Theta-Burst Stimulation to Treat Depression

Major Depressive Disorder (MDD) is a mental illness affecting millions of individuals worldwide and in Canada, and is a leading cause of morbidity, mortality, and disability. While antidepressant medications are effective in treating MDD, their efficacy is moderate and systemic side-effects persist, such as sexual dysfunction, drowsiness, weight gain, and dry mouth. Thus, more effective treatments are needed for MDD. Neuroimaging techniques have implicated the dysregulation of brain plasticity in depression. In particular, long-term potentiation (LTP)-like activity in the dorsolateral prefrontal cortex (DLPFC) and the motor cortex is known to be impaired in MDD. As such, transcranial magnetic stimulation (TMS)-based interventions, which aim to modify underlying cortical activity, are now established treatments of depression. Intermittent theta-burst stimulation (iTBS), a novel form of repetitive TMS (rTMS) approved by the US Food and Drug Administration (FDA) for the treatment of depression, delivers intermittent, high-frequency theta bursts. It has been demonstrated to induce sustained plasticity in the DLPFC and the motor cortex. Studies have shown that iTBS is equally effective as conventional rTMS in terms of response rates, and its adverse effects are comparable to iTBS-sh and active rTMS. One key advantage of iTBS over rTMS is its time efficiency, with each session lasting approximately 3 min compared to up to 40 min with rTMS. Notwithstanding its efficiency, systematic reviews of RCTs indicate no significant difference in remission rates - defined as a reduction in symptoms below a threshold indicating euthymic state - between iTBS (\~26%) and iTBS-sh (\~19%), or between iTBS (32%) and rTMS (27%). Thus, while iTBS is well tolerated, efficient and effective in reducing symptoms of MDD, its efficacy is still far from optimal as is the case for other treatments of depression. Based on promising research conducted in the hippocampus of rodents, the investigators believe that modifying some parameters of the iTBS protocol may be more effective in inducing plasticity than the currently used iTBS protocol. Thus, in this trial researchers aim to determine whether an optimized iTBS protocol will result in better LTP-like activity in the DLPFC of adults with MDD. The objectives and hypotheses of the study are as follows: Objective 1: To compare the ability of iTBS-c vs. iTBS-s to induce DLPFC LTP-like activity in depressed adults as measured using TMS-electroencephalography (EEG). Hypothesis 1a: iTBS-s will induce stronger DLPFC LTP-like activity compared to iTBS-c. Hypothesis 1b: iTBS-s will induce longer-lasting DLPFC LTP-like activity compared to iTBS-c. Objective 2: To evaluate the relationship between DLPFC LTP-like activity and changes in depression severity, as measured by the Montgomery-Asberg Depression Rating Scale (MADRS). Hypothesis 2a: DLPFC LTP-like activity will be inversely associated with baseline depressive symptoms. Hypothesis 2b: DLPFC LTP-like activity will be associated with improvement in depressive symptoms on Visits 3 to 5. Objective 3: To compare the effect of iTBS-c vs. iTBS-c within-subjects on DLPFC LTP-like activity in depressed adults as measured using TMS-EEG during the cross-over phase of the study. Hypothesis 3: iTBS-s will induce stronger DLPFC LTP-like activity compared to iTBS-c within-subjects.