Focal Task-specific Dystonia-From Early Descriptions to a New, Modern Formulation

Abstract

Background: Vivid descriptions of the phenomenology of focal task-specific dystonia (FTSD) date back to the late nineteenth century.

Methods: In this review, I summarize the natural history, phenomenology, and treatment of FTSD, focusing on nineteenth-century neurologists' descriptions of the phenomenology, etiology, treatment, and mechanism.

Results: Examining these texts through a twenty-first-century lens, the "modern" ideas of a dystonic endophenotype, disordered physiology, and dystonic metabolic networks actually appeared in these texts more than a century ago.

Discussion: By incorporating these ideas with recent investigations, I present a new conceptual model for understanding this mysterious malady.

Keywords: Dystonia; musician; task-specific; writer's cramp.

Figure 1. A New Working Model of Focal Task-specific Dystonia.

The y-axis represents the stability of the sensory motor network. An elevation in the y-axis depicts the activation energy barrier to be overcome to achieve a new network. Time is represented on the x-axis. Starting from the left side of the graph, factors such as age, training regimen, gender, peripheral biomechanics, family history, and susceptibility to plasticity (surround inhibition) impact the stability of the baseline endophenotype network. A family history of dystonia, male gender, and late age of onset of training push the baseline endophenotype higher on the y-axis, making the network less stable and making an individual more susceptible to develop dystonia. At some point, a peripheral trigger or trauma may push the network up from left to right, over the activation energy barriers (blue hills), resulting in an eventual slide down (orange line) to a meta-stable state of a dystonic prodrome. This dystonic prodrome network is not stable, tending to degenerate (red line) and progressing to the right to a formed dystonic network (far right). Treatments such as sensory motor retraining, limb immobilization, medications, botulinum toxin injections, and stereotactic surgery are aimed at pushing the network from right to left, back up the activation energy scheme (solid blue line with arrows), and in the best-case scenario, returning the patient to a stable non-dystonic network.