Short Interval Intracortical Inhibition Responses to Low-Frequency Repetitive Transcranial Magnetic Stimulation Under Multiple Interstimulus Intervals and Conditioning Intensities.

BACKGROUND: The extent to which short interval intracortical inhibition (SICI) responds to low-frequency repetitive transcranial magnetic stimulation (rTMS) remains inconclusive with reports of increased, decreased and unchanged response following modulation. The aim of this study was to systematically investigate if the variability of SICI following rTMS is explained by the interstimulus interval (ISI) and/or the conditioning stimulus intensity (CSI). METHODS: Two experiments with pretesting/posttesting and an rTMS session (1 Hz, 90% RMT, 900 pulses) were done. Experiment I (N = 15): SICI with multiple ISIs (1.0-4.0 msec, 0.2 msec increment). Experiment II (N = 15): SICI with CSIs (50-95% of RMT, 5% increment). In both experiments, the cortical silent period (cSP) was also collected. RESULTS: After low-frequency rTMS, no significant change (p > 0.10) in SICI at any specific ISI or CSI was observed, nor did the optimal ISI or CSI change. However, a significant decrease was observed in SICI responses when assessed under the range of ISIs (p = 0.0001), but not CSIs. cSP inhibition increased significantly (p < 0.0015) for both experiments. CONCLUSIONS: The optimal ISI or CSI did not shift or reveal SICI changes after inhibitory rTMS. However, when the whole curve of SICI responses were evaluated from a wide range of ISIs, a decrease in inhibition was found. The contrast between the results of individual ISI tests and the wide range of ISI assessment may be due to higher intersubject variability of SICI and/or sample size, rendering traditional SICI testing methods ineffective for measuring changes in inhibition. Further, it is possible that rTMS modulates GABAA and GABAB mediated inhibitory processes differently, which would explain the conflicting results for SICI and cSP.

EMG breakthrough during cortical silent period in congenital hemiparesis: a descriptive case series.

BACKGROUND: The cortical silent period is a transient suppression of electromyographic activity after a transcranial magnetic stimulation pulse, attributed to spinal and supraspinal inhibitory mechanisms. Electromyographic breakthrough activity has been observed in healthy adults as a result of a spinal reflex response within the cortical silent period. OBJECTIVES: The objective of this case series is to report the ipsilesional and contralesional cortical silent period and the electromyographic breakthrough activity of 7 children with congenital hemiparesis. METHODS: TMS was delivered over the ipsilesional and contralesional primary motor cortices with resting motor threshold and cortical silent period measures recorded from first dorsal interosseous muscle. RESULTS: Seven children (13±2 years) were included. Ipsilesional and contralesional resting motor thresholds ranged from 49 to 80% and from 38 to 63% of maximum stimulator output, respectively. Ipsilesional (n=4) and contralesional (n=7) cortical silent period duration ranged from 49 to 206ms and 81 to 150ms, respectively. Electromyographic breakthrough activity was observed ipsilesionally in 3/4 (75%) and contralesionally in 3/7 (42.8%) participants. In the 3 children with ipsilesional breakthrough activity during the cortical silent period, all testing trials showed breakthrough. Contralesional breakthrough activity was observed in only one of the analyzable trials in each of those 3 participants. The mean peak amplitude of breakthrough activity ranged from 45 to 214µV (ipsilesional) and from 23 to 93µV (contralesional). CONCLUSION: Further research is warranted to understand the mechanisms and significance of electromyographic breakthrough activity within the cortical silent period in congenital hemiparesis. Understanding these mechanisms may lead to the design of tailored neuromodulation interventions for physical rehabilitation. TRIAL REGISTRATION: NCT02250092 (https://clinicaltrials.gov/ct2/show/NCT02250092).