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Ivan Donaldson

, C. David Marsden

, Susanne A. Schneider

, and Kailash P. Bhatia

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date: 01 July 2022

As defined in Chapter 35, primary or idiopathic dystonias are disorders in which dystonia is the major neurological feature and it is not caused by other recognized diseases. The most important of these is primary generalized dystonia, which is also called idiopathic torsion dystonia or dystonia musculorum deformans. It is now known that the classical type, with early onset and initial involvement of the lower limbs, is often associated with abnormality of the DYT1 gene located on chromosome 9q34, or in some cases with mutations in the THAP1 gene (DYT6 dystonia), whereas the non-classical type, with late onset and commencement in the head and neck, and atypical types are usually not due to this genetic mechanism. This group of disorders is discussed in Chapter 35. In recent years it has become apparent that even within the rubric of primary torsion dystonia it is becoming possible to separate out specific entities with particular characteristics that suggest separate diseases. For example, dystonia with marked diurnal fluctuation [dopa-responsive dystonia or Segawa’s disease (DYT5)], hereditary myoclonic dystonia, rapid onset dystonia-parkinsonism (DYT12), and X-linked recessively inherited dystonia (Lubag/DYT3) are distinctive entities. These conditions will be considered in Chapter 36 under ‘Other primary dystonias’. There is ample evidence to suggest that the primary focal dystonias are a closely related group of disorders and some may in fact be partial expressions of the same genetic abnormalities which underlie the primary generalized dystonias (phenotypic heterogeneity). The high incidence of focal dystonia amongst the relatives of patients with generalized primary dystonia, plus the occasional development of the generalized disorder in children of subjects with otherwise uncomplicated focal dystonia, are in keeping with this. The site and extent of dystonia, which seems to be determined by age of onset, are the only clinical features separating these disorders and the individual movements themselves are identical. The focal dystonias include cranial dystonia (blepharospasm and oromandibular dystonia) and spasmodic dysphonia, spasmodic torticollis and truncal-axial dystonia, and writer’s and craft cramps. Paroxysmal dystonias, such as paroxysmal dystonic dyskinesia and ‘paroxysmal hypnic dystonia’ are dealt with in Chapter 49, in Section 11 on ‘Episodic Movement Disorders’. The way in which primary dystonias are subdivided and dealt with in this book is outlined in Table 1. It seems likely, however, that as knowledge grows more such subsets of the group now known as ‘primary dystonias’ will be identified.

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