Delayed effects of noise on the ear.

The possibility that a delayed onset hearing loss may occur after noise exposure was investigated in 20 normotensive (N) and 20 spontaneously hypertensive (SH) Wistar rats. Half of the rats of each strain were first exposed to noise at 3 months of age and half were first exposed at 12 months of age. All rats were exposed for a total of 3 months to a noise of 100 dB Leq (lin) (a narrowband noise, sweeping from 3 to 30 kHz at a rate of 0.5 Hz). Hearing thresholds were determined with a behavioral technique, and also on certain selected animals by brainstem audiometry, using pulses of 1/3-octave filtered sine waves. One to two weeks after exposure to noise animals had a 30-50 dB hearing loss. The hearing loss remained stable or progressed only slightly in normotensive animals up to 12 months after end of exposure. Generally the SH animals suffered a slight additional loss of hearing over the 12 months after noise exposure. When tested at 18 months of age 3 of the 10 SH rats exposed to noise from 12 to 15 months of age suffered a hearing loss which covered the whole range of observed frequencies from 1.5 to 44 kHz and which had progressed noticeably. These results suggest that while the delayed effects of noise are normally small, exposure might trigger a pronounced hearing loss in subjects with fragile ears.