Non-invasive brain stimulation enhances motor and cognitive performances during dual tasks in patients with Parkinson's disease: a systematic review and meta-analysis.

Only four studies fed the cognitive result, and the authors themselves note that data synthesis is usually considered more trustworthy with at least five studies. Seven of ten studies gave a single session of stimulation, so the results describe a short-lived change rather than something that lasts. Randomisation and blinding were poorly reported: four studies did not describe how participants were allocated or whether the people delivering and assessing treatment were blinded, and the review rated the randomisation process, deviations from the intended intervention, and outcome measurement as raising some concerns. The subgroup that carried the headline, stimulation over the dorsolateral prefrontal cortex, showed evidence of publication bias in the motor analysis (P = 0.003), and the motor cortex analysis did too (P < 0.001), so the difference between the two targets should be read with care. All participants had mild to moderate Parkinson's disease with relatively normal cognition, so the findings say nothing about people with more advanced disease or dementia. No rTMS study showed benefit, and the authors state their results are limited to tDCS effects. In six studies stimulation was given alongside another intervention such as treadmill walking, dual-task walking or physical therapy, so how much of the improvement came from the stimulation itself cannot be separated from the training.

The article text supplied contains no funding statement and no conflict-of-interest declaration, so it is not possible to say who paid for this work or what the authors declared.

Do not read this as 'brain stimulation works for dual-task problems in Parkinson's'. The average effect was small, most studies gave only a single session, only four studies contributed to the cognitive result, and stimulation aimed at the motor cortex showed no benefit at all. The subgroup that carried the headline, front-of-brain stimulation, also showed signs of publication bias.