[Tinnitus in noise-induced hearing impairment].

The analysis of the epidemiological data indicates that exposure to noise is widespread and it is one of the most common causes of tinnitus, estimated at about 20.7% according to Hazell; 28% according to Axelsson; and 42% according to Palmer. Bearing in mind the scantiness of reliable data on the incidence and nature of tinnitus in persons exposed to industrial noise, and especially the need for the objectivity of this subjective symptom, the authors have decided to undertake the study aimed at assessing the interrelation between tinnitus, the magnitude and kind of hearing impairment, and otoacoustic emission. The study group included 191 persons aged 42.5 +/- 7.6 years (range, 25 to 65), occupationally exposed to noise at the levels of 88-92 dB(A) for 26.9 +/- 4.6 years (range, 9 to 30) who had reported hearing disorders and tinnitus. The control group, matched by similar age and duration of employment, consisted of 80 persons with perceptive hearing impairment induced by industrial noise who had not complained of tinnitus. The results of the study revealed that in 59.7% of the study subjects, noise proved to be one of the most probable factors responsible for the development of tinnitus. The presence of tinnitus was found in 22.5% and in 46% of the study subjects after 10 years and 11-20 years of noise exposure, respectively. In 95.8% of workers, tinnitus was associated with hearing loss, and only in 4.2% of cases it occurred in ears with normal hearing. In persons exposed to noise, tinnitus was most frequently (59.2%) bilateral and permanent. Following the audiologic examinations, verified by objective audiometry (tympanometry, ABR), cochlear hearing impairment was found in 68.6%; retrochochlear in 8.37%; mixed and other forms of impairment, e.g. presbyacousis, in 19.4% of subjects. The audiologic assessment of tinnitus demonstrated that in 62.3% of persons, tinnitus occurred at high frequencies and correlated with the magnitude of hearing impairment in the tonal audiogram. The tinnitus intensity ranged between 10-15 dB and 45 dB. In 40.3% of those under study, noise was not the only tinnitus-risk factor. In this group of persons, the presence of predisposing diseases was also observed, e.g. hypertension, diabetes, atherosclerosis, disturbed lipid metabolism and other etiologic factors that might have impact on the tinnitus incidence, ototoxic drugs, for example. The measurements of evoked otoacoustic emission (EOAE and DPOAE) revealed in 58.63% of persons significant differences (p < 0.01) in the amplitude and spectrum of EOAE in the ears with tinnitus as compared to the ears without tinnitus with a similar hearing threshold. Whereas in 27.74% of subjects, no differences in the EOAE measurements in the ears with or without tinnitus were observed. The results of DPOAE measurements showed in 62% of subjects significant differences in DP-grams in the ears with tinnitus as compared to the ears without tinnitus (p < 0.01). Interestingly, the differences in measurements of both types of evoked emissions (EOAE and DPOAE), expressed by the lowered amplitude, narrowed spectrum, reduction of emission or its complete fading in a limited area of high frequencies, were demonstrated in the ears with tinnitus only in retrocochlear hearing impairment, as compared to those free from tinnitus. The evaluation of the EOAE and DPOAE measurements seems to prove that this method may be useful in assessing the contribution of the cochlear mechanisms to the incidence of tinnitus and in distinguishing between tinnitus generated in cochlea and tinnitus with the source at other levels of the hearing organ or beyond it. Our study failed to determine the interrelation between tinnitus and spontaneous emissions as the emission was registered only in about 12% of persons exposed to noise, including 2% of those with normal hearing.