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.