THE RELATIONSHIP BETWEEN CHRONIC ENVIRONMENTAL LEAD EXPOSURE AND AUDITORY NERVE FUNCTION: THE NORMATIVE AGING STUDY

Abstract

ISEE-358 Objectives: This study evaluated the cross-sectional relationship between biological markers of chronic environmental lead exposure and hearing impairment amongst a community-based sample of elderly men. It was also designed to evaluate this relation in view of other covariates that may affect auditory function such as age, smoking and occupation. Methods: The participants of this study were derived from the Normative Aging Study, a longitudinal study of aging established by the Veterans Administration since 1963. We measured bone lead concentration using a K-X-ray fluorescence technique as a quantitative index of chronic lead exposure and blood lead using graphite furnace atomic absorption spectroscopy as an index of short-term lead exposure. A pure-tone audiometer was used to measure air and bone conduction hearing in decibel units at frequencies of 250, 500, 1000, 2000, 4000 and 8000 Hz. Results: In this study, 734 subjects were examined who had a mean age of 67.2 (SD 7.2) years, and mean (SD) values for blood, tibia and patella bone lead of 6.04 (3.9) mg/dL, 21.6 (13.4) mg/g, and 31.1 (19.4) mg/g respectively. We considered a subject to have hearing impairment if both ears had hearing thresholds greater than 20 Db at frequencies of 500, 2000 and 4000 Hz. Age was significantly higher in the hearing impaired group at the all frequencies studied (p>0.001). Blood, tibia and patella bone lead were significantly higher in the hearing impaired group at the higher frequencies 2 and 4 KHz for both air and bone conduction (p>0.01). Using multivariate logistic regression blood and patella lead, but not tibia lead, increased the odds of air and bone conduction hearing impairment at 2 KHz after adjustment for age, cumulative smoking and occupation (p<0.01 and p>0.05 respectively). At 4 KHz, both tibia and patella lead levels increase odds of air conduction hearing impairment (p>0.01). At 500 Hz, neither of these covariates seems to increase the odds of hearing impairment. Conclusion: We conclude that environmental lead exposure increases the odds of both air and bone conduction hearing impairment at high frequencies (2 KHz and 4 KHz) more than at lower frequency (500 Hz). Both blood and bone lead levels are good predictors of hearing impairment at higher frequencies.