Figure 1. Models of tremor.

ET has been viewed as a single but heterogeneous disorder (A, model adapted from Louis et al⁶⁰) and as an “isolated action tremor syndrome” encompassing other movement disorders (B; accounting for cases where “classic ET is too narrowly defined”⁶¹). An alternative model (C, brought to the fore in the reclassification effort for dystonia⁵³) relies on the phenomenological characterization of tremor as a movement presented in isolation or in combination with other movement disorders. ET: essential tremor, BTP: benign tremulous parkinsonism; DIP: drug-induced parkinsonism; DYT: designation for genes associated with dystonia; PD; Parkinson disease; FXTAS: Fragile X tremor ataxia syndrome; SCA: spinocerebellar ataxia