Objective: To characterize clinically and genetically a series of 23 patients with myoclonus dystonia syndrome (MDS) and to explore the Unified Myoclonus Rating Scale (UMRS) to assess disease severity and evolution after bilateral deep brain stimulation of the internal globus pallidum (GPi-DBS).

Background: MDS is a genetic movement disorder characterized by dystonia and myoclonic jerks, often associated with psychiatric comorbidities. Common onset is on the first two decades of life. Daily life activities and quality of life might be affected, mainly due to gait impairment , feeding/drinking and writing difficulties. Pharmacological treatment has limited efficacy and poor tolerability. GPi-DBS has been described as an effective option with potentially long-term benefit.

Methods: Cross-sectional study of patients with MDS seen at a reference center for movement disorders in Spain. The UMRS was used to assess myoclonus severity. Genetic testing of SGCE mutations was performed by Sanger method.

Results: 23 patients (11 males/12 females) from 16 families were included (median age at movement disorder onset: 2.7y, range 1-8y). Symptoms at debut were mainly myoclonus (n=16), followed by combination of myoclonus and dystonia (n=6). Mean age at clinical assessment was 12,7y (range 3-46y). Neuropsychiatric symptoms were observed in 14 patients, being anxiety the most frequent (n=12). UMRS was applied to 19 children with a mean age of 8,5y (range 3 -16y) and 4 adults with a median age of 29,2y (range 19-46y). Predominantly difficulties secondary to movement disorder, were reported in eating, drinking and walking (figure 1). Action myoclonus was observed predominantly in upper limbs, legs and neck (figure 2). In functional tests, patients showed more difficulties in Archimedes spiral drawing with left hand and pouring water (figure 3). SGCE mutations were identified in all patients. Genetic testing in 11 families identified the same mutation in 7 asymptomatic fathers. Two teenagers treated with GPi-DBS showed a significant reduction in action myoclonus and functional performance according to the UMRS (table 1).

Conclusions: Our series expands the clinical and genetic spectrum of MDS due to SGCE mutations. The UMDRS seems a valid method to assess myoclonus severity in children. The beneficial use of DBS in MDS is encouraging and should be indicated and further explored in the paediatric population.

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