Course of hearing loss and occurrence of tinnitus

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.