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Dystonia is characterized by involuntary muscular contractions causing twisting movements and abnormal postures. As described in the previous chapter, dystonia can be classified into primary (dystonic movements and postures in the absence of other motor neurological abnormalities, with the exception of tremor) and secondary forms (environmental causes or symptomatic; dystonia plus syndromes and heredodegenerative diseases).1 In other chapters in this book, childhood-onset PTD (Chapter 31) and the symptomatic dystonias (Chapter 34) are covered in detail.
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The clinical spectrum of PTD in adults is considerably different from that observed in childhood (Table 29–1). It generally involves the upper body—the cranial musculature, neck, or arm—and the spasms tend to remain focal or spread only to the adjacent musculature. In this chapter we cover PTD of adult onset, discussing the clinical manifestations of the various focal dystonias and current thoughts on the underlying pathophysiology.
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Most adult-onset focal dystonias (AOFDs) are sporadic, although, on occasion, more than one member in a family may have a focal dystonia. Table 29–2 lists the terminology used to describe the various forms of AOFD. The AOFDs are much more common than previously recognized. Their prevalence has been estimated by Nutt and colleagues,13 in the population living in Rochester, Minnesota, at 30 per 100,000, compared to generalized PTD at 3.4 per 100,000. A study in the western area of Tottori Prefecture in Japan encountered a lower prevalence (6.2 per 100,000) for focal dystonias than in western countries.14 A study of primary dystonia in several European countries showed a prevalence rate of 152 per million, with the majority being focal dystonia (117 per million).15 The reasons for these geographical differences in prevalence are unclear and may reflect the difficulty in identifying cases: patients with focal dystonias may go undiagnosed, and others do not seek medical help. Although the prevalence of generalized PTD is higher among the Ashkenazi Jewish population, this is not the case for AOFD.16
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