Transcranial magnetic stimulation is a pain-free, non-invasive technique used to stimulate the central nervous system. The electric currents necessary to stimulate the brain are produced by rapidly changing magnetic fields that are initiated by a brief high-intensity electric current that passes through a wire coil held over the scalp. The subsequent magnetic field is projected perpendicular to the electric current and is able to passes through the layers of human tissue (skin, bone, cortex) with very little impedence. TMS can be delivered via single-pulse, double-pulse, paired-pulse and repetitive pulse (rTMS). rTMS is the method currently under investigation for use as a treatment for stroke mainly due to its ability to modulate excitability in the cerebral cortex over longer time periods (compared to other types of TMS). It can also enhance some cognitive processes, regulate activity in specific brain regions and provide causal information about the roles of different cortical regions in behavioural performance. The use of rTMS can also enhance neuroplasticity during motor training. Theta burst stimulation is a type of rTMS that has been found to effectively induce synaptic long-term potentiation and depression and is also currently under investigation for use as a treatment therapy for stroke. According to some experimental studies, a stroke would cause a relative hyperactivity of the unaffected hemisphere due to the release from reciprocal inhibition by the opposite hemisphere which would explain some of the dysfunctions observed in this population (Brighina et al, 2003). This phenomenon is called “interhemispheric inhibitory interactions”. Thus inhibitory stimulation (low frequency rTMS) to the unaffected hemisphere could work to curb this problem. In addition, other researchers like Talelli et al. (2007) suggest that excitation of the affected hemisphere (with high frequency rTMS) enhances corticospinal output and leads to promising therapeutic results. Nevertheless, there is still a clear lack of knowledge on the exact mechanisms of TMS.
Note: Only the studies that looked at rTMS as a rehabilitation intervention were considered in this module.