Consistency of inclusion criteria for functional movement disorder clinical research studies: A systematic review.

Background: Functional movement disorders (FMDs) are a common cause of disability. With an increasing research interest in FMD, including the emergence of intervention trials, it is crucial that research methodology be examined, and standardized protocols be developed.
Objective: To characterize the current inclusion criteria used to select patients for FMD research studies and review the consistency and appropriateness of these criteria.
Methods: We identified studies of potential biomarkers for FMD that were published over the last two decades and performed a qualitative analysis on the finally included studies.
Results: We identified 79 articles and found inconsistent inclusion criteria. The Fahn-Williams and DSM-IV criteria were the most commonly applied, but neither accounted for the majority (Fahn-Williams 46%, DSM-IV 32% of the total). The selection of the inclusion criteria depended in part on the phenotype of FMD under investigation. We also identified inclusion methodologies that were not appropriate, such as the inclusion of low-certainty diagnoses and diagnosing by excluding specific biomarkers rather than including patients based on clinical characteristics that commonly are thought to suggest FMD.
Conclusions: Significant variability exists with the inclusion criteria for FMD research studies. This variability could limit reproducibility and the appropriate aggregation of data for meta-analysis. Advancing FMD rehabilitation research will need standardized inclusion criteria. We make some suggestions.