Finger muscle control in children with dystonia

Abstract

Background: Childhood dystonia is a disorder that involves inappropriate muscle activation during attempts at voluntary movement. Few studies have investigated the muscle activity associated with dystonia in children, and none have done so in the hands.

Methods: In this study, we measured surface electromyographic activity in four intrinsic hand muscles while participants attempted to perform an isometric tracking task using one of the muscles.

Results: Children with dystonia had greater tracking error with the task-related muscle and greater overflow to non-task muscles. Both tracking error and overflow correlated with the Barry-Albright Dystonia scale of the respective upper limb. Overflow also decreased when participants received visual feedback of non-task muscle activity.

Discussion: We conclude that two of the motor deficits in childhood dystonia--motor overflow and difficulties in actively controlling muscles--can be seen in the surface electromyographic activity of individual muscles during an isometric task. As expected from results in adults, overflow is an important feature of childhood dystonia. However, overflow may be at least partially dependent on an individual's level of awareness of their muscle activity. Most importantly, poor single-muscle tracking shows that children with dystonia have deficits of individual muscle control in addition to overflow or co-contraction. These results provide the first quantitative measures of the muscle activity associated with hand dystonia in children, and they suggest possible directions for control of dystonic symptoms.

Conflict of interest statement

Potential conflict of interest: Nothing to report.

Copyright © 2011 Movement Disorder Society.

Figures

FIG. 1

Experimental setup. (A) Hand placement on table. Rectangular blocks prevented abduction of the second and fifth fingers, and EMG electrodes (labeled E) were placed on the ADM and FDI muscles. Left hand is shown, and right hand was arranged in the same way. (B and C) Tracking task: The horizontal gray bar acted as a target, moving vertically with a randomly distorted sinusoidal motion. Participants tracked the target with one of the dark horizontal bars (cursor), activating their muscle to move the cursor upward, and relaxing their muscle to move it downward. The order of bars on the screen corresponded to the order of muscles with hands lying on table (left to right): LH ADM, LH FDI, RH FDI, and RH ADM. (B) Display for block 1 contained only the target and cursor (LH FDI shown in this case). (C) Display for block 2 also included all non-task muscles.

FIG. 2

Representative EMG recordings from all trials in the first block (no feedback). For each participant, EMG recordings of all four muscles during four 60 s trials are organized in a matrix of trials (rows) and muscles (columns). The tracking muscles for each trial are located on the diagonal of the matrix, and non-task muscles are located off of the diagonal. A: Typical participant from the control group shows little muscle activity in non-task muscles. B: Typical participant from the dystonic group shows substantial muscle activity in non-task muscles. In particular, this subject showed significant overflow within each hand and overflow from the left hand to right hand.

FIG. 3

Kaplan-Meier plots showing mean tracking error (A) and overflow (B) in block 1 for individuals in the control (dashed line) and dystonic (solid line) groups. These plots illustrate the distribution of the measured values for each group, especially the difference between groups in the tails of the data.

FIG. 4

Mean tracking error (A) and overflow (B) in both blocks for the Control (squares with dashed line) and Dystonic (circles with solid line) groups. Error bars indicate standard errors of the mean for all participants.

FIG. 5

Relation of tracking error (A) and overflow (B) to the BAD rating of the respective arm for participants in the dystonic group during block 1. Points are plotted with a log scale and jittered about their location on the x-axis to illustrate their density. The dotted line is the best-fitting regression line.