Course of hearing loss and occurrence of tinnitus

Abstract

Chronic tinnitus is often accompanied by a hearing impairment, but it is still unknown whether hearing loss can actually cause tinnitus. The association between the pitch of the tinnitus sensation and the audiogram edge in patients with high-frequency hearing loss suggests a functional relation, but a large fraction of patients with hearing loss does not present symptoms of tinnitus. We therefore, investigated how the occurrence of tinnitus is related to the shape of the audiogram. We analyzed a sample where all patients had noise-induced hearing loss, containing 30 patients without tinnitus, 24 patients with tone-like tinnitus, and 17 patients with noise-like tinnitus. All patients had moderate to severe high-frequency hearing loss, and only minor to moderate hearing loss at low frequencies. We found that tinnitus patients had less overall hearing loss than patients without tinnitus. Moreover, the maximum steepness of the audiogram was higher in patients with tinnitus (−52.9 ± 1.9 dB/octave) compared to patients without tinnitus (−43.1 ± 2.4 dB/octave). Differences in overall hearing loss and maximum steepness between tone-like and noise-like tinnitus were not significant. For tone-like tinnitus, there was a clear association between the tinnitus pitch and the edge of the audiogram, with tinnitus pitch being on average 1.48 ± 0.12 octaves above the audiogram edge frequency, and 0.81 ± 0.1 octaves above the frequency with the steepest slope. Our results suggest that the occurrence of tinnitus is promoted by a steep audiogram slope. A steep slope leads to abrupt discontinuities in the activity along the tonotopic axis of the auditory system, which could be misinterpreted as sound.

Introduction

Hearing loss is a risk factor for tinnitus (Chung et al., 1984, Sindhusake et al., 2003, Sindhusake et al., 2004), and even tinnitus patients with normal audiograms might have restricted cochlear damage (Shiomi et al., 1997, Weisz et al., 2005) or hearing loss at frequencies above 8 kHz, which is not detected by normal clinical audiometry (Roberts et al., 2006). The tinnitus sensation and the frequency range of hearing loss are related: when subjects match their tinnitus pitch to a pure tone, most of the matches are at frequencies at which hearing is impaired (Henry et al., 1999); when subjects are asked to judge the contribution of comparison tones to their tinnitus sensation, the resulting tinnitus spectra span wide frequency ranges that correspond to the frequencies where hearing loss is present (Noreña et al., 2002). However, not all patients with hearing loss develop tinnitus, as demonstrated by the higher prevalence of hearing loss compared to tinnitus (Lockwood et al., 2002). It is therefore unclear, which factors of hearing loss contribute to the occurrence of tinnitus.

Tinnitus is believed to arise from alterations in the spontaneous activity of neurons in the auditory system (see, e.g., Eggermont, 2003). In animals, hearing loss through cochlear damage reduces the spontaneous firing rate in the affected auditory nerve fibers (Liberman and Dodds, 1984). This reduction can cause discontinuities in the profile of spontaneous activity along the tonotopic axis in the auditory pathway. Modeling studies suggest that such discontinuities could be exaggerated by lateral inhibition (Gerken, 1996) and homeostatic changes (Schaette and Kempter, 2006), leading to activity patterns in the models that are consistent with tone-like tinnitus.

Such concepts suggest that the development of tinnitus depends on the course of the audiogram, in that some audiogram shapes are more likely to lead to tinnitus than others. To investigate this hypothesis, we analyzed a sample of patients that all had noise-induced hearing loss from chronic noise exposure in the workplace. This choice of subjects eliminates possible confounding factors that could arise from mixed etiologies of hearing loss. Moreover, none of the patients used a hearing aid at the time of examination.