Tinnitus

Underlying Mechanisms

In some people, the development of tinnitus is an indication of significant otological pathology, such as an acoustic neuroma, Ménière's disease, or oto-sclerosis. As such, an interested and informed clinical opinion should be sought by a person with troublesome tinnitus, especially when it is unilateral or pulsatile. In other cases, there is no demonstrable pathology, and the mechanisms of the ignition of the tinnitus are complex. In some cases, the person becomes aware of the background auditory neuronal activity as previously described. In other cases, a hearing loss (usually of cochlear origin) is associated with the tinnitus, partly as the reduction in external auditory information increases the perceived intensity and clarity of the internal tinnitus signal. Physiological theories of tinnitus generation abound, and this reflects the great variety of experience seen in the clinical population of people with troublesome tinnitus. Examples of ignition sites for tinnitus include cochlear hair cell damage following noise-induced hearing loss, disinhibition of central auditory pathways following reduced input to the dorsal cochlear nucleus due to hearing loss, and aberrant plastic reorganization of higher auditory centers following cochlear dysfunction.

Describing these physiological mechanisms of tinnitus does not adequately address the clinical picture of severe, debilitating tinnitus. The consensus view is that the mammalian auditory system has a protective and defensive function, and is vigilant for sounds that might be associated with intrusion or threat, irrespective of intensity. Once such a signal is perceived, systems of reaction and arousal become active, and the animal experiences an orientating response to the signal, to which it pays intent attention. These responses are instinctive and extremely rapid, and are evident in human beings that hear a possible intruder at night or a change in breathing of a newborn child and are instantly alert and vigilant. Once a sound is identified as background or harmless, the animal habituates to that stimulus.

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