Ballistic movement overflow myoclonus a form of essential myoclonus.

The clinical and electrophysiological findings in 2 cases of familial essential myoclonus are presented. The myoclonus was inherited apparently as an autosomal dominant trait, onset was in the first decade of life and the course was benign without the development of other significant neurological deficits. The electroencephalogram was unremarkable. The electromyographic appearance of the myoclonus was 50-100 ms complex bursts, usually occurring alternately in agonists and antagonists, similar to a normal ballistic movement. Muscles throughout the body were activated synchronously. In one of the cases myoclonus occurred at rest, but in both cases the myoclonus could be regularly produced by attempting a rapid movement. The myoclonic activity occurred simultaneously with the appropriate muscle activity which initiated the ballistic movement. It was as if the command to generate a ballistic movement overflowed into an excessive number of muscles. From a review of the literature it was concluded that this physiological mechanism was probably responsible for a definite sub-group of essential myoclonus. The relation of this type of myoclonus to other types of myoclonus and other involuntary movement disorders is discussed.