Functional neurological disorder: new subtypes and shared mechanisms

Abstract

Functional neurological disorder is common in neurological practice. A new approach to the positive diagnosis of this disorder focuses on recognisable patterns of genuinely experienced symptoms and signs that show variability within the same task and between different tasks over time. Psychological stressors are common risk factors for functional neurological disorder, but are often absent. Four entities-functional seizures, functional movement disorders, persistent perceptual postural dizziness, and functional cognitive disorder-show similarities in aetiology and pathophysiology and are variants of a disorder at the interface between neurology and psychiatry. All four entities have distinctive features and can be diagnosed with the support of clinical neurophysiological studies and other biomarkers. The pathophysiology of functional neurological disorder includes overactivity of the limbic system, the development of an internal symptom model as part of a predictive coding framework, and dysfunction of brain networks that gives movement the sense of voluntariness. Evidence supports tailored multidisciplinary treatment that can involve physical and psychological therapy approaches.

Figure 1.

Functional Neurological Disorder can be triggered by pathophysiological and/or psychophysiological events. Acknowledgements to Stoyan Popkirov for the graphic concept.

Figure 2.. Neural mechanisms of FND.

This scheme relates to FMD but its principles are applicable to all FNDs. Movements are generated by motor cortex after planning/preparation in SMA. This produces feedforward signals to be compared to feedback from interoceptive and external signals after action. If signals don’t match, movement will not be appreciated as voluntary. The brain has a model of the body and world which adds predictive coding to this multimodal integration. Feedback signals that don’t match predictive coding create prediction error, which modifies the model so that predictive coding matches subsequent feedback. In FND, it is hypothesized that prediction error is not accurately updated, perpetuating dysfunction. FMD = functional movement disorder; DLPFC=Dorsolateral Prefrontal Cortex; SMA=Supplementary Motor Area; M1= Primary Motor Area; rTPJ=Right Temporoparietal Junction