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Dystonia is characterized by sustained or intermittent muscle contractions causing abnormal movements and/or postures. The overall prevalence of primary dystonia is reported to be 16.4 per 100,000 by one meta-analysis of population-based studies [19]. There exists a plethora of dystonia phenotypes, ranging from involvement of a single body site to generalized syndromes associated with additional neurologic signs. The functional impact of dystonia ranges from mild to disabling. Dystonic movements are usually exacerbated by anxiety, stress, fatigue, and voluntary movements, and disappear during sleep. In many cases, the movement can be suppressed by a specific tactile stimulation known as “sensory trick” in which touching the affected body part or an adjoining body part diminishes the abnormal muscle contractions. Dystonia is not usually painful with the exception of cervical dystonia.

Classification of dystonia by clinical characteristics or etiology is helpful for prognostication and optimization of treatment. Various classification schemes identify age of onset, body distribution, temporal pattern, and additional neurologic features as useful clinical parameters. Onset in childhood is most likely to be generalized dystonia with an identifiable cause, such as the monogenic disorders labeled as DYTs. At the time of this writing there have been 23 DYTs identified, designated DYT1 through DYT25. Sporadic dystonias tend to emerge in late adulthood. By body distribution, dystonia may be focal, segmental, multifocal, generalized or involve a hemi body. Focal dystonia, in which only one body site is affected, is the most common form, and the most frequently affected sites include cervical (spasmodic torticollis), orbicularis oculi (blepharospasm), hand (writer’s cramp), jaw (oromandibular dystonia), and vocal cords (spasmodic dysphonia). Of these, cervical dystonia is by far the most common focal dystonia, with age of onset usually between 20 and 50 years.

Primary dystonia traditionally refers to the presence of dystonia as the only sign (although accompanying tremor is generally accepted) after symptomatic causes have been excluded. It may be familial or sporadic. Primary dystonia typically begins focally, and while the majority of adult onset patients remain focal, a subset of patients will experience spread to other body regions – usually contiguously – to become segmental dystonia. Younger age of onset is associated with increased probability of spread. However, most patients with primary dystonia usually reach a plateau where there is no further worsening or spread.

Distinct from primary dystonia, which is regarded as “pure” dystonia, disorders falling under the label of dystonia-plus syndromes feature neurologic signs, in addition to dystonia. Examples include dopa-responsive dystonia and myoclonus-dystonia. Similar to primary dystonia, there is no known neurodegeneration. In contrast, another category of disorders known as the heredodegenerative dystonias have a hereditary basis and are associated with a progressive course of motor deterioration and development of other degenerative features, such as cognitive impairment. These conditions include Huntington disease, dentatorubral-pallidoluysian atrophy, Wilson disease, neuroacanthocytosis, and certain mitochondrial disorders.

Secondary, or acquired dystonias, are the result of an environmental exposure or insult, and etiologies include drug-induced (tardive), stroke, encephalitis, and anoxic brain injury. Tardive dystonia is the most common form of secondary dystonia observed in most referral centers. As with other tardive syndromes, the abnormal movements develop during chronic (at least 3 months) exposure to dopamine-receptor blocking agents, typically neuroleptics or anti-emetics, or up to 6 months after discontinuation. Tardive dystonia may be focal, segmental or generalized. While it may present identically to idiopathic dystonia, retrocollis and backward trunk arching are more characteristic of tardive dystonia. An important clue to the etiology is the frequent coexistence of other tardive symptoms, such as oro-buccal-lingual movements (tardive dyskinesia) or the inability to remain still due to restlessness (akathisia). Slow withdrawal of the offending agent is performed when feasible but is unlikely to lead to the resolution of symptoms, as most studies report remission rates of less than 20%.

Treatment for dystonia is symptomatic and the optimal approach depends primarily on the distribution of dystonia. For most types of focal dystonia, chemodenervation using botulinum toxin injections is considered first line therapy and is usually highly efficacious. Segmental dystonia is generally amenable to botulinum toxin therapy as well. For more widespread forms, oral medication and DBS are the preferable options. A trial of levodopa is usually given in childhood onset dystonia, and even some adult onset cases, given the potential for dramatic improvement at a relatively low risk. Anticholinergics such as trihexyphenidyl are a common choice for initial oral therapy, although the evidence for benefit in adult onset dystonia is limited. Anticholinergic side effects such as cognitive dysfunction and constipation are often dose limiting. Anti-spasmodics such as baclofen and clonazepam have been used with variable success. Dopamine-depleting agents such as tetrabenazine have been demonstrated to be effective, particularly in patients with tardive dystonia.

Pallidotomy and DBS of the globus pallidus internus (GPi) have been shown to be effective treatment options for various forms of dystonia, but the benefit is less robust or consistent than that seen with PD or ET. Primary dystonias clearly respond better than secondary dystonias, and the best potential for benefit is seen in generalized or segmental dystonias. DBS has become favored over ablative techniques for dystonia and has been shown to produce major improvement in clinical and functional measures long-term [20]. In contrast to tremor or PD, following DBS for dystonia a sustained benefit is typically not seen until weeks or months after stimulation is initiated. GPi DBS has been performed in patients with tardive dystonia with reportedly good outcomes and safety [21].