Development of an audiometric technique to identify individuals' susceptibility to noise
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Introduction. As shown in previous studies (Borg, Nilsson, & Engström, 1983; Zakrisson, Borg, Liden & Nilsson, 1980) the absence of acoustic reflex (AR) increases the hearing hazards from noise exposure. Past researchers (Stach, 1987; Colletti & Sittoni, 1986; Stach & Jerger, 1984) have studied the AR as a potential indicator of individuals’ susceptibility to noise. The Colletti & Sittoni (1986) study indicates the possibility of a relationship between the strength of the acoustic reflex contraction and the ear’s resistance to noise. However, no significant relationship has been established so far between the acoustic reflex characteristics and susceptibility to develop a noise-induced hearing loss. The goal of this project is to evaluate whether the strength of contraction of the acoustic reflex in workers exposed to high level noise is correlated with their industrial acquired hearing loss. Method. Retrospective analysis was conducted on 82 workers exposed to noise environments (Leq) of approximately 90 dBA. Participants were divided into four groups according to their hearing threshold (HT) in relation to their predicted ISO 1999 distribution (predicted hearing threshold based on a normal distribution with a given age as well as time and level of exposure). Group 1 was composed of workers with excellent hearing (HT falling between the 75 th and the 90 th percentile of the ISO 1999 distribution), Group 2 with just above average hearing workers (HT between the 50 th and the 75 th percentile), and Group 3 with just below average hearing workers (HT between the 25 th and the 50 th percentile. Group 4, was composed of workers that had developed the same amount of hearing loss as Group 2 workers but in a much shorter period of time (3 to 10 years in contrast to 13 to 20 years for Group 2 workers). All participants underwent an audiometric testing as well as an acoustic reflex measurement using a newly developed Interacoustic Wideband Reflectance instrument. Wideband acoustic reflexes were evaluated using 500, 1000 and 2000 Hz pure tones as well as a broad band noise. The acoustic reflex thresholds, strength of contraction as well as dynamic range were compared between the four groups for all stimulus presentations. Results. Observed results tend to show that Group 1 (more resistant to noise) has on average a stronger acoustic reflex (lower acoustic reflex threshold, stronger contraction) than Group 4 (least resistant to noise). However, due to a large inter-individual variability and relatively small number of subjects for a field study, the observed trends are not statistically significant. The need for further investigation to establish a correlation or a lack of correlation between AR and susceptibility to noise-induced hearing loss is clear. It would be of great interest to be able to prevent noise-induced hearing loss before it happens.