Strategies for replacing non-invasive brain stimulation sessions: recommendations for designing neurostimulation clinical trials.

INTRODUCTION: Despite the potential impact of missed visits on the outcomes of neuromodulation treatments, it is not clear how this issue has been addressed in clinical trials. Given this gap in the literature, we reviewed articles on non-invasive brain stimulation in participants with depression or chronic pain, and investigated how missed visits were handled. Areas covered: We performed a search on PUBMED/MEDLINE using the keywords: 'tDCS', 'transcranial direct current stimulation', 'transcranial magnetic stimulation', 'depression', and 'pain'. We included studies with a minimum of five participants who were diagnosed with depression or chronic pain, who underwent a minimum of five tDCS or TMS sessions. A total of 181 studies matched our inclusion criteria, 112 on depression and 69 on chronic pain. Of these, only fifteen (8%) articles reported or had a protocol addressing missed visits. This review demonstrates that, in most of the trials, there is no reported plan to handle missed visits. Expert commentary: Based on our findings and previous studies, we developed suggestions on how to handle missed visits in neuromodulation protocols. A maximum of 20% of missing sessions should be allowed before excluding a patient and these sessions should be replaced at the end of the stimulation period.