Objective: To discuss the etiological differential diagnosis of the clinical phenotype characterized by myoclonus and parkinsonism.

Background: The presence of myoclonus is increasingly recognized as a manifestation of diseases that primarily affect the basal ganglia. When present in association with parkinsonism, they usually suggest a less common diagnosis of parkinsonism.

Method: Case report

Results: We present the case of a 63-year-old man, with no relevant personal history, admitted for progressive onset of generalized, positive and negative, stimulus-sensitive myoclonus. No other neurological abnormalities were found on examination. He started on levetiracetam 500mg with resolution of the symptoms. EEG, MRI with contrast enhancement, and analytical evaluation with systemic autoimmunity, antineuronal, and antimembrane studies were normal. About 8 months after the initial presentation, he began to complain of erectile dysfunction, urinary urgency, difficulty with fine motor skills of the left hand, and sleep disturbance with vocalizations and dream enactment, suggesting REM sleep behavior disorder. On examination, he had asymmetric akinetic-rigid syndrome, postural instability, mild gait ataxia, hyperreflexia, and orthostatic hypotension. A new brain MRI was normal. IBZM showed bilateral dysfunction of the nigrostriatal pathway. Polysomnography revealed REM sleep without muscle atonia. Levodopa 300mg/day was associated with clinical worsening of orthostatic hypotension, without improvement of parkinsonism. With this case, we aim to discuss the possible differential diagnoses of a patient with generalized myoclonus and parkinsonism.

Conclusion: The concomitant presence of myoclonus and parkinsonism is associated with a limited differential diagnosis. According to the diagnostic criteria for Multiple System Atrophy (MSA) proposed by the Movement Disorder Society in 2022, this patient would fulfill criteria for possible MSA. Although the presence of myoclonus is described in MSA, particularly as minipolymyoclonus, the presence of generalized myoclonus is uncommon, especially as a presentation form as in this case.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/an-atypical-presentation-of-multiple-system-atrophy-msa/