[Behavior of the stapedius reflex in long-term acoustic stimulation]

The present examination shows the action of the acoustic reflex on stimulation with a pure tone of 500 cps which lies 10 dB above the reflex threshold by impedance measurement. The duration of stimulation lasts 5 min. The test results of persons with normal hearing, persons with cochlear and retrocochlear lesions were measured according to the amount, the duration, the beginning, and the speed of reflex decay. The preliminary results permit the following conclusions: Persons with noise-induced hearing loss show no reflex decay for the entire duration of stimulation. Persons with normal hearing show a reflex decay after 2 min. After another 135 sec a constant reflex amplitude is attained. Subjects with cochlear lesions which are not caused by noise show similar reflex behavior. The amount of decay is larger, and the decay begins earlier. Persons with retrocochlear lesions show heterogeneous results, but the average amplitudinal decay is rapid and exceeds 50 per cent. The underlying mechanisms are discussed. In our opinion muscle fatigue plays an important role, which is absent in noise-induced hearing loss due to a permanent muscle training caused by the chronic noise exposure. The main result for audiology lies in the possibility to distinguish clearly between noise-induced hearing loss and other forms of cochlear lesions.