- 1 Clinically Relevant Anatomy
- 2 Mechanism of Injury / Pathological Process
- 3 Clinical Presentation
- 4 Physical Examination
- 5 Diagnostic Procedures
- 6 Treatment
- 7 Differential Diagnosis
- 8 References
Clinically Relevant Anatomy
Myoclonus is sudden, brief, jerky, shock-like, involuntary movements arising from the central nervous system and involving extremities, face, and trunk. 
According to the pathophysiologic mechanisms, myoclonus is classified into three main categories; cortical, subcortical, and spinal. Among these three categories, cortical myoclonus is most commonly encountered. 
Cortical myoclonus is caused by a hyperexcitable focus within the sensory-motor cortex. It typically involves a limb or the face and is triggered by action of intention.
Reticular myoclonus may occur spontaneously, in response to various peripheral stimuli of during voluntary action. The myoclonic jerks and tend to be generalised myoclonic jerks. Axial and proximal muscles are mainly involved, causing neck flexion, shoulder elevation with trunk and knee extension.
Negative (Asterixis) myoclonus
Negative or asterixis myoclonus occurs when a muscle contraction is suddenly interrupted and can be seen in either cortical or subcortical lesions. It is usually associated with metabolic or toxic encephalopathy, but unilateral asterixis myoclonus can be seen in patients with ischemic or hemorrhagic disorders, especially those involving the thalamus.
Mechanism of Injury / Pathological Process
Cortical myoclonus is not disease-specific. It is most commonly seen in a group of diseases such as progressive myoclonic epilepsy (PME), and also seen in juvenile myoclonic epilepsy, postanoxic myoclonus, corticobasal degeneration, Alzheimer’s disease, olivopontocerebellar atrophy, advanced Creuzfeldt-Jakob disease (CJD), metabolic encephalopathy, Rett syndrome, and celiac disease. 
Myoclonic jerks are usually arrhythmic (without rhythm or regularity) and can be described as action myoclonus (activated by voluntary movement), reflex myoclonus (activated by sensory stimulation). 
Rhythmic segmental myoclonus and brainstem myoclonus persisted during sleep. Myoclonic jerks usually represent brief muscle contractions (positive myoclonus) but may also be produced by equally brief lapses of muscle contraction (negative myoclonus or asterixis (a tremor of the hand when the wrist is extended)). In other words, positive myoclonus jerks originate from rapid, active contractions of a muscle or group of muscles. 
The physiologic characteristics of cortical myoclonus are:
1) an associated EMG discharge of very short duration (usually less than 50 ms)
(2) an EEG spike preceding the myoclonus. A short interval (20 ms in case of hand myoclonus) and localised to the area of the contralateral central region corresponding to the involved muscle (around C3 and C4 in case of hand myoclonus) detected by back averaging technique. 
It is important to check whether myoclonus appears at rest, on posture (keeping the arms outstretched) or during action and to note its distribution.
Myoclonus at rest indicates a spinal or brainstem source, whereas action-induced myoclonus points to a cortical origin. Focal and multifocal jerks, occurring during voluntary action, are typical of cortical myoclonus. Spinal segmental myoclonus is also focal, although contrary to cortical myoclonus, it is not action-induced and is occasionally stimulus sensitive. Generalized myoclonus is usually subcortical (brainstem or propriospinal myoclonus) or less frequently cortical. The amplitude of myoclonus varies considerably. Very small, hardly visible distal myoclonic jerks (mini polymyoclonus) are typical for Multiple System Atrophy (MSA), whereas very large amplitudes are typical for progressive myoclonic epilepsies (PME). 
Myoclonus is distinguished from tics because the latter can be controlled by an effort of will, at least temporarily, whereas myoclonus cannot.
Myoclonus classification is based on its anatomic origin: cortical, subcortical, spinal, and peripheral myoclonus.
Treatment of myoclonus is most effective when a reversible underlying cause can be found that can be treated — such as another condition, a medication or a toxin.
If the underlying cause can't be cured or eliminated, then treatment is aimed at easing myoclonus symptoms, especially when they're disabling. There are no drugs specifically designed to treat myoclonus. More than one drug may be needed to control the symptoms.
Medications that doctors commonly prescribe for myoclonus include:
- Clonazepam (Klonopin), a tranquilizer, is the most common drug used to combat myoclonus symptoms. Clonazepam may cause side effects such as loss of coordination and drowsiness.
- Anticonvulsant have proved helpful in reducing myoclonus symptoms. The most common anticonvulsants used for myoclonus are levetiracetam (Keppra, Roweepra, Spritam), valproic acid (Depakene) and primidone (Mysoline). Piracetam is another anticonvulsant that's been found to be effective, but it's not available in the United States. 
Rhythmic myoclonus may be confused with tremor. Its frequency is often slower than the commonly observed tremors, it is present at rest, is not modified significantly by voluntary movements and often persists during sleep. 
Myoclonus may also be confused with chorea, especially if multifocal and asynchronous, but in Chorea the movements continue in a constant flow, randomly distributed over the body and randomly distributed in time. 
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