Genetics and pharmacological treatment of dystonia

Dystonia consists of involuntary repetitive twisting (torsion) or directional movements, sometimes leading to sustained postures. The movements are stereotyped and characterized by co-contraction of agonist and antagonist muscles. There is a broad clinical spectrum of dystonia which derives in part from the differential distribution of involvement. Dystonia may be localized, affecting a single body region, or generalized, affecting multiple extremities along with the trunk. Intermediate dystonic involvement can be described as segmental, designating two affected contiguous body regions, or multifocal, designating two or more noncontiguous affected body regions. Hemidystonia refers to dystonia affecting only one side of the body. Dystonia can also be categorized by age of onset and etiology. Early onset dystonia, occurring in childhood or adolescence (in some studies younger than 26 years old), is associated with more progressive disease [Greene et al. (1995). Mov. Disord. 10, 143]. In this age group, dystonia usually first appears in a limb and then spreads to involve other limbs and axial muscles; some early-onset patients may have involvement of laryngeal and other cranial muscles. Adult or late-onset dystonia typically begins in the neck, arm, or cranial muscles. Compared to early-onset dystonia, the area of involvement is more likely to remain focal or segmental. Dystonia can be considered either primary or nonprimary. Primary torsion dystonia (PTD), historically called dystonia musculorum deformans and Oppenheim's dystonia, describes dystonia in isolation, excepting tremor, without brain degeneration and without an identified acquired cause. Nonprimary or secondary dystonia encompasses a heterogeneous group of syndromes and etiologies including inherited (with or without brain degeneration), acquired, and complex neurological disorders. Monogenic forms of dystonia are labeled DYT and enumerated in the order in which they were discovered. The current 20 DYT loci comprise a heterogeneous group of disorders. (Table I) They can be divided into PTDs, dystonia-plus syndromes without brain degeneration, dystonia-parkinsonism with brain degeneration (i.e. DYT3), and paroxysmal dyskinesias. There are many neurodegenerative genetic disorders that share dystonia as a common feature of disease (Table II). This chapter will review the genetics of PTD, dystonia-plus syndromes without brain degeneration, and X-linked dystonia-parkinsonism. Other genetic dystonia-parkinsonism syndromes and the paroxysmal dyskinesias will not be discussed.