Objective: To evaluate if transcranial magnetic stimulation (TMS) measures correlate with disease severity and predict functional decline in frontotemporal dementia (FTD) phenotypes. Methods: Paired-pulse TMS was used to investigate the activity of different intracortical circuits in 171 FTD patients (122 bvFTD, 31 avPPA, 18 svPPA) and 74 healthy controls. Pearson's correlations were used to analyze the association between TMS measures and disease severity, while multiple regression analysis was used to identify the best clinical or neurophysiological measure to predict functional decline at 12 months. Results: We observed significant strong correlations between TMS measures [short interval intracortical inhibition-facilitation (SICI-ICF) and long interval intracortical inhibition (LICI)], and disease severity (evaluated with the FTLD-CDR) (all r > 0.5, p < 0.005). SICI-ICF, short interval intracortical facilitation (SICF) and LICI were also significant predictors of functional decline, evaluated as the change in FTLD-CDR scores at 12 months (all p < 0.005), while at the stepwise multiple regression analysis, SICI was the best predictor of disease progression, accounting for 72.5% of the variation in FTLD-CDR scores at 12 months (adjusted R2 = 0.72, p < 0.001). Conclusions: The present study has shown that the dysfunction of inhibitory and facilitatory intracortical circuits, evaluated with TMS, correlates with disease severity and progression, accurately predicting functional decline at 12 months, better than any other investigated marker.
