騒音受傷性と耳小骨筋反射

抄録

Effects of noise exposure on aural reflex (AR) were investigated. The threshold of AR (ART) was measured in two groups exposed to occupational noise (Group A and B) by the electro-acoustic impedance bridge (Madsen, ZO-72). White noise (WN), and pure tones of 1 and 4 kHz were selected as activating stimuli for AR, and ARTs for these stimuli (ART_WN, ART_1k, ART_4k) were determined respectively. The results were compared with the ARTs in the normal group (Group C) reported by the authors previously. Group A consisted of 31 male workers exposed mainly to steady noise, and Group B included 65 male workers exposed maily to impulsive noise. Group C consisted of 50 males and 50 females, who had normal hearing acuity without any history of occupational noise exposure (Fig. 12). Mean age was 37.2 years in Group A, 49.9 in Group B. Mean period of noise exposure was 13.3 years in Group A, 26.1 in Group B. In both groups, the hearing loss was observed in the higher frequency region at 4 and/or 8 kHz, while normal hearing acuity was reserved at 1 kHz (Fig. 3 and 4). Because of no significant difference between Group A and B in each of the mean ARTs (Table 3), Groups A and B were summarized as the exposed group to noise. ARTs for three different stimuli (ART_WN, ART_1k, ART_4k) in the exposed group showed significant differences compared to those in the non-exposed group (Group C); i.e. mean ART_WN was 92.0 dB SPL in the exposed group and 83.8 in Group C, mean ART_1k was 93.8 dB SPL in the exposed group and 96.2 in the non-exposed, mean ART_4k was 102.1 dB SPL in the former and 97.5 in the latter group (Table 5). There was a significant negative correlation between the hearing level at 4 kHz (HL_4k) and the value of [ART_4k-HL_4k] (Fig. 14). The difference between ART_1k and ART_WN, [ART_1k-ART_WN], was significantly reduced in the exposed group; i.e. 1.9 dB in the exposed group and 12.4 in Group C (Table 6). For the purpose of our study, the exposed group was re-classified into another two groups according to their HLs at 4 and 8 kHz; i.e. Group D (HL_(4k+8k)/2<25 dB) and Group E (HL_(4k+8k)/2>__=25 dB). The mean values of ARTs and HLs in Groups D, E and C, which are summarized in Table 7, show two interesting points. First, the significant elevation of ART_WN is observed not only in Group E but also in Group D with normal hearing acuity. Second. the lowering of ART_1k is observed in Group E only. These results indicate that the elevation o ART_WN probably depends upon the exposure history to occupational noise but not upon the hearing loss, and that lowering of ART_1k may be caused by the hearing loss of the subjects in the higher frequency region. These estimations may be supported by the fact that [ART_1k-ART_WN] was 3.7 dB in group D and 0.6 dB in Group E (Table 8). Since damages of outher hair cells (OHC) in the Corti's organ do not always lead to hearing loss, it is conceivable that the reduction of [ART_1k-ART_WN] is the results of the widening of the critical band width for AR due largely to the damage of OHC caused by the occupational noise exposure. Under this conception, the value of [ART_1k-ART_WN] may be used as a sensitive and objectibe indicator for detecting and evaluating the early stages of noise-induced hearing impairment and individual susceptibility to noise.