Determinants of bilateral audiometric notches in noise-induced hearing loss

Objectives/Hypothesis: To explore the possible determinants of bilateral audiometric notches in noise-induced hearing loss (NIHL). Study Design: Prospectively enrolled cohort with cross-sectional analysis. Methods: Male workers (n = 412) with exposure to continuous loud noise (noise exposure level normalized to a nominal 8-hour working day >75 dBA) during work were included in this cohort study. The relationship between the presence of bilateral audiometric notches and age, noise exposure duration, noise exposure level, and shift from the predicted thresholds over different frequencies were determined by regression model analysis, receiver operating characteristic analysis, and area under the curve analysis. Results: The presence of bilateral audiometric notches was closely associated with the shifts from the predicted thresholds over 1, 2, 3, 4, and 6 kHz. The shift pattern was somewhat different between those who used hearing protection devices and those who did not. The shifts from the predicted thresholds demonstrated a stronger association with bilateral audiometric notches than did age, gender, noise exposure level, or noise exposure duration. Conclusions: The bilateral audiometric notches in NIHL could not be determined only on the basis of age, gender, noise exposure level, or noise exposure duration. The importance of other factors associated with bilateral audiometric notches should be recognized. Elucidation of the factors of formation of bilateral audiometric notches would help us to develop a more comprehensive protection strategy to prevent further NIHL.