Abstracts

Rationale: Over the last decades, transcranial magnetic stimulation (TMS) has been used to assess cortical excitability in epilepsy. Longitudinal studies can be used to monitor disease course over time, or to evaluate effects of anti-epileptic therapy. To detect changes in excitability, it is important to validate the repeatability of TMS outcomes for the same subject between different sessions. Previous studies in healthy subjects found mixed degrees of repeatability, ranging from poor to good agreement (Badawy 2012; Du 2014). Especially, the long intracortical inhibition (LICI) curve showed a large variability and poor reliability across sessions (Du 2014). In this study we evaluate the repeatability of LICI in healthy subjects using paired pulse TMS. Methods: Twenty-eight healthy subjects (9 males, mean age 28.78.4 yrs.) were studied two times (1 week apart) with paired pulse TMS combined with electromyography. Both motor hot spots (abductor digiti minimi) were stimulated with 50 paired pulses (intensity 120% of resting motor threshold) at interstimulus intervals (ISIs): 50, 100, 150, 200, 250 and 300 ms. In twenty subjects a figure-of-eight coil was positioned and held in place manually during both sessions, while in eight subjects a robotic arm was used. The concordance correlation coefficient (rho-c) was used to estimate the agreement between repeated sessions (Lin 1989). Results: The average LICI curve among all subjects showed a close agreement between both TMS sessions for manual (rho-c = 0.91) and robot-guided (rho-c = 0.94) positioning, see figure 1. However, the repeatability decreased when individual LICI curves were compared; manual (rho-c = 0.72) and robot-guided (rho-c = 0.86). There was a large variation in repeatability between subjects, ranging from poor to good levels of agreement, see figure 2. Individual ISIs also showed a large variation in repeatability, ranging from poor to good levels: manual (range rho-c: 0.32-0.72) versus robot-guided (range rho-c: 0.62-0.89). Conclusions: Repeatability of LICI was highly variable between healthy subjects. Good levels of agreement were found for analysis on group levels, but reproducibility decreased for the level of individual subjects or ISIs. Overall, repeatability of the LICI curve was higher for robot-guided than for manual coil positioning. References? Badawy et al. Epilepsy Res 2012;98:182-186? Du et al. J Mot Behav 2014;46-1:39-48 ?Lin. Biometrics 1989;45:255-268 Funding: This work was financed by the Dutch TWIN foundation for neuromodulation.