Myoclonus and neurodegenerative disease--what's in a name?

Myoclonus is a clinical symptom (or sign) defined as sudden, brief, shock-like, involuntary movements caused by muscular contractions or inhibitions. It may be classified by examination findings, etiology, or physiological characteristics. The main physiological categories for myocolonus are cortical, cortical-subcortical, subcortical, segmental, and peripheral. Neurodegenerative syndromes are potential causes of symptomatic myoclonus. Such syndromes include multiple system atrophy, corticobasal degeneration, progressive supranuclear palsy, frontotemporal dementia and parkinsonism linked to chromosome 17, Huntington's disease, dentato-rubro-pallido-luysian atrophy, Alzheimer's disease, and Parkinson's disease, and other Lewy body disorders. Each neurodegenerative syndrome can have overlapping as well as distinctive clinical neurophysiological properties. However, claims of differentiating between neurodegenerative disorders by using the presence or absence of small amplitude distal action myclonus appear unwarranted. When the myoclonus is small and repetitive, it may not be possible to distinguish it from tremor by phenotypic appearance alone. In this case, clinical neurophysiological offers an opportunity to provide greater differentiation of the phenomenon. More study of the myoclonus in neurodegenerative disease will lead to a better understanding of the processes that cause phenotypic variability among these disorders.