Myoclonus and movement disorders.

Myoclonus is a movement disorder characterized by the occurrence of an involuntary abrupt muscle contraction causing a sudden unexpected jerk. Many other movement disorders can present with the same jerky, shock-like appearance. This paper reviews the clinical and neurophysiologic arguments supporting the distinction between true myoclonus and various imitators, including chorea, ballism, tics, dystonia, stereotypy, tremor and restless limbs. To be differentiated from myoclonus, these movement disorders, despite their heterogeneity, are distinctive through the patterned profile of muscle activation, the longer duration of the muscle contraction, the conditions in which they occur, and their suppressibility at will.