Tinnitus in noise-induced hearing loss

Below is a summary of a paper published by the Nofer Institute of Occupational Medicine in Łódź, Poland.

An analysis of epidemiological data shows that noise exposure is a widespread and 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.

Given the scarcity of reliable data on the frequency and nature of tinnitus in people exposed to industrial noise, and especially the need to objectify this subjective symptom, the study set out to assess the relationship between tinnitus, the extent and type of hearing loss, and the behaviour of otoacoustic emissions. The study included 191 people exposed to occupational noise at levels of 88-92 dB(A) who reported hearing problems and tinnitus, aged 25-65 (mean 42.5 ± 7.6), with 9 to 30 years of work in noise (mean 25.9 ± 4.6). The control group consisted of 80 people with sensorineural hearing loss caused by industrial noise, of similar age and length of service, who did not report tinnitus.

In about 59.7% of the subjects, industrial noise was shown to be one of the most likely factors responsible for tinnitus. Tinnitus was found in 22.5% of subjects after 10 years of noise exposure and in 46% after 11-20 years of exposure. In 95.8% of subjects, tinnitus was associated with hearing loss, and in only 4.2% of cases did it occur in ears with normal hearing.

In people exposed to noise, tinnitus is most often (59.2%) bilateral and constant. Audiological tests verified with objective audiometry (tympanometry, ABR) found cochlear hearing loss in 68.06% of subjects, retrocochlear in 8.37%, and mixed hearing loss or other types of damage, such as presbycusis, in 19.36%. Audiological assessment showed that in 62.3% the tinnitus frequency lies in the high-frequency range and correlates with the degree of hearing loss on the pure-tone audiogram.

Tinnitus loudness ranges from 10-15 dB to 45 dB. In about 40.3% of subjects, noise was not the only risk factor. This group showed predisposing conditions such as hypertension, diabetes, atherosclerosis and lipid disorders, as well as other aetiological factors that may affect tinnitus, for example ototoxic drugs.

Measurements of evoked otoacoustic emissions (EOAE and DPOAE) showed significant differences (p < 0.01) in EOAE amplitude and spectrum in 58.63% of ears with tinnitus compared with ears without tinnitus at a similar hearing threshold. In 27.74% of subjects no differences were found. DPOAE measurements showed significant differences in DP-grams in 62% of subjects between ears with and without tinnitus (p < 0.01). Differences in both types of evoked emissions, such as reduced amplitude, a narrower spectrum, reduced or completely absent emission in a limited high-frequency region, were found in ears with tinnitus only in cases of cochlear hearing loss.

The results suggest that this method may be useful for assessing the role of the cochlear mechanism in tinnitus and for distinguishing tinnitus generated in the cochlea from tinnitus originating at other levels of the auditory system or outside it. No relationship was found between tinnitus and spontaneous emissions, as these were recorded in only about 12% of people exposed to noise, including 2% of people with normal hearing.

Source: a publication of the Nofer Institute of Occupational Medicine in Łódź, Poland (in Polish). The original PDF is no longer available on the Institute’s website.