Transcranial magnetic stimulation coregistered with MRI: a comparison of aguided versus blind stimulation technique and its effect on evoked compound muscle action potentials

Introduction and methods: Compound muscle action potentials (CMAPs) elicite d by transcranial magnetic stimulation (TMS) are characterized by enormous variability, even when attempts are made to stimulate the same scalp locati on. This report describes the results of a comparison of the spatial errors in coil placement and resulting CMAP characteristics using a guided and bl ind TMS stimulation technique. The former uses a coregistration system, whi ch displays the intersection of the peak TMS induced electric field with th e cortical surface. The latter consists of the conventional placement of th e TMS coil on the optimal scalp position for activation of the first dorsal interossei (FDI) muscle. Results: Guided stimulation resulted in significantly improved spatial prec ision for exciting the corticospinal projection to the FDI compared to blin d stimulation. This improved precision of coil placement was associated wit h a significantly increased probability of eliciting FDI responses. Althoug h these responses tended to have larger amplitudes and areas, the coefficie nt of variation between guided and blind stimulation induced CMAPs did not significantly differ. Conclusion: The results of this study demonstrate that guided stimulation i mproves the ability to precisely revisit previously stimulated cortical loc i as well as increasing the probability of eliciting TMS induced CMAPs. Res ponse variability, however, is due to factors other than coil placement. (C ) 2001 Published by Elsevier Science Ireland Ltd.