F147. Absolute and relative reliability of short-interval intracortical inhibition measured by threshold tracking transcranial magnetic stimulation

Abstract

Introduction Paired-pulse transcranial magnetic stimulation (TMS) is a useful tool to examine brain function. The technique relies on measuring the size of the motor evoked potential (MEP) generated using a fixed test stimulus, although MEP amplitude can vary by up to twofold per trial. TMS measures have shown significant within- and between-subject variability in ‘constant stimulus’ paired-pulse paradigms, raising concerns about reliability and potentially limiting clinical utility. As such, the present study was undertaken to assess whether threshold tracking TMS (TT-TMS) may overcome problems due to the variability of MEP amplitude. The aim of this study is to describe the absolute and relative reliability of short-interval intracortical inhibition (SICI) using TT-TMS. Methods TT-TMS studies were performed on twenty-six healthy subjects over three sessions (two recordings on the same day and one seven days apart), with MEPs recorded over abductor pollicis brevis. Absolute and relative reliability was established by calculating the standard error of the measurements (SEM), minimal detectable change (MDC) and intraclass correlation coefficient (ICC). Results No significant differences in mean resting motor threshold (RMT), SICI at 1 ms, SICI at 3 ms, and averaged SICI were observed between sessions. Averaged SICI presented the lowest SEM and highest ICCs for both within-day and between-day comparisons. Specifically, averaged SICI presented an excellent within-day (ICC = 0.88) and between-days (ICC = 0.95) reproducibility. SEM and MDC were large in all paired-pulse TT-TMS variables, setting a limit to the ability to track small changes in longitudinal studies. Additionally, SICI at 1 ms showed a higher SEM than SICI at 3 ms, suggesting different physiological processes between these two periods. The absolute and relative reliability was higher for tests undertaken seven days apart compared to tests performed on the same day. Conclusion Averaged SICI is the most reproducible variable across paired-pulse TT-TMS measures, showing an excellent ICC. TT-TMS measures demonstrated reduced variability when repeat tests were undertaken at the same time over different days compared to repeated testing at different times on the same day. It is recommended that changes in cortical excitability should be measured and averaged over a number of interstimulus intervals to further reduce variability.