Investigating EEG biomarkers of clinical response to low frequency rTMS in depression

Background: Repetitive transcranial magnetic stimulation (rTMS) is an effective intervention for major depressive disorder (MDD). Completing a full treatment course, however, is costly and time-consuming. Biomarkers of clinical outcome such as baseline resting-state brain activity measured with electroencephalography (EEG) may spare people futile treatment and conserve limited clinical resources. Additionally, investigating changes in EEG power post-treatment could provide insights into the working mechanism of rTMS. Methods: 39 MDD patients received 6 daily sessions of accelerated low-frequency (LF) rTMS over the right dorsolateral prefrontal cortex (DLPFC) for 5 days followed by a tapering course of 25 once-daily sessions. Resting-state EEG and heart rate (HR) measures were acquired immediately before and after a single rTMS session at 3 different timepoints: baseline, one week after the final accelerated session, and upon completion of the tapering course. The primary clinical outcome measure was the Beck Depression Inventory II (BDI-II). Results: High relative baseline theta power in prefrontal areas and high baseline HR were associated with poorer clinical outcome. HR decreased acutely at the beginning of the patients’ first rTMS session but this effect was not associated with treatment outcome. Limitations: The main limitations were small sample size and a lack of sham and healthy control group. Conclusion: Our results suggest that high relative theta power at baseline may be a marker of poorer response to right-sided LF rTMS. If validated, this easily applicable measure could inform rTMS protocol choice for the individual, thereby potentially speeding up patient recovery and saving clinical resources.