Transcranial Magnetic Stimulation as a Tool for Investigating and Treating Tinnitus

摘要

Transcranial magnetic stimulation (TMS) is a technique that allows non-invasive, painless stimulation of neuronal structures, based on electromagnetic induction of electric fields in excitable tissue. The technique has been applied to investigate normal physiology and disease states. TMS has therapeutic potential, as repetitive stimulation (RTMS) can produce long-term modulation of neuronal circuits. RTMS has been demonstrated to be an effective treatment for depression and is being developed for use in neurorehabilitation. TMS is delivered by discharge of high-voltage capacitors through copper coils applied to the scalp, with induction of a rapidly changing magnetic field that easily penetrates the skull. Besides brain stimulation, other effects of such a discharge are a clicking noise, stimulation of sensory fibers in the scalp, and heating of the coil and nearby conductors. It may be difficult to distinguish which effects are important in tinnitus investigations, particularly if the coil noise and scalp sensory stimulation are not controlled. Localization of stimulation is another problem in TMS work, as coils may be accurately placed over brain areas but then produce significant magnetic fields over several square centimeters of brain surface. Tinnitus is a condition defined by auditory experience in the absence of internal or external auditory input. While the experience is subjective, there is evidence from functional neuroimaging that it associated with activation of brain auditory pathways. RTMS can suppress tinnitus when applied over a variety of regions, including primary auditory cortex in the temporal lobe, and even reduce the tonic activation in auditory regions. There are now a number of clinical trials investigating RTMS methods in tinnitus and the latest published results will be discussed. It is less clear what TMS can tell us about the underlying pathophysiology of tinnitus as conflicting results have been obtained regarding measurable abnormalities in cortical physiology.