Exploring Non-invasive Brain Stimulation Effects on Physical Outcomes in People With Parkinson's Disease: An Umbrella Evidence Mapping Review With Meta-analyses.

The authors pooled the numbers published in existing reviews rather than going back to the original trials or re-running the analyses from raw data, so any errors in those reviews carry through. The reviews included were only moderate quality on average, 18 of the 41 were rated low quality, and only 4 high. Most failed to list the studies they excluded and half did not examine how publication bias might have affected their results. The headline motor result came with enormous variability between studies, which is one reason the effect fell from moderate to small once three studies at high risk of bias were removed. Only one meta-analysis compared stimulation during 'on' versus 'off' medication states, and most reviews either pooled both states together or looked only at people in the 'on' state, so whether the medication state changes the effect is unknown. The freezing of gait result rests on five meta-analyses with small trials and conflicting findings, one showing a large benefit and another showing none. No studies directly compared early versus advanced Parkinson's disease, so the findings say nothing about which stage might benefit most. Only English-language reviews were searched, so relevant reviews in other languages were missed. Stimulation site, dose and technique varied widely across the included reviews, so the pooled figures describe the general idea of stimulation rather than any particular protocol.

The authors declared no competing interests and said they received no funding for the research, writing or publication.

Do not take the pooled motor improvement as a reliable effect size. It fell from moderate to small (SMD −0.80 to −0.37) and heterogeneity collapsed once three studies at high risk of bias were removed, and the reviews it pools were only moderate quality on average. Do not assume stimulation helps balance either: the pooled result for static balance and postural control was null. The contribution of stimulation to walking cannot be separated from the medication and exercise it was usually given with.