Treating Refractory Major Depressive Disorder With Repetitive Transcranial Magnetic Stimulation

Major Depressive Disorder (MDD) is one of the most prevalent mental illnesses in North America, affecting approximately 4% of Canadians annually. Though a number of effective treatments are available, as many as 15% of those diagnosed with a depressive disorder die by suicide, 30% fail to respond to treatment and approximately 60% experience a relapse. These statistics emphasize the need to optimize treatment response, as well as to understand the neurobiological mechanisms mediating MDD, in order to improve therapeutic outcome.

To date, few alternatives have been available for the treatment of refractory symptoms - one alternative is electroconvulsive therapy (ECT); however, this treatment is associated with significant side effects, most notably memory impairment. Also, ECT requires the use of a general anesthetic, and, thus, is a relatively more invasive procedure with an increased risk of complications. In addition, the stigma associated with ECT often limits its widespread acceptance as a treatment for depressive symptoms. rTMS has been shown to be an effective therapeutic tool for the treatment of several neuropsychiatric disorders, including MDD and schizophrenia. In MDD, two types of rTMS treatment protocols have been shown to be effective. These include high frequency (10Hz) rTMS applied to the left dorsolateral prefrontal cortex (DLPFC) (HFL) and low frequency (1Hz) rTMS applied to the right DLPFC (LFR). More recently, preliminary studies combining LFR rTMS with HFL rTMS - in effect, Bilateral rTMS - have shown this method to be safe, well-tolerated, and superior to using either stimulation protocol alone. However, other studies have demonstrated equivocal efficacy of rTMS treatment for MDD. Several methodological limitations, however, have tainted most treatment studies, precluding the ability to make definitive conclusions regarding the efficacy of rTMS for MDD. These limitations include: 1) small sample sizes; 2) a lack of adequate double-blind conditions; 3) a lack of adequate treatment duration; 4) biased randomization; 5) patient heterogeneity; 6) a lack of maintenance treatment protocols; 7) an unclear understanding of the parameters necessary to optimize treatment; and 8) insufficient understanding of the neurophysiological mechanisms mediating the therapeutic efficacy of rTMS treatment.

With this study, we intend to rectify these methodological limitations by: including a large sample of treatment refractory patients, who meet pre-established criteria for treatment resistance; excluding patients with comorbid Axis II psychopathology; developing and maintaining a randomized and double-blind protocol prior to study initiation; extending active rTMS treatment courses; evaluating 2 different treatment protocols; and evaluating whether the induction of CI mediates the therapeutic effects of rTMS on depressive symptoms.

With regard to the latter objective, several lines of evidence support our hypothesis regarding a mechanistic role of CI in the therapeutic effects of rTMS. First, ECT-mediated increases in EEG slow wave activity (SWA) and cortical GABA in patients with MDD suggest that enhanced CI is related to clinical improvement. Second, MDD is a disorder that has been associated with deficits in CI. Third, deficits in CI, as indexed through cortical GABA, were rectified by supplementing antidepressant medication. In addition, a core deficit in MDD - cognitive inhibition - is conceptually related to impaired CI. Cognitive inhibition refers to the ability to ignore or inhibit mental events. Those with MDD typically experience a pronounced difficulty shifting thoughts away from negative ideas. In fact, impaired cognitive inhibition for depressogenic thoughts and information has been proposed as a mechanism and/or risk factor underlying the development and maintenance of MDD. Research in our event-related potential (ERP) lab has examined the neurophysiological correlates of CI in healthy adults and in clinical groups. During the Stroop task, CI is associated with an increased negative voltage shift peaking between 400 and 500 milliseconds over the frontocentral region of the scalp, with a decreased positivity over the left parietal region, referred to as the N450 or N500. The experimental manipulation in the present study is distinct from our ongoing MDD-ERP work in that we now have the ability to examine changes in the N450 response following anticipated rTMS-induced improvements in CI. Thus, if rTMS does bring about improvements in CI, and CI is related to cognitive inhibition, this should be associated with normalization of the N450 response in MDD.

Objectives

1. To evaluate the efficacy of an acute course of rTMS to treat patients with treatment refractory MDD.
2. To evaluate which stimulus protocol demonstrates superior therapeutic efficacy.
3. To evaluate whether the induction of CI mediates the therapeutic effects of rTMS for treatment refractory MDD.
4. To evaluate the efficacy of a maintenance course of rTMS, for those who responded to rTMS treatment, in preventing the recurrence of depressive symptoms.
5. To measure whether changes in CI are associated with changes in cognitive inhibition related to emotional information, as measured by ERP and the Emotional Stroop Task.

Hypotheses

1. In the acute treatment phase, active rTMS will be effective in treating refractory MDD compared to sham treatment.
2. Bilateral and HFL rTMS will be shown to have superior therapeutic efficacy to sham rTMS, although Bilateral rTMS will be shown to have superior therapeutic efficacy compared to HFL stimulation.
3. The induction of CI will be shown to mediate the therapeutic effects of rTMS on refractory symptoms in patients with MDD.
4. Biweekly maintenance rTMS will be effective in preventing the relapse of depressive symptoms.
5. Prior to rTMS treatment, patients will exhibit diminished neurophysiological indices of cognitive inhibition, as measured by the N450 component of the ERP. If rTMS effectively increases CI, this increase should be associated with improvements in cognitive inhibition, as measured by normalization of the N450.