Tyler L. Danielson1, Shawn M. Reese1, Claire M. Smith1, Jason M. DeFreitas1

1Oklahoma State University, Stillwater, Oklahoma

Transcallosal conduction time (TCT) is a relatively new assessment of how well the brain’s 2 hemispheres communicate with each other. TCT represents the time for a volley of action potentials to go from the cortex of one cerebral hemisphere to the contralateral cortex, via the corpus callosum (CC). The CC is the only connection between the 2 hemispheres within the mammalian brain. TCT can be calculated from transcranial magnetic stimulation (TMS) by recording the ipsilateral silent period (iSP) onset latency and the motor evoked potential (MEP) onset latency from a target muscle. This variable is currently being used in clinical populations to evaluate progression of disease within the CC, with little known regarding TCT’s reproducibility and variability. PURPOSE: The purpose of this study was to evaluate the reproducibility of these measures (iSP latency, MEP latency, and TCT) at the abductor pollicis brevis muscle (APB) within a healthy population and with the use of single-pulse TMS. METHODS: A figure-eight TMS coil was used to stimulate both the left and right primary motor cortices in 10 healthy subjects (20-38yrs). This cortical stimulation was given contralateral and ipsilateral for both APB muscles. Ten stimulations were given over each hemisphere and recorded at each hand, resulting in 40 total. This was repeated 48-72 hrs later by the same investigator. TCT was calculated for each trial where both an iSP and MEP latency could be established to acquire within-subject coefficients of variation (CV%). Reproducibility for TCT was calculated using intraclass correlation coefficients (ICC3,1). RESULTS: The CV% of TCT ranged from 18.96 to 122.23%. This was likely due to the higher variability of the iSP latency (range: 8.4-76.6%) than the MEP latency (range: 1.6-12.9%). The resulting ICC values of TCT were -0.05 and -0.12 for the left and right hands, respectively. CONCLUSION: The resulting CV% and ICC values for TCT calculations showed that the measures were highly variable and not reproducible. The variation among iSP latencies was notably higher with a wider range than that of the MEP latencies. The lack of reproducible TCT could be due to error in the measurement of iSP latencies, or possibly due to variations within the body between testing days. This should be investigated further before continued clinical use.

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