Previously, we examined the relation between total energy in the noise exposure & percentage losses of outer (OHC) & inner (IHC) hair cells in the apical & basal halves of 607 chinchilla cochleae (Harding & Bohne, JASA 115, 2004). The animals had been exposed continuously to either a 4-kHz octave band of noise (OBN) at 47-108 dB SPL for 0.5 hr-36 days, or a 0.5-kHz OBN at 65-128 dB SPL for 3.5 hr-433 days. Interrupted exposures were also employed for both OBNs. Post-exposure recovery times ranged from 0-913 days. Cluster analysis was used to separate the data into 3 magnitudes of damage. The data were also separated into recovery times of 0 days (acute) & > 0 days (chronic). It was found that moderate-level, moderate-duration exposures produced OHC & IHC losses that were related to total energy, while hair-cell losses from high-level, short-duration exposures were not related to total energy. A substantial part of the hair-cell losses occurred in focal lesions (i.e., >/= 50% loss of IHCs, OHCs or both cell types over a distance of >/= 0.03 mm). This abstract describes, within the same 3 clusters, the apex-to-base distribution of 1820 lesions found in 468 of 660 noise-exposed cochleae. In these cochleae, Organ-of-Corti (OC) length in mm was converted to % distance from the apex. The data were analyzed to determine % distance from the apex and size (mm) of the lesions. In 55 of 140 non-noise-exposed OCs, there were 186 lesions, the characteristics of which were also determined. Focal lesions involved IHCs only, OHCs only or combined OHCs & IHCs. The OHC only and combined lesions were grouped for the analysis. The distributions of lesion location were tallied in 2 %-distance bins. In controls, focal lesions were uniformly distributed from apex to base & 70% of them were pure IHC lesions. In cochleae exposed to the 4-kHz OBN, lesions were distributed in the basal half of the OC. In cochleae exposed to the 0.5-kHz OBN, lesions occurred in both halves of the OC. With continuous exposures, 74% of the lesions were pure OHC or combined lesions. With interrupted exposures, 48% of the lesions were pure IHC lesions. Lesion size was larger in the chronic compared to acute cochleae with similar exposures. There was a minimum total energy at which focal lesions began to appear & slightly higher energies resulted in nearly all exposed cochleae having focal lesions.
Abstracts of the 31st Midwinter Research Meeting of the Association for Research in Otolaryngology, Febuary 16-21, 2008, Phoenix, Arizona