Variability of Movement Disorders: The Influence of Sensation, Action, Cognition, and Emotions

Patients with movement disorders experience fluctuations unrelated to disease progression or treatment. Extrinsic factors that contribute to the variable expression of movement disorders are environment related. They influence the expression of movement disorders through sensory-motor interactions and include somatosensory, visual, and auditory stimuli. Examples of somatosensory effects are stimulus sensitivity of myoclonus on touch and sensory amelioration in dystonia but also some less-appreciated effects on parkinsonian tremor and gait. Changes in visual input may affect practically all types of movement disorders, either by loss of its compensatory role or by disease-related alterations in the pathways subserving visuomotor integration. The interaction between auditory input and motor function is reflected in simple protective reflexes and in complex behaviors such as singing or dancing. Various expressions range from the effect of music on parkinsonian bradykinesia to tics. Changes in body position affect muscle tone and may result in marked fluctuations of rigidity or may affect dystonic manifestations. Factors intrinsic to the patient are related to their voluntary activity and cognitive, motivational, and emotional states. Depending on the situation or disease, they may improve or worsen movement disorders. We discuss various factors that can influence the phenotypic variability of movement disorders, highlighting the potential mechanisms underlying these manifestations. We also describe how motor fluctuations can be provoked during the clinical assessment to help reach the diagnosis and appreciated to understand complaints that seem discrepant with objective findings. We summarize advice and interventions based on the variability of movement disorders that may improve patients' functioning in everyday life. © 2020 International Parkinson and Movement Disorder Society.