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  • Updated 05.07.2020
  • Released 04.07.1994
  • Expires For CME 05.07.2023

Myoclonus

Introduction

Overview

Myoclonus is an involuntary shock-like contraction of a muscle or a group of muscles, followed by relaxation. Myoclonic jerks are irregular in rhythm and amplitude. This article presents an overview of myoclonus with an emphasis on differential diagnosis, etiology, and treatment. Myoclonus occurs as a symptom or sign in numerous diseases and conditions, necessitating an organized approach to diagnostic evaluation. Noting the clinical circumstances that surround the myoclonus is key. The first step in focusing on the myoclonus is to determine its category of clinical classification. Subsequent phases of testing can then be added as necessary. The physiological classification of myoclonus compliments the other information for diagnostic purposes. Once the diagnosis is determined, treatment of the underlying cause or symptomatic treatment of the myoclonus can be undertaken. For this update, the author has organized the treatment of myoclonus according to its pathophysiology with new treatment information added. This includes instances where deep brain stimulation or botulinum toxin injection may treat myoclonus.

Key points

• Myoclonus can be a symptom or a sign, but it is not a diagnosis.

• Myoclonus occurs in numerous diseases and conditions, necessitating an organized approach to diagnostic evaluation.

• Classification with regard to exam findings, clinical circumstances, and pathophysiology offers complimentary, not redundant, information.

• Symptomatic treatment approach is best strategized on the basis of the physiology classification of the myoclonus.

• There should be a thorough search for myoclonus etiology and consideration for etiology treatment before symptomatic treatment is considered.

Historical note and terminology

Friedreich first described myoclonus as "paramyoklonus multiplex" in a patient with myoclonus (68). He used the term “myo” to distinguish quick movements from the epileptic disorders; “para” to indicate symmetry; and “multiplex” to stress the multifocal quality of the disorder. Lowenfeld proposed that "paramyoklonus multiplex" be shortened to "myoklonus" (84). Rabot described nonprogressive familial myoclonic epilepsy in 1899. In 1903, Lundborg classified myoclonus into 3 groups: essential, symptomatic, and familial myoclonic epilepsy.

Literally, myoclonus means “a quick movement of muscle.” Myoclonus refers to sudden, brief, shocklike involuntary movements caused by muscular contraction (positive myoclonus) or inhibitions (negative myoclonus) usually arising from the central nervous system (35; 32). Myoclonus may be classified according to clinical presentation/etiology, examination findings, or physiology. Four categories are used for classification into clinical presentation: physiologic, essential, epileptic, or symptomatic myoclonus. Each of these clinical presentation categories contains various etiologies. On exam, myoclonus may be rhythmic, in which case it is referred to by some movement disorder specialists as tremor; more typically, it is arrhythmic. Stimulus sensitive myoclonus is termed “reflex myoclonus” and action sensitive myoclonus is termed “action (or intention) myoclonus.” Myoclonus can be classified according to the distribution: focal or segmental (confined to 1 particular region of the body), multifocal (different parts of the body affected, not necessarily at the same time), or generalized (whole body part affected in a single jerk). The classification according to the physiological type or classification of electrical discharge is: cortical, cortical-subcortical, subcortical-nonsegmental, segmental, or peripheral.

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