P138 Influence of the difference of induced current direction on measurement of corticospinal excitability changes after continuous and intermittent theta burst stimulation

Abstract

Introduction The after-effects of continuous and intermittent theta burst stimulation (cTBS and iTBS) are highly variable between individuals and only about 50% of subjects respond in expected ways (i.e. depression of motor evoked potentials (MEPs) in cTBS and facilitation in iTBS). TBS is usually applied using a biphasic stimulus pulse. It preferentially activates the brain during the second depolarizing phase of the current in the brain, that is, anterior-posterior (AP) current. In contrast, the after-effects of any plasticity inducing protocols are usually evaluated by a monophasic stimulus pulse with posterior-anterior (PA) current. Here we hypothesized that such difference in magnetic pulse configuration of transcranial magnetic stimulation (TMS) influences variability of TBS effects. Objectives To investigate whether the after-effects of TBS should be clearer if tested with MEPs generated by AP currents or biphasic currents. Methods Twenty-three healthy right-handed volunteers participated in this study. TMS was applied to the hot spot of the right first dorsal interosseous muscle and MEPs were measured at the stimulus intensities set to evoke MEPs of about 1 mV in monophasic PA, monophasic AP, and biphasic currents. Thirty MEPs for each direction were measured before and up to 30 min after cTBS or iTBS (Huang et al., 2005). Responder (i.e. depression in cTBS and facilitation in iTBS) was defined according to the grand average of MEPs. Results In cTBS, responder rates were 23.9% (5/21) in PA current, 38.1% (8/21) in AP current, and 14.2% (3/21) in biphasic current. 42.9% (9/21) of subjects showed opposite responses between PA and AP currents. As for non-responders both in PA and AP currents (47.6%, 10/21), the difference of baseline MEP latencies at rest between monophasic PA and biphasic currents was smaller than the others. In iTBS, responder rates were 60.9% (14/23) in PA current, 65.2% (15/23) in AP current, and 47.8% (11/23) in biphasic current. 47.8% (11/23) of subjects showed opposite responses between PA and AP currents. Conclusion The after-effects of TBS were highly variable regardless of current direction in measurement of MEPs. In cTBS, a tendency to early I wave recruitment in biphasic pulse might relate to MEP facilitation both in PA and AP currents.