Hypothesis: Patients with subjective nonpulsatile tinnitus and a normal conventional audiogram have more objective audiologic evidence of hidden hearing loss and deafferentation-related pathology than patients without tinnitus.
Background: The aim of this study was to assess the epidemiologic characteristics and audiologic profiles, including auditory brainstem response (ABR), distortion product otoacoustic emission, and threshold-equalizing noise test results, in patients with tinnitus and a normal audiogram.
Methods: The test results for 20 patients complaining of nonpulsatile chronic tinnitus were compared with those of 91 subjects with normal hearing and no tinnitus.
Results: Patients with unilateral tinnitus had higher tinnitus handicap inventory scores than those with bilateral tinnitus (p < 0.05). Threshold-equalizing noise tests were normal in all study participants. In patients with unilateral tinnitus, the ABR and distortion product otoacoustic emission test results were similar to those of controls. In contrast, patients with bilateral tinnitus showed a shortening of latency in wave III of the ABR on the right (p = 0.047) and in wave V on the left (p = 0.024). Logistic regression analysis revealed that enhanced wave III/I (p = 0.018) and V/I (p = 0.012) ratios on the left and poorer pure-tone average on the right were significant risk factors for bilateral tinnitus.
Conclusion: The mechanism involved in the development of tinnitus may depend on its laterality. Bilateral tinnitus may be associated with hyperactivity at the level of the cochlear nucleus whereas a higher-order cortical area may be involved in unilateral tinnitus.