Objective: Tinnitus may be caused by a lesion or dysfunction at any level of the auditory system. This study explores cochlear mechanics using otoacoustic emissions in patients with tinnitus after head injury, in whom there seems to be evidence to support dysfunction within the CNS.
Methods: The study included 20 patients with tinnitus and other auditory symptoms, such as hyperacusis and difficulty in listening in background noise, after head injury, in the presence of an "intact" auditory periphery (normal or near normal audiometric thresholds). They were compared with 20 normal subjects and 12 subjects with head injury, but without tinnitus, who had similar audiometric thresholds. In all subjects otoacoustic emissions, including transient click-evoked (TEOAEs) and spontaneous otoacoustic emissions (SOAEs), were recorded, and a test of efferent medial olivocochlear suppression, consisting of recording of TEOAEs under contralateral stimulation, was performed.
Results: A significantly higher prevalence of SOAEs (100%), higher TEOAE response amplitudes, and reduced medial olivocochlear suppression in patients with tinnitus in comparison with subjects without tinnitus have been found.
Conclusion: These findings have been interpreted to be an extracochlear phenomenon, in which the reduction in central efferent suppression of cochlear mechanics, leading to an increase in cochlear amplifier gain, was subsequent to head injury. Auditory symptoms in these patients seemed to constitute the "disinhibition syndrome".