Transcranial direct current stimulation for the reduction of clinical and experimentally induced pain: A systematic review and meta-analysis

Objectives: To evaluate the effectiveness of transcranial direct current stimulation on clinical and experimental pain, and to identify the most beneficial stimulation parameters. Methods: Predefined search using key terms of information sources including: MEDLINE, EMBASE, CAB Abstracts, and PsychINFO, Cochrane Register of Controlled Trials, CINAHL, and PeDRO databases; reference lists of retrieved articles, journal contents, and conference proceedings. Two reviewers independently searched and evaluated publications. English and non-English controlled trials that applied direct current stimulation to the brain published before September 30, 2010 were included. Studies using magnetic stimulation or pulsed currents were excluded. Results: Trials investigating experimental pain in healthy participants (n=6) used a wide variety of stimulation and outcome parameters that did not allow a synthesis across outcome parameters. Trials investigating chronic pain (n=8) used anodal motor cortex stimulation of 1 or 2 mA intensity, either as a single dose or on a maximum of 10 consecutive days. Four trials on chronic pain were excluded due to a high risk of bias. A meta-analysis of 4 trials on chronic pain found a pooled effect size of -2.29 with a 95% confidence interval of -3.5 to -1.08. This effect does just reach minimal clinically important difference recommendations. Discussion: The level of evidence for the efficacy of transcranial direct current stimulation in experimental and chronic pain reduction is low. Evidence from high quality randomized controlled trials is required before this treatment should be recommended.