Possibilities of improving the diagnosis of noise-induced hearing damage by means of directional audiometry, the dichotic speech discrimination test, and the EEG1

Abstract

Several audiometric tests were made of 85 noise workers exposed to an intensive impulse noise during pressing and punching. These tests aimed at finding out noise-induced central hearing damages (directional audiometry, dichotic speech discrimination test according to Feldmann and checking of the induction of the impedance jump). In addition, a clinical EEG was taken from each subject. Having obtained the necessary norms with series of tests in subjects with normal hearing abilities for their respective ages, the following results have been obtained: (1) directional hearing has hardly been affected by noise-induced hearing damage. It behaves like age changes in the group compared with it; (2) the dichotic speech discrimination test shows increasingly pathological values with progressive noise-induced hearing damage; this was already obvious in the age group 41-50. The impedance jump is less and less produced with progressive noise-induced hearing damage and finally disappears at a noise intensity of up to 110 dB. Because of these findings a progressive degeneration of the spiral ganglion and hearing nerve in noise-induced hearing damage is discussed as a cause of a deteriorated speech discrimination and the absence of the impedance jump. The EEG findings show changes in the alpha-rhythm in about 50% of the cases at an exposure time of 10 years onwards which may be possibly due to damage to the reticular formation, once increasing noise-induced hearing damage no longer exists.