Myoclonus

Clinical Classification

Classification—Anatomic Distribution
Another basis for classification of myoclonus is related to the anatomic distribution of the movements. Focal myoclonus consists of movements that are restricted to a defined body part. The most common types of focal myoclonus are spinal myoclonus, propriospinal myoclonus, arm myoclonus, and palatal myoclonus (although given the rapid movements of palatal myoclonus [150 times per minute], this may be classified as a tremor). The rhythmic movements of palatal myoclonus can be unilateral or bilateral and can be essential or symptomatic.

Segmental myoclonus involves adjacent areas of the body and is usually caused by damage to the spinal cord through trauma, inflammation, or tumor. Multifocal myoclonus involves two or more nonadjacent areas of the body. Generalized myoclonus involves synchronous jerks of one or more major muscle groups.

• Focal
• Segmental
• Multifocal
• Generalized

Classification—Provocative Factors
Myoclonic movements may also be classified based on their response to stimulus or movement. They are either spontaneous, in which case no stimulus is needed to elicit the jerks, or may appear in response to a stimulus (reflex), or with volitional movement (action or intention). Spontaneous jerks may be focal, multifocal, or generalized. Common causes of spontaneous myoclonus are metabolic encephalopathies or Creutzfeldt-Jakob disease.

Reflex myoclonus occurs in response to an auditory, somasthetic, or verbal stimulus. Hyperexplexia, an exaggerated startle reflex, can be idiopathic or have a genetic basis. The jerks of reflex myoclonus can be focal or generalized.

Of the types of myoclonus that are associated with provocative factors, action myoclonus is the most disabling. The jerks are triggered by voluntary movement or the intention to move, thereby preventing or disrupting volitional movement. The myoclonic movements range from simple focal to generalized severely disabling jerks. The primary cause of stimulus-sensitive myoclonus is a hypoxic or anoxic event—Lance Adams syndrome or posthypoxic myoclonus.

• Spontaneous
• Stimulus sensitive
  • Auditory
  • Somasthetic
  • Verbal
• Reflex
• Action

Classification—Contraction Pattern
Identifying the rhythmicity of the movements in myoclonus can assist the clinician in ascertaining the underlying pathophysiology. Jerks may be rhythmic, with a set regular pattern; arrhythmic, without a proscribed pattern; or oscillatory, which can often resemble a very fast tremor.

• Rhythmic
• Arrhythmic
• Oscillatory