Longitudinal threshold changes in older men with audiometric notches

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.