Repetitive Transcranial Magnetic Stimulation (rTMS) for Treatment Resistant Depressive Disorder

Several studies have demonstrated that rTMS is an efficacious treatment for treatment resistant major depressive disorder (TRD). However, recent meta-analyses, and more recent, large, multi-centre studies, have provided evidence suggesting that rTMS, at best, provides modest therapeutic efficacy compared to sham stimulation.

Several reasons may account for this modest therapeutic effect. First, the majority of these studies involved left-sided treatment alone to the dorsolateral prefrontal cortex (DLPFC), which is a significant limitation when considering that electroconvulsive therapy - another form of brain stimulation used in TRD - has been shown to be less efficacious when used unilaterally, compared to bilaterally.

Second, sub-optimal methods were utilized to target the DLPFC (i.e., '5-cm anterior method'), limiting the treatment potential of what is inherently a targeted form of treatment. In this regard, recent data from the investigators suggests that treatment directly targeting the DLPFC provides enhanced therapeutic efficacy compared to the '5-cm anterior' method.

Third, treatment durations were typically short (i.e., 2-4 weeks). Fourth, stimulation intensity may have been insufficient by not taking into consideration coil-to-cortex distance, which is of particular importance when considering that this parameter may contribute significantly to rTMS-induced antidepressant response. Fifth, there has not been a study that has examined TRD across the lifespan in a way that addresses the differences between older and younger adults. Therefore, the investigators propose to conduct a study evaluating the efficacy of rTMS for TRD that directly addresses all 5 of these major limitations.

This study will compare bilateral rTMS to unilateral rTMS and will involve targeting the DLPFC using cortical co-registration techniques, as well as treatments of optimal duration (i.e., up to 6 weeks) and intensity parameters (i.e., adjusted for coil-to-cortex distance). The use of distance cortical co-registration and adjustment for coil-to-cortex distance addresses the major limitation (lack of stimulation intensity to compensate for age-related prefrontal atrophy) in studies that have examined rTMS in an elderly sample. Preliminary data from this research group provide compelling evidence that rTMS may, indeed, be effective when some of these limitations are optimized in both younger and older adults. Finally, it is also essential that research investigate the mechanisms of therapeutic efficacy, so that increases in understanding can be translated into enhanced treatment. For several reasons, cortical excitability may represent a neurophysiological process through which the therapeutic effects of rTMS are mediated. Recent advances in electroencephalography (EEG) technology now permit direct measurement of excitability from the DLPFC; thus, we are now able to ascertain whether these are mechanisms through which the therapeutic effects of rTMS in TRD are mediated.