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Abstract
Age-related hearing loss (presbycusis) is a multifactorial process that results chiefly
from the accumulating effects of noise damage and aging on the cochlea. Noise damage
is typically evidenced clinically by a discrete elevation (notch) of the auditory
thresholds in the 3-6 kHz region of the audiogram whereas aging affects the highest
frequencies first. To determine whether the presence of such high-frequency notches
influences auditory aging, we examined the 15 year change in audiometric thresholds
in 203 men from the Framingham Heart Study cohort. The mean age at the first hearing
test was 64 years (range 58-80). Occupational and recreational noise exposure over
the 15 years was assumed to be minimal due to the age of the subjects. The presence
or absence of a notch was determined using a piecewise linear/parabolic curve fitting
strategy. A discrete elevation of the pure-tone thresholds of 15-34 dB in the 3-6
kHz region was deemed a small notch (N1), and elevations of 35 dB or greater were
deemed large notches (N2). Absence of a notch (N0) was encoded those ears with <15
dB elevation in the 3-6 kHz region. The presence and absence of notches correlated
with the subjects' history of noise exposure. The 15 year pattern of change in age-adjusted
pure-tone thresholds varied significantly by notch category. There was less change
over time in the notch frequencies (3-6 kHz) and significantly greater change in the
adjacent frequency of 2 kHz in the N2 group as compared to the N0 and N1 groups. The
adjacent frequency of 8 kHz showed a significant, but smaller, change in the N1 group
as compared to the N0 and N2 groups. The change at 2 kHz was independent of the starting
hearing level at E15, whereas the changes at 4-8 kHz were influenced by the hearing
level at E15. These data suggest that the noise-damaged ear does not 'age' at the
same rate as the non-noise damaged ear. The finding of increased loss at 2 kHz suggests
that the effects of noise damage may continue long after the noise exposure has stopped.
The mechanism for this finding is unknown but presumably results from prior noise-induced
damage to the cochlea.