM traveling wave
along the cochlea must necessarily be greatly reduced under this condition.
The findings that a reduction of the BM traveling wave leaves hearing
thresholds unchanged, but increases vulnerability against acoustic overload,
constitute a direct experimental confirmation of a completely
new theory of cochlear functions that was first published 16 years
ago. (Comment Martin Braun)
for a PDF of the full paper by Perez et al. (2009)
Confirmed by Perez et al. (2010)