These movement disorders are involuntary and cannot be inhibited, thereby differing from habit spasms or tics. At one time, torticollis was thought to be a psychological disorder but all now agree that it is a localized form of dystonia. It is characteristic of focal dystonias to display a simultaneous activation of agonist and antagonist muscles (co-contraction) and to have a tendency for the spasm to spread to adjacent muscle groups that are not normally activated in the movement (overflow), but these features tend not to be as prominent in focal dystonias as in the generalized varieties described earlier. Sometimes, focal dystonias include an arrhythmic intermixed tremor, which may be the prominent incipient feature. The tremor in particular may cause difficulty in diagnosis if the slight degree of underlying dystonia is not appreciated by careful observation and by palpation of the involved muscles.
The pathogenesis of the idiopathic focal dystonias is uncertain, although there is evidence that some of them, like the generalized dystonias, are genetically determined. Authoritative commentators, including Marsden, classified the apparently idiopathic adult-onset focal dystonias with genetically determined generalized torsion dystonia. This view is based on several lines of evidence: the recognition that each of the focal dystonias may appear as an early component of generalized syndrome in children, the occurrence of focal and segmental dystonias in family members of these children, as well as a tendency of the dystonia in some adult patients to spread to other body parts. Perhaps the most compelling observation in this regard has been the finding that there are families in which the only manifestation of the DYT1 mutation (the gene associated with generalized torsion dystonia) is a late-onset writer’s cramp or other focal dystonia. Whether this explains most or even many of the cases of adult onset focal dystonia is unclear but it does emphasize the phenotypic variability associated with the DYT1 mutation. The genetics of primary torsion dystonia is more complex than portrayed here, and is reviewed in Chap. 38.
It is noteworthy that no consistent pathologic changes have been demonstrated in any of the idiopathic or genetically determined dystonias (see Zeman). Most physiologists cast the disorder in terms of reduced cortical inhibition of unwanted muscle contractions, as summarized by Berardelli and colleagues. Moreover, physiologic changes in the cortical sensory areas that are pertinent to the dystonias associated with overuse of body parts (occupational dystonias) are described further on.
A focal dystonia rarely emerges transiently after a stroke that involves the striatopallidal system, mainly the internal segment of the pallidum or the thalamus, but the varied locations of these infarctions makes it difficult to draw conclusions about the mechanism of dystonia. It will be noticed that the same disturbances that cause chorea, as discussed in an earlier section, may produce focal dystonias (see Table 4-4). Focal dystonias may also occur in metabolic diseases such as Wilson disease and nonwilsonian hepatolenticular degeneration. Any of the typical forms of restricted dystonia may represent a tardive dyskinesia; that is, they complicate treatment with high-potency dopamine antagonists and other medications that are used primarily for the treatment of psychosis and nausea (see further on under “Drug-Induced Dyskinesias”). Dystonias of the hand or foot often emerge as components of a number of degenerative diseases—Parkinson disease in particular but also corticobasal ganglionic degeneration, and progressive supranuclear palsy (described in Chap. 38). Such cases that fall into the category of symptomatic or secondary dystonias are described by Krystkowiak and colleagues and by Munchau and colleagues. Janavs and Aminoff have summarized several focal dystonias that are caused by acquired systemic disorders, such as drugs, and by autoantibodies, including from systemic lupus erythematosus. It is the last of these that we have encountered most often in clinical practice.