Inflection points in hearing deterioration: clinical characteristics of NIHL from steady-state noise exposure.

Objectives: To explore the clinical characteristics of noise-induced hearing loss (NIHL) caused by long-term exposure to steady-state noise and find a possible inflection point time leading to hearing deterioration.

Material and methods: Subjects exposed to steady-state noise were selected as the noise-exposed group and matched with a control group of individuals not exposed to noise. Both groups underwent pure-tone audiometry (PTA) and distortion product otoacoustic emissions (DPOAE), and their hearing conditions were analyzed. The time inflection point with the most significant disparities in NIHL between early and late exposure was evaluated. The noise-exposed subjects were divided into 2 groups based on cumulative exposure time: the early exposure group (group A) and the late exposure group (group B). Retrospective analyses of clinical characteristics of hearing loss were conducted.

Results: The noise-exposed group exhibited significantly higher hearing thresholds and reduced otoacoustic emissions compared to the control group, with high-frequency hearing loss being the most prominent. The most significant disparity in high-frequency hearing loss in PTA was observed before and after 5 years of cumulative steady-state noise exposure. Among the 78 noise-exposed subjects, 37 were in group A (≤5 years) and 41 in group B (>5 years). In DPOAE, the most significant disparity occurred before and after 4 years of acexposure, with 33 subjects in group A (≤4 years) and 45 in group B (>4 years). Distortion product otoacoustic emissions identified the time inflection point of significant hearing deterioration 1 year earlier than PTA.

Conclusions: Hearing loss caused by long-term exposure to steady-state noise showed evident deterioration after 4-5 years. The DPOAE can illustrate the inflection point of hearing deterioration 1 year earlier than PTA. Int J Occup Med Environ Health. 2025;38(1):57-69.