[The development of endogenous inner ear hearing loss due to occupational noise exposure]

Pure tone audiograms of 80 workmen with preexisting endogenous cochlear hearing loss were investigated over an average of 16 years (range 10 to 26) of occupational exposure to noise. Further hearing deterioration was moderate in the majority of cases more severe hearing losses more often affected the low tone range. The pure tone threshold 0.25 to 8 kHz remained unaltered in at least one ear in 24% of cases, in the high tone range 3 to 6 kHz in 51%. The average hearing loss was least at 3 kHz, followed by 4 kHz. The average hearing loss was somewhat greater in the initially better hearing ear. Differences between the two sides mostly developed in the high and the low tone region of trough-shaped thresholds, and in the low tone region of sloping thresholds. Hearing losses were greater when preexisting impairment did not exceed 30 dB HL than in preexisting damage over 30 dB HL. The average hearing losses for 0.5, 1, 2, 3 and 4 kHz were all lower than the respective average values for noise-induced hearing loss. Thus there was no evidence of increased noise susceptibility in endogenous cochlear hearing loss, either in comparison with controls or in a comparison of the sides or frequency ranges.