Myoclonus and extrapyramidal diseases.

Parkinsonism or dystonia are associated with myoclonus in several extrapyramidal diseases. Although the latter symptom is not always prominent, stimulus-sensitive, distal, or focal reflex myoclonus is frequently observed. This review will consider the clinical and electrophysiological features of myoclonus in Parkinson's disease, multiple system atrophy, corticobasal degeneration, progressive supranuclear palsy, Huntington's disease, dentatorubral-pallidoluysian atrophy, Lewy body dementia, and myoclonus with dystonia. The evidence of a long-latency reflex response, the presence of giant somatosensory evoked potentials, and the demonstration of a back-averaged premyoclonus focal cortical EEG activity often lead to classify myoclonus as a cortical phenomenon. However, a subcortical origin cannot always be ruled out.