Objective: To provide guidance for current debates regarding the nosology of tremor in dystonia.

Background: Dystonia is defined as a disorder with sustained or intermittent muscle contractions causing abnormal, often repetitive and patterned movements or postures. In contrast, tremor is defined as a disorder with regular and rhythmical oscillation of a body region. While dystonia and tremor are viewed as distinct disorders, they have close epidemiological and phenomenological relationships. Further, there are disagreements about the optimal operational definition of dystonic tremor.

Methods: Subjects were recruited through the Dystonia Coalition by 38 sites in North America, Europe and Australia. Subjects were at least 18 years of age and had a diagnosis of isolated dystonia. Any body region could be affected, alone or in various combinations (focal, segmental, multifocal, and generalized). As part of an ongoing natural history study, all sites collected standardized, detailed clinical data, including an exam that detailed presence or absence of dystonia and tremor in each body part. And if tremor was present, what type and whether tremor is the dominant clinical feature was collected.

Results: 2,301 subjects were included in the study analyses. In keeping with previous reports, a large proportion of dystonia subjects had tremor upon exam, with co-prevalence varying according to body region affected, how dystonic tremor is defined, and variations in enrolling sites.

Conclusions: Dystonia and tremor occur together frequently. There are 3 hypothetical models to explain the epidemiological and biological relationships between tremor and dystonia. (1) Tremor and dystonia are biologically unrelated disorders, but a movement disorder indistinguishable from isolated tremor may sometimes occur in dystonia (Figure 1a). (2) Tremor and dystonia are caused by the same biological process, with the overt clinical manifestations being pleiomorphic consequences falling at different ends of a continuous spectrum (Figure 1b). (3) Tremor and dystonia are caused by different biological processes, but they frequently overlap because the biological pathways are partially related (Figure 1c). Current definitions for dystonia and tremor imply they are mutually exclusive disorders. Identifying the biological bases for both dystonia and tremor, such as responsible genes, may help clarify which model best represents the underlying relationship between these two diseases.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/dystonia-tremor-and-dystonic-tremor/