In contrast to objective tinnitus, subjective tinnitus appears to be an inappropriate activation of the auditory system. At some point in the multilink pathway of translation of sound pressure waves in the air to neural code in the brain, there is activity that is not normally present. The higher centers in the brain interpret this inappropriate activity as a sound, despite the absence of a true sound stimulus. It appears that there can be triggers for the inappropriate activity anywhere along the chain of structures in the auditory system. Perhaps the most common cause of tinnitus is not a systemic disease, but a local one: hearing loss. It is theorized (4) that autoregulatory mechanisms in the auditory system trying to compensate for the hearing loss are overactive resulting in generation of signals that the higher cortical auditory centers interpret as sound. The sound sensations in subjective tinnitus are quite varied but are commonly described as "buzzing," "ringing," "hissing," or similar sounds. The phenomenon of tinnitus with hearing loss is analogous to the "phantom limb pain" phenomenon, where cortical stimulation is perceived in the absence of external input. Our understanding of tinnitus causes is still rudimentary, however, as a large number of people with tinnitus appear to have no known auditory or systemic pathology.
Through understanding that there can be inappropriate activations of the auditory system presenting as tinnitus, we can determine possible pathogenic links from systemic diseases that result in the generation of inappropriate sound sensations. In our review of systemic diseases that can cause tinnitus, we will go by body systems and their known pathologies.
Interestingly, most tumors of the nervous system are silent with respect to the auditory system. However, tumors located on the eighth nerve can cause an insidious compression of the nerve and progressive loss of hearing, and result in the appearance of tinnitus. These
TABLE 2 Etiologies of Objective Nonpulsatile Tinnitus
Temporomandibular joint dysfunction Superior canal dehiscence syndrome Posterior canal dehiscence syndrome acoustic neuromas are well described elsewhere (5). The very common condition of migraine can present with tinnitus. Migraine may be a genetic calcium channel defect of neurons and can present with multiple unusual manifestations (see Chapter 23 for detailed discussion of migraines). Cure (6) gives a report of multiple sclerosis (MS) patients presenting with tinnitus. MS lesions can appear throughout the brain and spinal cord and thus could be expected to affect locations in the auditory system, reduce signal transmission, and activate the compensatory mechanisms that result in tinnitus.
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