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Hector A. Gonzalez-Usigli

, MD, HE UMAE Centro Médico Nacional de Occidente

Last full review/revision May 2020| Content last modified May 2020
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Myoclonus is a brief, shocklike contraction of a muscle or group of muscles. Diagnosis is clinical and sometimes confirmed by electromyographic testing. Treatment includes correction of reversible causes and, when necessary, oral drugs to relieve symptoms.

Classification of myoclonus

Myoclonus may be classified as physiologic (benign) or pathologic (1).

Physiologic myoclonus may occur when a person is falling asleep and during early sleep phases (called hypnic myoclonus). Hypnic myoclonus can be focal, multifocal, segmental, or generalized (see below) and may resemble a startle reaction. Another type of physiologic myoclonus is hiccuping (diaphragmatic myoclonus).

Pathologic myoclonus can result from various disorders and drugs (see table Some Causes of Myoclonus). The most common causes are

  • Hypoxia

  • Drug toxicity

  • Metabolic disturbances

Other causes of pathologic myoclonus include degenerative disorders affecting the basal ganglia and some dementias.

Myoclonus may also be classified as follows:

  • By its distribution: Focal, segmental (contiguous areas), multifocal (noncontiguous areas), or generalized

  • By its site of origin: Cortical, subcortical, segmental, or peripheral

  • By clinical presentation: Positive or negative

  • By its etiology: Essential (primary), acquired, or idiopathic

  • By its trigger: Sensory or spontaneous

Myoclonus classified by site of origin is as follows:

  • Cortical: Cortical myoclonus is associated with cerebral cortex damage or epilepsy. Photic visual stimuli or touching may trigger myoclonic jerks, which may cause abnormalities on an EEG (eg, focal or generalized spike-and-wave or polyspike-and-wave epileptiform discharges, giant somatosensory evoked potentials). The myoclonic jerks may be less evident at rest but aggravated during motor action. This type of myoclonus can severely impair speech and gait.

  • Subcortical: Subcortical myoclonus is associated with disorders that affect the basal ganglia or other subcortical structures. Its effects are similar to those of cortical myoclonus. However, there are no EEG abnormalities or giant somatosensory evoked potentials, and photic visual stimuli are not a trigger. The category of subcortical myoclonus includes essential myoclonus, myoclonus-dystonia, reticular reflex myoclonus, startle syndromes, Creutzfeldt–Jakob disease, and subacute sclerosing panencephalitis (1).

  • Segmental and peripheral: Forms of segmental or peripheral myoclonus are relatively rare. Segmental myoclonus includes spinal segmental and propriospinal myoclonus. Spinal segmental myoclonus refers to myoclonus in spinal muscles of one or several contiguous segments of the spinal cord. Propriospinal myoclonus is characterized by slowly propagated movements that spare the face, often with a burst duration incompatible with other types of myoclonus. Palatal myoclonus, now mainly considered a misnomer, has been reclassified as palatal tremor. The most common peripheral myoclonus is hemifacial spasm; it results primarily from vascular compression of the facial nerve as it exits from the brain stem or from compression by cerebellopontine angle tumors. Hemimasticatory spasm is much rarer; it is characterized by unilateral, paroxysmal contractions of the jaw muscles. It may be caused by compression of the motor branch of the trigeminal nerve Myoclonus : Classification reference.

Classifying myoclonus based on site of origin is thought to be the most helpful when choosing the most effective treatment.

Clinical presentation of patients with myoclonus may be classified as positive or negative:

  • Positive: Patients have an active muscle twitch leading to a jerk.

  • Negative: Muscle tone suddenly lapses (with electrographic silence on EMG); when antigravity muscles lose muscle tone, the patient may fall. Negative myoclonus includes asterixis (eg, flapping of the hands that occurs in patients with severe liver failure).

Positive and negative myoclonus often occur in the same patient.

Etiology of myoclonus may be essential (primary), acquired (most common), or idiopathic.

  • Essential (primary) myoclonus has no identifiable cause and/or is suspected to involve genetic factors.

