Review of: Razza LB et al, Depress Anxiety 2020;37(7):594–608

Study Type: Meta-analysis of randomized sham-controlled trials

Transcranial direct-current stimulation (tDCS) is a neuromodulatory intervention with putative benefits in depression. The device consists of a headband with two electrodes that delivers a weak electrical current across the brain, generated by a smartphone-sized stimulator. These devices have regulatory approval for depression in Europe, Australia, and Mexico, and have gained popularity in the US among patients who appreciate their low cost (around $500), limited side effects, and ability to self-administer the treatment. tDCS is well tolerated with commonly reported side effects of tingling sensations, headache, fatigue, and scalp erythema. Psychiatrists, however, have been reluctant to adopt tDCS because of concerns over efficacy. The available studies have been few with small sample sizes and mixed results, including one where escitalopram outperformed tDCS. This new meta-analysis aggregated data from available tDCS trials to assess its efficacy in depressive symptoms.

The authors identified 23 randomized, sham-controlled trials with a total of 1,092 patients who were assigned to either active or sham tDCS. Two of the studies were large (> 100 patients), three included bipolar depression, and eight focused on treatment-resistant depression. All studies used anodal tDCS, which uses a positive-ion current as opposed to the negative ions of cathodal stimulation. Anodal stimulation depolarizes the neurons while cathodal stimulation hyperpolarizes the neurons. The treatments were delivered over the left dorsolateral prefrontal cortex at current intensities between 0.5 and 2.5 mA with subjects receiving 5 to 20 treatments lasting 10 to 30 minutes per session. The primary outcome was the effect size for depression scores. Secondary analyses included response and remission rates.

Compared to sham, the active tDCS group showed an improvement in depression scores with a moderate effect size (0.46). Response (33.3% vs 16.56%; number needed to treat of 6) and remission (19.12% vs 9.78%; number needed to treat of 11) rates were also better with active tDCS.

The main limitation of these findings is the difficulty of lumping together many small studies, each with its own design that could influence the sham or placebo response. Another limitation was that most studies evaluated outcomes after a few weeks, but the full effects of tDCS may not be seen for a few months.

TCPR’s Take
tDCS is a promising treatment for depression, but we still need large, well-designed controlled trials to be confident in its effects. While we await more evidence, we can consider tDCS selectively for patients who decline, don’t tolerate, or don’t respond adequately to antidepressants. tDCS is also a convenient, well-tolerated option for patients who are wary of other neuromodulatory interventions like ECT and TMS.