Regional specificity in the hand area of the primary motor cortex for healthy individuals and individuals with focal hand dystonia.

Abstract

Idiopathic focal hand dystonia is a movement disorder characterized by abnormal postures and loss of motor control of the affected limb. Currently, the underlying pathophysiology responsible for these motor manifestations is not fully understood. Recent empirical evidence suggests a link between deficits of cortical inhibition, maladaptive plasticity and abnormal sensory and motor processing in individuals with dystonia. These factors may contribute to an atypical organization of the hand knob area of the primary motor cortex. The current literature lacks a well designed method to clearly define and quantify healthy cortical activation. The purpose of this study was to first establish a definition of healthy cortical activation in the primary motor cortex during a finger tapping task and then use this baseline for comparison to a group of participants with focal hand dystonia. Methods. Functional magnetic resonance imaging was used to compare the cortical activation of six participants diagnosed with idiopathic focal hand dystonia to eight healthy individuals during a randomly ordered finger tapping task. Quantification of the cortical activation was performed with GLM beta weight analysis to examine for main effects and through a `Selectivity Index' that allowed for activation of a single digit (in the hemisphere contralateral to the moving digit) to be measured relative to the activation of the four other digits of the hand. Results. In the beta weight analysis differences in cortical activation was found at the group, finger and hemisphere levels (p < 0.05). Significant interaction effects were found in activation for group x finger x hemisphere (p = 0.02 ). The analysis demonstrated less selectivity of individual finger activation in both hemispheres for the dystonic group compared to the healthy group (right: p = 0.0017, left p = 0.0105). Discussion. This study is the first to define a method for determining the degree to which a cortical area is associated with the movement of one digit related to another. Importantly, it elucidates a potential neuropathophysiological substrate related to individual finger activation in the primary motor cortex in humans with focal dystonia.