IS THE OLDER EAR MORE SUSCEPTIBLE TO NOISE DAMAGE

Eight chinchillas aged 8.9 to 12.8 years were used to examine the effe ct of noise on the aging ear. The left malleus/incus complex was remov ed to produce a 50-dB conductive hearing loss which protected those ea rs 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 pr essure level of 95 dB. After 1 hour (n=2) or 1 month (n=6) of recovery , their cochleas were prepared for microscopic examination. The percen tages of missing inner hair cells (IHCs) were 7.4 +/- 0.0% and 7.8 +/- 5.1% for their protected and exposed ears, respectively. Outer hair c ell (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 reve aled that OHC loss was significantly greater (P=.003) in the older-exp osed compared to the older-protected ears whereas IHC loss was not sig nificantly different. For younger-exposed ears (i.e., 1 to 3 years), t he percentages of missing MCs and OHCs averaged 2.6 +/- 2.0% and 12.3 +/- 4.6%, respectively. When the aging-related cell loss was subtracte d from total loss in the younger- and older-exposed ears, the noise-in duced loss of sensory cells in the older ears was not significantly di fferent from that in the younger ears. Therefore, it is concluded that older chinchilla ears are not more susceptible to noise damage than y ounger ears.