Which component of TMS-induced electric field contributes to the antidepressant effects?

Abstract Background: Recent research suggests that the diversity of the transcranial magnetic stimulation (TMS) induced electric fields (EF) in the brain due to individual differences in head morphology may partly explain the response variation to TMS. Objective: We aimed to investigate the relationship between TMS-induced EF characteristics and antidepressant effects in patients with treatment resistant depression (TRD). Methods: Data from twelve TRD patients who received 3 weeks of daily sequential bilateral theta burst stimulation (TBS) were included for analysis. TMS-induced EF were computed in the finite element models created based on each subject’s head. Left and right stimulation regions in the DLPFC were chosen as regions of interest (ROIs). The strength (||E||), tangential (E||), and normal (E⊥) component of EF were calculated as the sum of the respective average values within the left and right ROI to simulate the real stimulation “dose” that patients received. The magnitude of E⊥ component (|E⊥|) was defined as the sum of the average absolute value of both left and right E⊥ inside an ROI. Results: Generalized linear mixed model showed a significant interaction effect between time and |E⊥|sum on The Inventory of Depressive Symptomatology, Clinician Rating (IDS-C) scores (F=8.911, P=0.01), but not for other EF characteristics. Pearson correlation analyses confirmed that the change in IDS-C scores was only significantly correlated with |E⊥|sum component (r=-0.752, P=0.005). Conclusions: Our results indicate that the magnitude of E⊥ component of the TMS-induced EF is associated with the antidepressant response to bilateral sequential TBS treatment in patients with TRD. Future studies using a tailored TMS treatment approach may consider maximizing the |E⊥| component in order to enhance clinical effects. Research Category and Technology and Methods Translational Research: 10. Transcranial Magnetic Stimulation (TMS) Keywords: theta burst stimulation, electric field modeling, major depressive disorder, disorder, finite element method