Impaired performance of patients with writer’s cramp in complex fine motor tasks

•Patients with writer’s cramp revealed abnormalities in complex fine motor performance.•Patients showed impaired performance in visuomotor tracking and fast force changes.•Findings suggest deficits in visuomotor integration, fine motor coordination and cognitive aspects. Writer’s cramp (WC) is a task-specific focal dystonia. WC is characterized by involuntary contractions of muscles of the hand and arm during handwriting, resulting in impaired writing with exaggerated finger forces. The generalization of symptoms to other fine motor tasks is widely discussed. The aim of the study was to determine affected fine motor aspects with an extensive testing battery. Twelve people with WC and twelve healthy controls were examined. Performance in the Jebsen-Taylor Hand Function Test, Nine-Hole-Peg Test and 2-point discrimination was evaluated. To analyze object manipulation skills, we examined grip forces, temporal measures and other aspects of force control during (1) lifting actions with variations of weight and surface (2) cyclic movements (3) visuomotor tracking (4) fast force changes and (5) grip strength. In addition, correlation between the dependent variables of the fine motor tasks and the handwriting deficits was assessed. WC patients had impaired performance in the visuomotor tracking task (root mean square error (RMSE), p = 0.03 and time lag, p = 0.05) and the fast force changes (frequency, p = 0.01). There were no statistically significant group differences in the other tasks. We found a correlation between the RMSE of the tracking task and the time needed to write the test sentence (r = 0.643, p = 0.01). WC patients revealed abnormalities in complex fine motor performance in tasks with high demands on coordination and visual components, specifically in tracking and fast force changes. This suggests a deficit in visuomotor integration, coordination and cognitive aspects related to movement processing particularly with respect to low forces. These insights may prove useful in the development of targeted training approaches.