Clinical Classification

Anatomic Distribution
Another basis for classification of myoclonus is related to where in the body the movements occur (anatomic distribution).

Focal myoclonus consists of movements that are limited to a specific body part. The most common types of focal myoclonus are

• Spinal myoclonus
• Propriospinal myoclonus
• Arm myoclonus
• Palatal myoclonus

The rapid movements associated with palatal myoclonus (about 150 times per minute) may be classified as a tremor. The rhythmic movements of the soft palate (the rear of the roof of the mouth) may occur on one side (unilateral) or both sides (bilateral).

Segmental myoclonus involves areas of the body that are near or next to each other. This form of myoclonus is usually caused by damage to the spinal cord through trauma, inflammation, or a tumor.

Multifocal myoclonus involves two or more areas of the body that are not next to each other.

Generalized myoclonus involves jerky movements of one or more major muscle groups, all at the same time.

Provocative Factors
Myoclonic movements may also be classified based on their response to stimulus or movement (provocation). The movements are either unprompted (spontaneous), in which case they no stimulus is required to bring on the jerks, or they may appear in response to a stimulus (reflex) or with a voluntary movement (action or intention). Spontaneous jerks may be focal, multifocal, or generalized. Common causes of spontaneous myoclonus are metabolic disorders that affect the brain (encephalopathies) or Creutzfeldt-Jakob disease.

Reflex myoclonus may occur in response to certain sounds or words or to weakness and fatigue. Hyperexplexia, which is an exaggerated startle reflex, may occur for no known reason or it may have a genetic basis. The jerks of reflex myoclonus may be focal or generalized.

Action myoclonus is the most disabling form of myoclonus associated with provocative factors. The jerks are triggered by voluntary movement or the intention to move; these jerks then prevent or disrupt the movement. The myoclonic movements range from simple, localized focal jerks to generalized, disabling jerks. The primary cause of stimulus-sensitive myoclonus is an event that decreases the oxygen supply to the brain (hypoxia) or totally stops the flow of oxygen to the brain (anoxia); the resulting myoclonus is called Lance Adams syndrome or posthypoxic myoclonus.

Contraction Pattern
Identifying the rhythmic nature of the movements in myoclonus may assist in determining the underlying cause. Jerks may have a set regular pattern (rhythmic); have no definite pattern (arrhythmic); or resemble a very fast tremor (oscillatory).

• Rhythmic
• Arrhythmic
• Oscillatory

	Etiologic Classification