In most transcranial magnetic stimulation (TMS) studies stimulus intensity is kept constant and evoked muscular response amplitudes are measured and averaged. In threshold-tracking (TT) TMS, stimulation intensity is measured and continuously adjusted to maintain a predefined target response, thus monitoring and controlling the biological fluctuations in corticospinal excitability and technical factors. Although TT-TMS is emerging as a useful diagnostic tool (Menon et al. 2015), little is known about its reliability and comparability with conventional techniques. Our aim was to evaluate the reproducibility of TT-TMS for short interval intracortical inhibition (SICI) and compare it with standard constant stimulus measurements.
SICI was measured in 12 healthy male volunteers (24±4 years) from the intrinsic muscles of the dominant hand using an automated stimulation protocol incorporating (i) conventional, fixed-intensity (A-SICI) and (ii) threshold tracking (T-SICI) methods at conditioning stimuli (CS) of 50-80% of resting motor threshold (RMT) at a paired-pulse interval of 2.5 ms and interstimulus intervals of 4.1 s. Baseline parameters of corticospinal excitability were determined by threshold tracking: RMT was defined as a stimulation intensity required to evoke a response of 0.05 mV (for A-SICI) or 0.2 mV (for T-SICI), test stimulus (TS) intensity was set to evoke a response of 1 mV. Measurements were repeated four times at approximately 9-10 day intervals by a single operator.
No significant differences in mean RMT, TS, A-SICI and T-SICI were observed between sessions. RMT and TS, measured by threshold-tracking technique, showed excellent reproducibility over four sessions. SICI parameters, obtained by threshold tracking, showed significantly smaller between- and within subject variability compared to constant stimulus technique (between-subject coefficient of variation (CV) 31-62% for A-SICI and 5-10% for T-SICI; within-subject CV 22-47% for A-SICI and 3-7% for T-SICI). A-SICI measurements had adequate reproducibility at CS of 60% and 70% RMT (intraclass correlation coefficient (ICC) 0.448 and 0.654, respectively), while T-SICI had adequate reproducibility at all CS intensities (ICC 0.49-0.715). The A-SICI protocol took approximately 9 minutes, while the T-SICI protocol – 5 minutes to complete.
In conclusion, SICI measured by threshold tracking shows good interday reproducibility and significantly smaller variability compared to conventional techniques. Automation of the procedure and allowance for naturally occurring fluctuations in cortical excitability by threshold tracking appear to increase the reproducibility of SICI measurements.
