Total remission or persistence of tinnitus and decreased sound level tolerance in adolescents with normal audiograms: A follow-up study

Background: Tinnitus may reflect hidden cochlear synaptic injury that does not express in the audiogram, but leads to neuroplastic changes in auditory pathways that, in turn, reduce tolerance to sounds. Such injury may follow the exposure to loud sounds. The aim of this study was to follow-up adolescents enrolled in a private school to evaluate the prevalence of tinnitus and reduced sound level tolerance (SLT) with 1-year interval, as well as to observe rates of tinnitus persistence, remission and incidence of new cases by repeat measurements. Methods: In Study 1 (Sanchez et al., 2016), we evaluated 170 adolescents by a questionnaire about tinnitus and reduced tolerance to ordinary sounds and by measurements in a sound booth: audiometry (0.25–16 kHz), Loudness Discomfort Levels (LDL, 0.5–4 kHz) and tinnitus pitch/loudness matching (if present). Tinnitus measured in the booth was then called “confirmed tinnitus.” In Study 2, we revaluated 54 adolescents who returned voluntarily 1 year later to repeat all measurements. Results: From Study 1 to 2, the prevalence of confirmed tinnitus reduced from 28.8% (49/170) to 14.9% (8/54) in retested subjects, which includes the cases of persisting tinnitus (confirmed tinnitus in both studies, n = 6/54 = 11.2%) and the new cases of tinnitus (confirmed tinnitus just at Study 2, n = 2/54 = 3.7%). Among the 15 adolescents with confirmed tinnitus at Study 1 who returned for Study 2, 40% had persistent tinnitus (n = 6) and 60% did not (remitted tinnitus, n = 9). SLT was reduced by 17.3 dB in cases with persistent tinnitus (P < 0.0002), similar to the findings of Study 1, and returned to normal levels in subjects with remitted tinnitus. Hearing thresholds averaged 4.37 dBHL and were < 20 dBHL in 97% of ears and all frequencies. At 14 and 16 kHz thresholds were bilaterally elevated at Study 1 (5.07 dB) and 2 (5.56 dB) in adolescents with confirmed tinnitus. Conclusions: Tinnitus and reduced sound tolerance could feature early signals of hidden synaptic injury that is prevalent among adolescents and hidden from the audiogram. The strong relationship between both symptoms, in addition to low-level increases in hearing thresholds at high frequencies in the extended audiogram, poses a challenge for future hearing health and should be further evaluated as a possible intrinsic vulnerability for lesions following exposure to loud sounds. Moreover, the relationship between their spontaneous remissions may signal a possible synaptic repair, which has been reported in animal models.