Is the older ear more susceptible to noise damage?

Eight chinchillas aged 8.9 to 12.8 years were used to examine the effect of noise on the aging ear. The left malleus/incus complex was removed to produce a 50-dB conductive hearing loss which protected those ears from noise damage. The animals were then exposed for 36 days to an octave band of noise with a center frequency of 0.5 kHz and a sound pressure level of 95 dB. After 1 hour (n = 2) or 1 month (n = 6) of recovery, their cochleas were prepared for microscopic examination. The percentages of missing inner hair cells (IHCs) were 7.4 +/- 6.0% and 7.8 +/- 5.1% for their protected and exposed ears, respectively. Outer hair cell (OHC) loss was 12.8 +/- 8.7% and 20.6 +/- 7.8% for their protected and exposed ears, respectively. A paired sample Student's t test revealed that OHC loss was significantly greater (P = .003) in the older-exposed compared to the older-protected ears whereas IHC loss was not significantly different. For younger-exposed ears (i.e., 1 to 3 years), the percentages of missing IHCs and OHCs averaged 2.6 +/- 2.0% and 12.3 +/- 4.6%, respectively. When the aging-related cell loss was subtracted from total loss in the younger- and older-exposed ears, the noise-induced loss of sensory cells in the older ears was not significantly different from that in the younger ears. Therefore, it is concluded that older chinchilla ears are not more susceptible to noise damage than younger ears.