Short-interval intracortical inhibition: Comparison between conventional and threshold-tracking techniques

BACKGROUND Short-interval intracortical inhibition (SICI) is conventionally measured as the relative amplitude reduction of motor evoked potentials (MEPs) by subthreshold conditioning stimuli. In threshold-tracking SICI (T-SICI), stimulus intensity is instead adjusted repeatedly to maintain a constant MEP and inhibition is measured as the relative threshold increase. T-SICI is emerging as a useful diagnostic test, but its relationship to conventional amplitude SICI (A-SICI) is unclear. OBJECTIVE To compare T-SICI and its reliability with conventional A-SICI measurements. METHODS In twelve healthy volunteers (6 men, median age 30 years), conventional and T-SICI were recorded at conditioning stimuli (CS) of 50-80% resting motor threshold (RMT) and interstimulus interval of 2.5 ms. Measurements were repeated on the same day and at least a week later by a single operator. RESULTS Across the CS range, mean group T-SICI showed a strong linear relationship to the mean group values measured by conventional technique (y = 29.7-0.3x, R2 = 0.99), but there was considerable interindividual variability. At CS 60-80% RMT, T-SICI had excellent intraday (intraclass correlation coefficient, ICC, 0.81-0.92) and adequate-to-excellent interday (ICC 0.61-0.88) reproducibility. Conventional SICI took longer to complete (median of 5.8 vs 3.8 min, p < 0.001) and tended to have poorer reproducibility (ICC 0.17-0.42 intraday, 0.37-0.51 interday). With T-SICI, smaller sample sizes were calculated for equally powered interventional studies. CONCLUSION The close relationship between conventional and T-SICI suggests that both techniques reflect similar cortical inhibitory mechanisms. Threshold-tracking measurements of SICI may be able to improve reproducibility, to shorten acquisition time and to reduce sample sizes for interventional studies compared with the conventional technique.