Review articleCognitive functioning and deep transcranial magnetic stimulation (DTMS) in major psychiatric disorders: A systematic review
Introduction
Brain stimulation methods have gained popularity and acceptance as promising non-pharmacological interventions to treat various psychiatric disorders. One example of such a method is a non-invasive, repetitive transcranial magnetic stimulation (rTMS), which has consistent antidepressant properties during and beyond the daily treatment phase (Berlim et al., 2014b, Kedzior et al., 2014, Kedzior et al., 2015c). RTMS is typically administered using figure-of-eight coils, which can stimulate selective, but relatively superficial, cortical regions (Zangen et al., 2005). A novel alternative to rTMS is a non-invasive, deep transcranial magnetic stimulation (DTMS), which is delivered using the H-coil system (Roth et al., 2002). Unlike the figure-of-eight coil, the H-coil stimulates wider, and presumably deeper, brain regions (Roth et al., 2014, Zangen et al., 2005). According to a randomised-controlled trial (RCT) (Levkovitz et al., 2015) and two meta-analyses (Kedzior et al., 2015a, Kedzior et al., 2015b), high-frequency DTMS appears to have short-term antidepressant and anxiolytic properties in the treatment of major depression. However, it remains unclear if this therapeutic intervention is also able to improve the cognitive impairments associated with major depression (Rock et al., 2014) and other psychiatric disorders. To date, only one systematic review (Minichino et al., 2012) addressed this issue. According to this review, high-frequency DTMS is associated with small, short-term improvements in sustained attention and cognitive planning as well as larger improvements in spatial and visuospatial memory based on data from two open-label studies including patients with treatment-resistant, medication-free, unipolar depression (Minichino et al., 2012). Similar conclusions regarding the putative cognitive effects of the conventional, high-frequency rTMS have been drawn in two other systematic reviews (Guse et al., 2010, Serafini et al., 2015). Indeed, both reviews reported consistent trends towards improvement in some cognitive functions, such as visuospatial memory and verbal memory, especially in studies including patients with major depression (Guse et al., 2010, Serafini et al., 2015). Given the growing interest in the application of DTMS in psychiatry and the increasing number of published studies, it is important to assess the magnitude of its cognitive effects. Therefore, the aim of the current study was to systematically review the literature on the cognitive effects of DTMS applied with the H-coil system in major psychiatric disorders.
Section snippets
Material and methods
A systematic literature search was conducted in PsycInfo and PubMed databases from any date until December 7, 2015 (Table S1 in the online supplementary materials). A total of 32 relevant studies (with terms “deep transcranial magnetic stimulation”, DTMS, or H-coil in titles and “cognitive” or “cognition” in text) were identified from the electronic search and the hand search of bibliographies of these relevant studies.
Study and participant characteristics
The majority of the 13 studies were performed in Israel (k = 7), where the DTMS system was developed, followed by Canada and Italy (k = 2 each), and the USA and Australia (k = 1 each; Table 1). Most studies with clinical samples have used open-label or single-blind designs and did not include sham-control groups (Table 1) except for one RCT with schizophrenia patients (Rabany et al., 2014). Three studies were case reports with one patient each (Table 1).
Of the 13 studies, 10 included clinical
Discussion
Results of the current study suggest that 20 sessions of high-frequency DTMS might improve cognitive deficits associated with major depression and, possibly, schizophrenia. In contrast, DTMS does not seem to affect cognitive functioning in healthy adults. Although these results are based on a qualitative synthesis of outcomes in mostly uncontrolled studies with relatively small samples, a consistent worsening of cognitive functioning was not observed after DTMS in any of the 13 studies included
Conclusions
High-frequency DTMS tends to improve cognitive performance particularly in studies with major depression patients. RCTs with sham control groups are required to assess the magnitude of cognitive effects of DTMS compared to placebo in major psychiatric disorders. Furthermore, durability of the cognitive effects needs to be investigated beyond the daily stimulation phase.
Conflicts of interest
Dr. Kedzior, Ms. Gierke, and Ms. Gellersen have nothing to disclose. Dr Berlim has received a researcher-initiated grant from Brainsway, Inc., to study the neural basis of DTMS in MDD using neuroimaging.
Authors' contributions
KKK and HMG conceptualised the study; LG and KKK performed the search; KKK, LG, HMG assessed the studies and extracted data; all authors contributed to writing of the manuscript.
Role of funding source
There was no external funding for this study.
Acknowledgements
None.
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