Myoclonus and Dystonia as Recurrent Presenting Features in Patients with the SCA21-Associated p.Pro170Leu Variant.

Background: Spinocerebellar ataxia 21 (SCA21) is a rare neurological disorder caused by heterozygous variants in . A growing, yet still limited number of reports suggested that hyperkinetic movements should be considered a defining component of the disease.

Case Series: We describe two newly identified families harboring the recurrent pathogenic p.Pro170Leu variant. Both index patients and the mother of the first proband developed movement disorders, manifesting as myoclonic dystonia and action-induced dystonia without co-occurring ataxia in one case, and pancerebellar syndrome complicated by action-induced dystonia in the other. We reviewed the literature on variants linked to hyperkinetic disorders, comparing our cases to described phenotypes.

Discussion: Adding to prior preliminary observations, our series highlights the relevance of hyperkinetic movements as clinically meaningful features of SCA21. mutation should be included in the differential diagnosis of myoclonic dystonia and ataxia-dystonia syndromes.