Fatigability of the stapedius reflex in industrial noise. A field study.

The depression of the stapedius reflex after a whole workday in industrial noise was studied in 11 normal-hearing employees at a shipbuilding yard. The reflex activity was assessed with simultaneous bilateral recordings of impedance change induced by stimulation with 2 000 and 500 Hz tones. One ear was exposed for approximately 7 hours in the plant, the other one was protected. The dose was measured individually at 89--103 dB(A) equivalent level. Utilization of bilateral recordings together with a unilateral exposure enabled an analysis based on the whole stimulus response curves. The results show that the reflex is depressed on the average 4 dB at 2 000 Hz 10 min after end of workday. By extrapolation the stapedius reflex was estimated to be depressed less than 10 dB at the end of the exposure. Since the stimulus-response curves are shifted in parallel, the contraction during the high-level components of the noise were largely unaffected. No evidence was found for creation of transtympanic pressure differences during noise exposure. It was concluded that the reflex fulfils one necessary condition for influencing the individual pattern of noise damage to the ear, fatigue resistance in the presence of long-term exposure to hazardous noise.