Increased task-uncorrelated muscle activity in childhood dystonia

Abstract

BackgroundEven if movement abnormalities in dystonia are obvious on observation-based examinations, objective measures to characterize dystonia and to gain insights into its pathophysiology are still strongly needed. We hypothesize that motor abnormalities in childhood dystonia are partially due to the inability to suppress involuntary variable muscle activity irrelevant to the achievement of the desired motor task, resulting in the superposition of unwanted motion components on the desired movement. However, it is difficult to separate and quantify appropriate and inappropriate motor signals combined in the same muscle, especially during movement.MethodsWe devise an innovative and practical method to objectively measure movement abnormalities during the performance of a continuous figure-eight writing task in 7 children with dystonia and 9 age-matched healthy controls. During the execution of a continuous writing task, muscle contractions should occur at frequencies that match the frequencies of the writing outcome. We compare the power spectra of kinematic trajectories and electromyographic signals of 8 upper limb muscles to separate muscle activity with the same frequency content of the figure-eight movement (task-correlated) from activity occurring at frequencies extraneous to the task (task-uncorrelated).ResultsChildren with dystonia present a greater magnitude of task-uncorrelated muscle components. The motor performance achieved by children with dystonia is characterized by an overall lower quality, with high spatial and temporal variability and an altered trade-off between speed and accuracy.ConclusionsFindings are consistent with the hypothesis that, in childhood dystonia, the ability to appropriately suppress variable and uncorrelated elements of movement is impaired. Here we present a proof-of-concept of a promising tool to characterize the phenomenology of movement disorders and to inform the design of neurorehabilitation therapies.

Children With and Without Dystonia Share Common Muscle Synergies While Performing Writing Tasks

“…In accordance with previous studies 20 , children with dystonia presented an impaired writing performance compared to their healthy peers, characterized by non-smooth and rather variable motor outcomes with an altered trade-off between speed and accuracy of movement. Surprisingly, notwithstanding the compromised kinematics of the writing performance, synergy analysis revealed no difference in the number of synergies between children with and without dystonia.…”

“…This suggests that muscle synergy analysis has the ability to capture the muscle components that are not corrupted by the noisy elements that typically affect the EMGs of patients with dystonia and can be instead identified, for instance, through frequency-domain methods 20 . With the analysis of muscle synergies, such noisy elements may be eliminated as part of the residual variance during the process of dimensionality reduction, or when reducing the frequency content for the linear envelope extraction.…”

“…Compared to healthy subjects, muscle activity in dystonia typically exhibits overflow into task-unrelated muscles and greater variability within task-related muscles 17,20,26 . The use of muscle synergy analysis as a tool to deepen our understanding of the CNS dysfunction in dystonia has not been leveraged yet.…”

Objective, computerized video-based rating of blepharospasm severity

“…Our results further highlight concerns about the reliability of inherently subjective methods for evaluating blepharospasm severity and support the need for more objective measures of motor symptoms in dystonia. 4,27 The 3 video reviewers, after conducting a joint practice session to work toward consensus on how to apply the rating scales immediately before their independent ratings, exhibited only moderate agreement (ICC z0.6) on the severity ratings across these patients. The video raters also exhibited only moderate correlation (Spearman r 5 0.58-0.61) with the live raters.…”

Current and emerging strategies for treatment of childhood dystonia

“…The mechanism of overflow in dystonia is not well understood but theoretical and neurophysiological studies suggest overflow could be due to poor awareness of muscle activation, such that during actions related but unwanted muscles or motor programs are not suppressed adequately. 5,31,37,38 …”