Comparing the two-axis classifications of hyperkinetic disorders.

This review analyzes the structure and content of the two-axis classification systems for hyperkinetic disorders. The first classification was developed for dystonia in 2013, a tremor classification appeared in 2018, and a myoclonus classification in 2025, together with a revision of the 2013 dystonia classification. In all these classifications, Axis I refers to clinical characteristics, and Axis II to etiology. However, for myoclonus there is also an Axis Ib dedicated to neurophysiology. The clinical features of dystonia are recognized by clinical assessment, whereas in the case of tremor, and particularly of myoclonus, neurophysiology plays a relevant role. In the tremor and myoclonus classifications, neurophysiology is included as an Axis I descriptor aside clinical assessment. The positioning of other laboratory tests differs among classifications. Neuroimaging is an etiological Axis II division in dystonia, whereas it is included aside other laboratory tests under Axis I in the tremor and myoclonus classifications. The results of genetic testing are included under Axis II in the dystonia and tremor classifications, instead under Axis I in the myoclonus classification. The positioning of functional movement disorders also differs, and for myoclonus a new Axis II etiology of "functional neurological disorders" has been added. These classification exercises highlight the specificity of different hyperkinetic disorders on the background of a comparable two-axis structure.