Plasticity and intracortical inhibition in dystonia--methodological reconsiderations

Abstract

Sir, I read with interest the article ‘Abnormal sensorimotor plasticity in organic but not psychogenic dystonia’ by Quartarone et al. published in Brain (2009; 132: 2871–7). Dystonia is a challenging condition for clinicians and neuroscientists alike. The presented symptoms are often bizarre and a diagnostic tool that reliably distinguishes between organic and psychogenic origin is highly desirable. In their manuscript, the authors describe that ‘plasticity’, which they refer to as long-term potentiation-like plasticity that can be induced by the paired-associative stimulation (PAS) protocol (Stefan et al. , 2000), is ‘abnormally high’ only in patients with organic dystonia, while short-interval intracortical inhibition, a parameter of GABA-ergic intracortical inhibition (Mueller-Dahlhaus et al. , 2008), is equally ‘abnormal’ in psychogenic and organic dystonia. They conclude that ‘abnormal plasticity is a hallmark of organic dystonia; furthermore it is not a consequence of reduced inhibition since the latter is seen in psychogenic patients who have normal plasticity’. This conclusion is appealing since it not only offers a diagnostic tool for organic dystonia but also a simple solution to the intensively researched and still not well understood pathophysiology of this movement disorder. Transcranial magnetic stimulation is now an easily accessible and intensively used research tool in human basic and clinical neuroscience. With increasing insights into the mechanisms and effects of transcranial magnetic stimulation, even well established protocols such as short-interval intracortical inhibition or PAS are scrutinized again and further refined in order to make our research findings more reliable and valid. With this in mind, the findings and conclusions proposed in the current paper by Quartarone et al. (2009) require and might benefit from some methodological and conceptual re-considerations. There is no doubt that transcranial magnetic stimulation protocols, particularly PAS (Stefan et al. , 2000), have contributed significantly to our understanding of plasticity of the …