P 13. Intersession reliability of cortical motor maps with navigated transcranial magnetic stimulation

Navigated transcranial magnetic stimulation (nTMS) is a non-invasive highly effective tool for mapping individual muscle representations in the motor cortex Julkunen et al., 2009, Picht et al., 2011. Optical navigation and model based estimates of intracortical target sites and stimulation strength suggest that nTMS should provide spatially more precise cartography than using external landmarks for defining a motor ‘hot-spot’. The remaining outstanding challenges are (a) a high inter-and intra-subject variability of motor evoked potentials (MEP) Schmidt et al., 2009, Brasil-Neto et al., 1992, (b) the extent to which anatomical mapping relative to the underlying gyrification is preferable and (c) hemispheric functional asymmetries possibly reflecting functional “lateralization”. These factors are particularly relevant in a clinical context, where typically cartography will be used with single pulses per stimulation location. Further, the definition of a representation “area” is strongly confounded by the variability of “fringe neurons” or overlapping cortical representations (Schieber and Hibbard, 1993). The aim was to study the extent to which single pulse cartography can assure reproducible results. In the present analysis we focus on the concept of the “hot-spot”, i.e. point of maximum MEP and Center of Gravity (CoG). 15 subjects were examined in two sessions by two different researchers. Stimulation was performed using a computer based navigated TMS system with a figure-of-eight-coil (eXimia, Nextim Ltd., Helsinki, Finland). Mapping of the FDI over both hemispheres was performed with perpendicular (fine) and changing (random) coil orientation with an average of 79 (±35.91) stimuli at 110% resting motor threshold. Margins were defined as scalp locations where a stimulus resulted in an MEP <50 μV. To assess intersession variability we calculated the intraclass correlation coefficient (ICC), which is considered to reflect high reliability when higher than 75% (Mortifee et al., 1994). Stimulus location for the FDI-“hot-spot” in the dominant hemisphere showed a high intersession correlation (ICC: 81–87%). In the non-dominant hemisphere the ICC was lower (45–73%). There was no significant difference in ICC using intracortical instead of scalp stimulus location. Weighted “hot-spots” as expressed by the CoG showed a very high intersession reliability (ICC: 84–88%), the side of maximum MEP a rather poor intersession reliability (ICC: 9–72%). Perpendicular mapping was associated with significant higher test–retest correlation than random mapping (ICC for random mapping 22–38%). Stimulus locations (“hot-spots”) in the dominant hemisphere are more precise than in the non-dominant hemisphere. Intracortical locations are not superior to scalp location for “hot-spot”-measurements. Weighted “hot-spots” are more precise than maximal MEPs for “hot-spot”-definition and inter-rater reproducible. Perpendicular mapping is clearly spatially more accurate than random mapping.