# Equivalent efficacy of left versus right hemisphere accelerated intermittent theta burst stimulation for major depressive disorder

## TL;DR

Stimulating either the left or right side of the brain with a specific type of stimulation reduces depression symptoms equally, but affects brain networks differently.

## Contribution

Demonstrates equivalent clinical efficacy of left and right hemisphere accelerated iTBS for depression in older adults.

## Key findings

- Depression, anxiety, and anhedonia symptoms improved similarly with left or right DLPFC aiTBS.
- Right DLPFC aiTBS caused significant connectivity changes between brain networks, unlike left DLPFC aiTBS.
- Symptom improvement was maintained in patients switched to contralateral stimulation if initial progress was insufficient.

## Abstract

Intermittent theta burst stimulation (iTBS) to the dorsolateral prefrontal cortex (DLPFC) for major depression has been FDA-approved in the United States since 2018. Accelerated iTBS (aiTBS) protocols of multiple treatments per day have shown promising response and remission rates for major depression, especially when combined with connectivity-guided targeting. Brain networks associated with emotion regulation demonstrate significant changes in connectivity after effective iTBS. However, these findings have been confined to treatment of the left DLPFC, despite literature suggesting equivalent outcomes with right side stimulation. To date there has not been a direct comparison of clinical outcomes and connectivity changes between left and right DLPFC aiTBS for depression.

Forty-four patients aged 50–79 with chronic major depressive disorder underwent open-label accelerated fMRI-guided aiTBS (45 sessions, 9 days) to the DLPFC (18 left, 26 right). Depression, anxiety, and anhedonia symptoms were assessed, and resting-state fMRI was obtained at baseline (Visit 1), after 15 sessions (Visit 2), and end of treatment (Visit 3). Patients who were not demonstrating at least 10% improvement in depression at Visit 2 were switched to contralateral stimulation for the remaining 30 sessions.

For the entire cohort (N = 44), mean depression (IDS-C30) scores decreased significantly over the course of treatment. Mean change in IDS-C30, GAD-7, TEPS, SHAPS, and BISBAS between participants with right-sided stimulation (N = 26) and those with left-sided stimulation (N = 18) were not statistically different. Functional connectivity analysis demonstrated significant decreases in connectivity from Visit 1 to Visit 3 between default mode network and limbic networks in patients receiving right DLPFC iTBS, whereas patients receiving left DLPFC iTBS demonstrated limited changes in connectivity.

Accelerated iTBS to the right DLPFC appears to have equivalent efficacy as aiTBS to the left DLPFC in terms of magnitude of reduction of depressive, anxious, and anhedonic symptoms in a late-life population. However, connectivity changes associated with treatment were asymmetric, and may reflect hemispheric lateralization of functional network responses to iTBS. Further work is needed to confirm the comparative efficacy and network dynamics of left versus right hemisphere aiTBS for depression.