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Angelo Quartarone,1 Vincenzo Rizzo,1 Sergio Bagnato,1,2 Francesca Morgante,1 Antonino Sant’Angelo,1 Marcello Romano,3 Domenica Crupi,1 Paolo Girlanda,1 John C. Rothwell4 and Hartwig R. Siebner5,6
1Department of Neuroscience, Psychiatric and Anaethesiological Sciences, University of Messina, 2Department of Neurological Sciences, and Istituto Neurologico Mediterraneo Neuromed IRCCS, Pozzilli (IS), University of Rome ‘La Sapienza’, 3Department of Neurology Azienda ospedaliera villa sofia cto of Palermo, Palermo, Italy, 4Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College of London, London, UK, 5Department of Neurology, Christian-Albrechts-University, Kiel and 6NeuroImageNord, Hamburg-Kiel-Lu ̈beck, Germany
Correspondence: Dr Angelo Quartarone, Clinica Neurologica 2, Policlinico Universitario, 98125 Messina, Italy E-mail: angelo.quartarone@unime.it

The excitability of inhibitory circuits in patients with writer’s cramp is reduced at multiple levels within the sensorimotor system, including the primary motor hand area (M1). Although this may play a major role in the pathophysiology of writer’s cramp, it is still unclear what factors may cause the imbalance between inhibition and excitation to arise. One possibility is that homeostatic mechanisms that keep cortical excitability within a normal physiological range are impaired. In eight patients with writer’s cramp and eight healthy age-matched controls, we combined low-frequency repetitive transcranial magnetic stimulation (rTMS) with transcranial direct current stimulation (TDCS) to probe regional homeostatic plasticity of the left M1. Confirming our previous study (Siebner et al., J Neurosci 2004; 24: 3379–85), ‘facilitatory’ preconditioning of the M1 with anodal TDCS enhanced the inhibitory effect of subsequent 1 Hz rTMS on corticospinal excitability. Conversely, ‘inhibitory’ preconditioning with cathodal TDCS reversed the after effect of 1 Hz rTMS, producing an increase in corticospinal excitability. The results were quite different in patients with writer’s cramp. Following pre- conditioning with TDCS, 1 Hz rTMS induced no consistent changes in corticospinal excitability, indicating a loss of the normal ‘homeostatic’ response pattern. In addition, the normal inhibitory effect of preconditioning with cathodal TDCS was absent. The present data suggest that homeostatic mechanisms that stabilize excitability levels within a useful dynamic range are impaired in patients with writer’s cramp. We propose that a faulty homeostatic response to acute increases in corticospinal excitability favours maladaptive motor plasticity. The role of homeostatic-like plasticity in the pathophysiology of task-specific dystonias warrants further study.

Keywords: focal dystonia; homeostatic plasticity; repetitive transcranial magnetic stimulation; transcranial direct current stimulation; writer’s cramp.