Objective: In collaboration with the International Parkinson and Movement Disorder Task Force for Nomenclature of Genetic Movement Disorders, we present a new classification of genetically determined myoclonus syndromes aligned with the renewed naming system of the MDS Task Force.

Background: The Task Force was established in 2012 to renew and advocate the naming system of genetically determined movement disorders as the previous system (e.g. PARK, DYT designations) was problematic. Here, we present the nomenclature of genetically determined myoclonus syndromes of which currently no naming system is available. We have included genetic causes of myoclonus epilepsy as myoclonus can be present in the borderland of epilepsy and movement disorders; however, from the perspective of the movement disorder specialist.

Methods: A systematic review was performed using the search terms ‘myoclonus’, ‘myoclonic jerks’ or ‘startle’ with ‘genetic cause’ in Pubmed, OMIM and textbooks. The criteria for gene inclusion were adopted from the Task Force and included 1) the confirmed description of a monogenic disorder (no risk factor genes) and 2) myoclonus as a predominant feature reported by two independent groups of researchers. (1) The severity of the most common accompanying symptoms (epilepsy, ataxia, cognitive decline and dystonia) and clinical clues were stated for each gene.

Results: In total, 150 genes associated with myoclonus syndromes were identified and after administration of the inclusion criteria, 52 disease-causing genes were included. We divided the genetic disorders based on the occurence of predominant myoclonus in the majority or minority of patients. The disorders were further sorted based on anatomical origin of myoclonus creating three subgroups: 1) the cortex: 41 genes of which nine were described as myoclonus epilepsy; 2) undetermined (previously subcortical) areas: eight genes; and 3) the brainstem: three genes. Cortical myoclonus was most commonly accompanied by epilepsy, cognitive decline and ataxia resembling the progressive myoclonus epilepsy/ataxia phenotypes. Undetermined myoclonus was most commonly accompanied by dystonia resembling the myoclonus-dystonia phenotype.

Conclusions: The new table can be used as a diagnostic framework for physicians in clinical practice and can easily be extended by adding newly identified genes in the future.

References: 1. Marras C, Lang A, van de Warrenburg BP, et al. Nomenclature of genetic movement disorders: Recommendations of the International Parkinson and Movement Disorder Society Task Force. Mov. Disord. 2016; 31: 436–57.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/new-nomenclature-of-genetic-myoclonus-syndromes/