Functional effects of transcranial direct stimulation in musicians with focal dystonia

Focal dystonia in musicians is a task-specific movement disorder which presents itself as a loss of voluntary motor control in extensively trained movements while playing a musical instrument. The disorder represents one of the most serious conditions as yet to be resolved in the field of musician’s medicine. Musicians suffering from the disorder are highly disabled professionally, often leading to the termination of their musical career. Transcranial direct current stimulation (tDCS) is a non-invasive tool to guide neuroplasticity and modulate cortical function by a tonic stimulation with weak direct currents. Anodal tDCS enhances cortical excitability, whereas cathodal tDCS reduces it. The present thesis aims to investigate the functional effects of tDCS as new treatment approach for focal hand dystonia in musicians and to provide new insights into the pathophysiology of focal dystonia. The first experimental study of the present thesis addresses the question as to whether a single session of cathodal tDCS over the primary motor cortex facilitates fine motor control in a group of guitarists suffering from focal hand dystonia. We hypothesized that cathodal tDCS over the primary motor cortex reduces the overshooting activity in musician’s dystonia and improves fine motor control due to an enhanced signal-to-noise ratio in the motor system. Professional guitarists (n=10) with musician’s dystonia played exercises before, directly after and 60min after cathodal tDCS. Guitar exercises were video-documented and symptoms were evaluated by three independent experts. No beneficial effect of cathodal tDCS on fine motor control was found for the entire group. In the second experiment, tDCS was combined with another treatment approach to musician’s dystonia: Nine professional pianists with musician’s dystonia carried out sensorimotor retraining at the instrument while being stimulated over the primary motor cortex. Sensorimotor retraining is a therapeutic approach to focal dystonia in musicians and aims to establish new non-dystonic movements through motor learning processes. Anodal tDCS has been shown to facilitate motor learning, whereas cathodal tDCS improves performance in overlearned tasks. Thus, we tested both types of stimulation and their effects on sensorimotor retraining. Again no beneficial effects of a single-session tDCS-supported sensorimotor retraining on fine motor control were found. The third experiment investigated whether daily repeated tDCS-supported retraining facilitates fine motor control of a single pianist with focal hand dystonia. tDCS was applied for five consecutive days over the primary motor cortex while the pianist carried out sensorimotor retraining. Three conditions (anodal, placebo and cathodal) were tested in three weeks, with a time interval of several weeks between the conditions. Improvement of fine motor control after retraining and tDCS was found in all three conditions. Cathodal tDCS was found to prolong the effect of retraining. However, this result requires further verification as cumulative retraining effects in the cathodal stimulation week (third week) might have influenced this outcome. To summarize previous studies and the results of the present thesis, the application of tDCS when limited to a single session is not a helpful treatment option for musician’s dystonia. Future studies should concentrate on the effects of repeated cathodal stimulation applied over several days or even weeks. Sensorimotor retraining yielded very promising results in the single case report. Research studies should aim at indentifying optimal retraining strategies in a large number of affected musicians. As therapeutic options are limited in patients with musician’s dystonia, prevention should be addressed as an important goal in music high schools and orchestras.