Outcome Measures for Functional Neurological Disorder: A Review of the Theoretical Complexities

The development and selection of optimal outcome measures is increasingly recognized as a key component of evidence-based medicine, particularly the need for the development of a standardized set of measures for use in clinical trials. This process is particularly complex for functional neurological disorder (FND) for several reasons. FND can present with a wide range of symptoms that resemble the full spectrum of other neurological disorders. Additional physical (e.g., pain, fatigue) and psychological (e.g., depression, anxiety) symptoms are commonly associated with FND, which also can be highly disabling with implications for prognosis, and warrant concurrent assessment, despite an unclear etiological relationship with FND. Furthermore, several unique clinical aspects of FND make it likely that the usual prioritization of "objective" (or clinician-rated) over "subjective" (or patient-rated) measures might not be appropriate. Self-report measures may be more clinically meaningful in this patient population. Despite being a common and disabling disorder, there has been little research into outcome measures in FND, and to date trials have largely used measures designed for the assessment of other disorders. An international FND Core Outcome Measure group (FND-COM) has been established to develop a consensus battery of outcomes for FND: a "core outcome set." In this perspective article, the authors reviewed the process of outcome measure development and selection before considering the specific features of FND affecting the development of a core outcome set, as well as a research agenda to optimize outcome measurement in this complex neuropsychiatric disorder.

Outcome measurement in functional neurological disorder: a systematic review and recommendations

OBJECTIVES

We aimed to identify existing outcome measures for functional neurological disorder (FND), to inform the development of recommendations and to guide future research on FND outcomes.

METHODS

A systematic review was conducted to identify existing FND-specific outcome measures and the most common measurement domains and measures in previous treatment studies. Searches of Embase, MEDLINE and PsycINFO were conducted between January 1965 and June 2019. The findings were discussed during two international meetings of the FND-Core Outcome Measures group.

RESULTS

Five FND-specific measures were identified-three clinician-rated and two patient-rated-but their measurement properties have not been rigorously evaluated. No single measure was identified for use across the range of FND symptoms in adults. Across randomised controlled trials (k=40) and observational treatment studies (k=40), outcome measures most often assessed core FND symptom change. Other domains measured commonly were additional physical and psychological symptoms, life impact (ie, quality of life, disability and general functioning) and health economics/cost-utility (eg, healthcare resource use and quality-adjusted life years).

CONCLUSIONS

There are few well-validated FND-specific outcome measures. Thus, at present, we recommend that existing outcome measures, known to be reliable, valid and responsive in FND or closely related populations, are used to capture key outcome domains. Increased consistency in outcome measurement will facilitate comparison of treatment effects across FND symptom types and treatment modalities. Future work needs to more rigorously validate outcome measures used in this population.