Dystonia is a movement disorder characterized by sustained or intermittent muscle contractions causing abnormal, often repetitive, movements, postures, or both. Dystonic movements are typically patterned, twisting, and may be tremulous. Dystonia is often initiated or worsened by voluntary action and associated with overflow muscle activation.
The physical sign of dystonia is identified by the observation of dystonic postures and movements and of conditions that specifically activate, increase or decrease this phenomenology. A typical example is when a tactile stimulus or a voluntary geste (known as a sensory trick or geste antagoniste), such as touching the affected or an adjacent body part, can reduce the dystonia. These features are observed in a variety of dystonia syndromes.
The neural mechanism underlying dystonia involves many regions of the central nervous system (particularly: the basal ganglia, cerebellum, supplementary motor areas and sensorimotor cortex) leading to impaired inhibition, abnormal plasticity and sensorimotor dysfunction. Different levels of functional impairment lead to varying degrees of observed clinical severity.
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The clinical characteristics of dystonia include: age at onset (from infancy to late adulthood), body distribution (focal forms affect a single body part, segmental affect contiguous body parts , generalized forms involve the trunk and two other body regions), and temporal pattern (disease course static or progressive and variability related to voluntary actions or to diurnal fluctuations). Dystonia can be isolated or combined with another movement disorder, such as parkinsonism or myoclonus, or can be associated to other neurological or systemic manifestations.
Isolated dystonia with onset in childhood tends to progress to generalization, whereas dystonia arising in adulthood usually remains focal or segmental. Some focal/segmental dystonia syndromes have specific names, as they were originally considered independent diseases: blepharospasm, resulting in repetitive forceful eyelid closure; spasmodic dysphonia affecting speech; tongue, jaw opening and jaw closing called oromandibular dystonia; cervical dystonia or torticollis involving the neck.
Focal dystonia may also be task-specific, for example, writer's cramp is a dystonia affecting the hand/arm during the act of writing. Other examples of task specific dystonia include musician's dystonia that can affect the body part involved in playing an instrument.
There are many causes of dystonia. The etiological classification considers two main lines: evidence of nervous system pathology and whether dystonia is inherited or acquired. In many cases of isolated dystonia there is no evidence of degeneration or structural lesion (e.g., DYT1, DYT6; formerly called "primary" dystonias). Otherwise, there may be evidence of a static structural lesion causing dystonia, as in the case of dystonic cerebral palsy following perinatal brain injury, or of progressive degeneration (e.g., PKAN, PLAN; formerly called "dystonia plus").
Inherited dystonias can have all sorts of transmission patterns; acquired dystonias recognize a variety of causes (perinatal brain injury, infection, drug, toxic, vascular, etc.); idiopathic dystonias (whether sporadic or familial) are genetically unclassified forms, considered to be related to yet undiscovered genes.