New targets for rTMS in depression: a review of convergent evidence

Abstract

Although rTMS is moving steadily into the mainstream as a treatment for medically refractory depression, its efficacy continues to lag behind that of more invasive neuromodulation treatments such as ECT or DBS. Here we review evidence to suggest that a fruitful, but neglected, strategy for improving rTMS efficacy may be to explore alternatives to the conventional stimulation target in the dorsolateral prefrontal cortex (DLPFC). The convergent evidence of lesion, stimulation, connectivity, and correlative neuroimaging studies suggests that the DLPFC may have a relatively peripheral role in mood regulation, at least compared to several alternative areas within the prefrontal cortex. In particular, we consider the evidence base in support of four new potential targets for rTMS in depression: dorsomedial prefrontal cortex (DMPFC), frontopolar cortex (FPC), ventromedial prefrontal cortex (VMPFC), and ventrolateral prefrontal cortex (VLPFC). Each of these regions enjoys broader support, from a more diverse evidence base, than the DLPFC in terms of its role in emotion regulation in major depression. We discuss the relative merits of each of these novel targets, including potential obstacles to stimulation using currently available technologies, and potential strategies for overcoming these obstacles. It is hoped that this detailed review will spur a more vigorous exploration of new targets for rTMS in depression. The use of new targets may help to propel rTMS across the threshold of efficacy required of a first-line treatment, to assume a more widespread role in the treatment of depressed mood.