Myoclonus is defined as sudden, brief, shocklike movements caused by muscle contractions (positive myoclonus) or inhibitions (negative myoclonus). It can be confusing to clinicians because of its varied clinical appearance and disparate etiologies. Patients will usually call their myoclonus “jerks,” “jolts,” “shakes,” or “spasms.” Myoclonic movements are now recognized as having many possible etiologies, anatomical sources, and pathophysiologic features, including an association with epilepsy.1 However, this relationship with epilepsy has not always been clear. This chapter discusses that relationship and distinguishes the two disorders based on differences in phenotype, physiology, and treatment.
The term myoclonus was first described by Nikolaus Friedreich in 1881.2 He was confident that myoclonus should be distinguished from epilepsy.3 However, the 20th century saw increased study of both entities, and the two became intertwined. Because both myoclonus and epileptic seizures are manifested by paroxysmal, fast motor phenomena, the potential for overlap is obvious. Indeed, despite the close relationship between myoclonus and epilepsy, it has become evident that real distinctions between them are justified on the basis of phenotype, physiology, and treatment.
In Olmsted County, Minnesota, the average annual incidence rate for myoclonus as of 1976–1990 was 1.3 cases per 100, 000 person-years.4 The lifetime prevalence of myoclonus was 8.6 cases per 100,000 population. Symptomatic myoclonus (72%) was the most common clinical category, followed by epileptic myoclonus (17%) and essential myoclonus (11%).4 Posthypoxic state, neurodegenerative disease, and epilepsy syndromes are the most common causes of myoclonus. Toxic-metabolic and drug-induced cases are particularly common in the hospital setting.
Myoclonus may be classified by examination findings, physiology based on clinical neurophysiology testing, and clinical presentation/etiology.
The myoclonus examination findings are classified by distribution, temporal profile, and activation characteristics of the myoclonic movement. The distribution can be focal, multifocal, segmental, or generalized. A multifocal myoclonus distribution is widespread, noncontiguous, and irregular, but there may be bilaterally synchronous movements as well. A generalized myoclonic jerk refers to a simultaneous muscle contraction that is both diffuse and bilateral. The temporal profile of myoclonic jerking can be continuous or intermittent, as well as rhythmic or irregular. If intermittent, the myoclonus can occur as isolated or repetitive jerks. The activation of the myoclonus may be at rest (spontaneous), induced by various stimuli (reflex myoclonus), or induced by voluntary movement (action myoclonus), or some combination of these. All the above activation characteristics should be noted as absent or present.
Understanding the neurophysiological aspects of myoclonus is of critical importance. The term neurophysiology used here implies both the neuroanatomical source of the myoclonus and the activity within the motor circuits generating the jerk. Certain patterns can correlate with diagnostic classification as well as predict which ...