Multicentre validation of a patient-reported outcome measure for functional movement disorders

No disorder-specific patient-reported outcome measure (PROM) has yet been validated for functional movement disorders (FMDs), leaving a critical gap in clinical care and research.

To validate the FMD questionnaire (FMDQ) in a prospectively recruited sample through a multicentre study.

Confirmatory factorial analysis (CFA) tested the assumed structure of the questionnaire with factors reflecting severity of motor symptoms, impairment of everyday activities, impact of non-motor symptoms and impairment of social functioning. Internal consistency and floor/ceiling effects were examined. The 36-item short form health survey (SF-36), patient health questionnaire-15 (PHQ-15), the fatigue assessment scale (FAS) and a clinician-rated scale corresponding to motor symptom items of the FMDQ (FMDQ-CR) were used to test criterion and construct validity. The minimally clinically important difference (MCID) was assessed through distribution-based and anchor-based methods in a convenience sample of patients with follow-up assessments.

Complete datasets from 157 patients were analysed; follow-up assessments were available from 30 patients. CFA confirmed that a four-factor model provides a better fit to the data compared with a more restrictive one-factor model. Internal consistency was appropriate for all factors/subscales. No floor or ceiling effects were detected. Criterion and content validity were supported by significant correlations with respective SF-36 subscores, PHQ-15, FAS and FMDQ-CR. Anchor-based MCID was estimated at 8 to 20 points, with the central value aligning with the distribution-based MCID of 12 points (8% of the total score range).

The FMDQ is a psychometrically robust PROM, making it a useful tool for clinical practice and treatment trials.