Brain Advance Access originally published online on May 4, 2005
Brain 2005 128(8):1943-1950; doi:10.1093/brain/awh527
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Homeostatic-like plasticity of the primary motor hand area is impaired in focal hand dystonia
1 Department of Neuroscience, Psychiatric and Anaethesiological Sciences, University of Messina, 2 Department of Neurological Sciences, and Istituto Neurologico Mediterraneo Neuromed IRCCS, Pozzilli (IS), University of Rome ‘La Sapienza’, 3 Department of Neurology Azienda ospedaliera villa sofia cto of Palermo, Palermo, Italy, 4 Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College of London, London, UK, 5 Department of Neurology, Christian-Albrechts-University, Kiel and 6 NeuroImageNord, Hamburg-Kiel-Lübeck, Germany
Correspondence: Dr Angelo Quartarone, Clinica Neurologica 2, Policlinico Universitario, 98125 Messina, Italy E-mail: angelo.quartarone{at}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 preconditioning 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.
Key Words: focal dystonia; homeostatic plasticity; repetitive transcranial magnetic stimulation; transcranial direct current stimulation; writer's cramp
Abbreviations: AMT = active motor threshold; FDI muscle = first dorsal interosseus muscle; ICF = intracortical facilitation; ISI = interstimulus interval; LTD = long-term depression; LTP = long-term potentiation; MEP = motor-evoked potential; PAS = paired associative stimulation; RMT = resting motor threshold; rTMS = repetitive transcranial magnetic stimulation; SICI = short-latency intracortical inhibition; TDCS = transcranial direct current stimulation
Received December 22, 2004. Revised March 17, 2005. Accepted April 5, 2005.
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