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Correlation of cochlear pathology with ABR threshold shift and DPOAE level shift following exposure to low-frequency noise.

Snow-D; Harding-GW; Bohne-BA
Abstr 28th Midwinter Res Meet 2005 Feb; 28:202
ABR thresholds & DPOAE levels were determined before & 1-3 times after a 24-h exposure to a 0.5-kHz OBN at 95 dB SPL. At termination, the EP was recorded in the 1st turn & carbon tracer injected into the endolymphatic space (ES). After 45 min, the cochlea was fixed with OsO4, plastic-embedded & dissected as a flat preparation. Quantitative data on hair-cell losses were collected. Cytocochleograms were prepared with ABR threshold shifts (TS) & DPOAE level shifts (LS) overlaid. Based on the functional results, organ-of-Corti (OC) segments in the 2nd & 1st turns were sectioned at a radial angle for light & TEM study. Four chinchillas were terminated by 3 h post-exposure (0-d); two were terminated at 1 wk. At 0 d, the animals had a TTS of 20-60 dB over 0.5-10 kHz & a TLS of 10-50 dB over 2-10 kHz. EP averaged 84 mV, similar to controls. OHC loss ranged from 2.2-10.5% & 0.8-2.8% in the apical & basal halves of the OC, respectively. IHC loss was negligible throughout the OC. Viewed by phase contrast microscopy, most missing OHCs were found to have been replaced by immature phalangeal scars & cellular debris was seen in the ES & Nuel spaces. Most remaining hair cells had normal shapes & stereocilia arrays. Hair-cell loss & damage were insufficient to account for the TTS & TLS. Over the extent of the TTS & TLS, the nerve fibers & endings below the IHCs were not damaged, while the outer pillar bodies were either non-parallel or buckled. By 1 wk, the TS & LS had improved considerably; EP averaged mV. The pillars were buckled over a smaller linear distance in the OC that was aligned with the frequency range of the residual TS & LS. Immature scars were still visible in the reticular lamina but the amount of cellular debris was decreased. We conclude that in the chinchilla, the most consistent pathological correlates of noise-induced TTS & TLS are an accumulation of cellular debris in cochlear fluids & injury to supporting cells that uncouples the stereocilia from the tectorial membrane.
Noise-exposure; Exposure-levels; Noise-induced-hearing-loss; Noise-exposure; Noise; Hearing; Hearing-disorders; Hearing-loss; Laboratory-animals; Animals; Cell-damage; Cellular-reactions
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Abstracts of the 28th Midwinter Research Meeting
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Washington University, St. Louis