Temporary DPOAE level shifts, ABR threshold shifts and histopathological damage following below-critical-level noise exposures

Abstract

DPOAE temporary level shift (TLS) at 2 f 1 − f 2 and f 2 − f 1, ABR temporary threshold shift (TTS), and detailed histopathological findings were compared in three groups of chinchillas that were exposed for 24 h to an octave band of noise (OBN) centered at 4 kHz with a sound pressure level (SPL) of 80, 86 or 92 dB ( n = 3, 4, 6). DPOAE levels at 39 frequencies from f 1 = 0.3 to 16 kHz ( f 2/ f 1 = 1.23; L 2 and L 1 = 55, 65 and 75 dB, equal and differing by 10 dB) and ABR thresholds at 13 frequencies from 0.5 to 20 kHz were collected pre- and immediately post-exposure. The functional data were converted to pre- minus post-exposure shift and overlaid upon the cytocochleogram of cochlear damage using the frequency-place map for the chinchilla. The magnitude and frequency place of components in the 2 f 1− f 2 TLS patterns were determined and group averages for each OBN SPL and L 1, L 2 combination were calculated. The f 2 − f 1 TLS was also examined in ears with focal lesions equal to or greater than 0.4 mm. The 2 f 1 − f 2 TLS (plotted at f 1) and TTS aligned with the extent and location of damaged supporting cells. The TLS patterns over frequency had two features which were unexpected: (1) a peak at about a half octave above the center of the OBN with a valley just above and below it and (2) a peak (often showing enhancement) at the apical boundary of the supporting-cell damage. The magnitudes of the TLS and TTS generally increased with increasing SPL of the exposure. The peaks of the TLS and TTS, as well as the peaks and valleys of the TLS pattern moved apically as the SPL of the OBN was increased. However, there was little consistency in the pattern relations with differing L 1, L 2 combinations. In addition, neither the 2 f 1 − f 2 nor f 2 − f 1 TLS for any L 1, L 2 combination reliably detected focal lesions (100% OHC loss) from 0.4 to 1.2 mm in size. Often, the TLS went in the opposite direction from what would be expected at focal lesions. Recovery from TLS and TTS was also examined in seven animals. Both TLS and TTS recovered partially or completely, the magnitude depending upon exposure SPL.