Dystonia

Introduction Dystonias are central movement disorders that are characterized by sustained involuntary muscle contractions that lead to abnormal postures or repetitive movements. Usually different types of dystonia are categorized according to their etiology (primary, that is, without any known cause, or secondary, that is, due to another etiological factor such as neuropsychiatric drugs, cerebral ischemia, and others) and according to the clinical distribution of symptoms. Focal dystonias affect only parts of the body, such as the neck in cervical dystonia (CD), the orofacial muscles in blepharospasm/oromandibular dystonia (Meige’s syndrome), or the larynx in laryngeal dystonia (LD). In action-induced dystonia, patients only become symptomatic during a specific, usually highly automated or overlearnt task like writing (focal hand dystonia = writer’s cramp or WC), speaking (LD) or playing a musical instrument (musician’s dystonia), such as winding in professional brass players (“embouchure” dystonia). Dystonia is considered to represent a disorder of the basal ganglia combined with a cortical sensorimotor dysfunction [1]. Considering functional imaging and electrophysiological findings (see below), there appears to be a defective inhibitory control from the basal ganglia to cortical structures in focal dystonia. Deficient inhibitory control could lead to a failure in focusing motor output (deficient “center–surround” inhibition) and predispose to the development of abnormal cortical plasticity, resulting in altered cortical maps or increased cortical excitability [2–4]. An injury or the intensive use of the affected limb together with maladaptive sensorimotor reorganization could lead to the clinical manifestation of dystonia.