Myoclonus is a clinical sign that is characterized by brief, shock-like, involuntary movements caused by muscular contractions or inhibitions . Muscular contractions produce positive myoclonus, whereas muscular inhibitions produce negative myoclonus (ie, asterixis). Patients will usually describe myoclonus as consisting of "jerks," "shakes," or "spasms."
Myoclonic movements have many possible etiologies, anatomic sources, and pathophysiologic features . Myoclonus may be classified by clinical presentation, examination findings, clinical neurophysiology testing, and etiology.
This topic will discuss disorders associated with symptomatic (secondary) myoclonus. Other aspects of myoclonus are reviewed separately. (See "Classification and evaluation of myoclonus" and "Treatment of myoclonus".)
TERMINOLOGY AND EPIDEMIOLOGY
The classification scheme of Marsden and colleagues uses four major categories to organize the numerous etiologies of myoclonus (table 1) :