  • Acquired myoclonus has multiple causes, including many metabolic disorders (see table Some Causes of Myoclonus). Most cases of myoclonus are acquired.

  • Idiopathic myoclonus is myoclonus whose presence is completely unexplained.

Myoclonus may have a trigger or not:

  • Sensory sensitive: Myoclonus is triggered by a stimulus (eg, sudden noise, movement, light, visual threat), as can occur when a person is suddenly startled (startle response).

  • Spontaneous: Myoclonus occurs without a trigger, as often occurs when the cause is metabolic.


Some Causes of Myoclonus



Degeneration of the basal ganglia

Multiple system atrophy


Progressive myoclonic encephalopathies (eg, mitochondrial disorders, certain types of epilepsy, such as sialidosis, neuronal ceroid lipofuscinosis, Lafora disease, Unverricht-Lundborg disease)

Metabolic disturbances


Hyperglycemia, nonketotic


Physical and hypoxic encephalopathies


Toxic encephalopathies


Heavy metals (including bismuth)

Methyl bromide

Encephalitis lethargica

Herpes simplex encephalitis

Postinfectious encephalitis

Subacute sclerosing panencephalitis

Drugs at toxic or high doses






MAO inhibitors

Opioids (usually dose related)



Tricyclic antidepressants



* With long-term treatment; dose-related.

DDT = dichlorodiphenyltrichloroethane; MAO = monoamine oxidase; SSRI = selective serotonin reuptake inhibitor.

Classification reference

  • 1. Eberhardt O, Topka H: Myoclonic disorders. Brain Sci7 (8): 103, 2017. doi: 10.3390/brainsci7080103.

Symptoms and Signs of Myoclonus

Myoclonus can vary in amplitude, frequency, and distribution.

Muscle jerks may occur spontaneously or be induced by a stimulus (eg, sudden noise, movement, light, visual threat).

Myoclonus that occurs when patients are suddenly startled (startle myoclonus) may be an early symptom of Creutzfeldt-Jacob disease.

Myoclonus due to severe closed head trauma or hypoxic-ischemic brain damage may worsen with purposeful movements (action myoclonus) or may occur spontaneously when movement is limited because of injury.

Myoclonus due to metabolic disturbances may be multifocal, asymmetric, and stimulus-induced; it usually involves facial or proximal limb muscles. If the disturbance persists, generalized myoclonic jerks and, ultimately, seizures may occur.

Diagnosis of Myoclonus

  • Clinical evaluation

Diagnosis of myoclonus is clinical. Testing is done based on clinically suspected causes. Brain imaging is done.

  • Correcting the metabolic disturbance or other cause if possible

  • Stopping or reducing the dose of the causative drug

  • Drug therapy to relieve symptoms

Treatment of myoclonus begins with correction of underlying metabolic disturbances or other causes if correctable. If a drug is the cause, the drug is stopped, or the dose is reduced.

For symptom relief, clonazepam 0.5 to 2 mg orally 3 times a day is often effective. Valproate 250 to 500 mg orally 2 times a day or levetiracetam 250 to 500 mg orally 1 or 2 times a day may be effective; rarely, other antiseizure drugs help. Doses of clonazepam or valproate may need to be lower in older patients.

Site of origin for myoclonus can help guide treatment. For example, valproate, levetiracetam, and piracetam tend to be effective in cortical myoclonus but ineffective in other types of myoclonus. Clonazepam may be effective in all types of myoclonus. In some cases, a combination of drugs is necessary.

In the past, many types of myoclonus seemed to respond to the serotonin precursor 5-hydroxytryptophan, which was used with the oral decarboxylase inhibitor carbidopa, but 5-hydroxytryptophan is not used anymore because better drugs are now available.

Key Points

  • Myoclonus is a brief, shocklike muscle contraction that can vary in severity and distribution.

  • Myoclonus can be physiologic (eg, hiccuping, sleep-related muscle contractions) or secondary to various brain disorders, systemic disorders, or drugs.

  • If a metabolic disturbance is the cause, correct it, and when necessary, give drugs (eg, clonazepam, valproate, levetiracetam) to relieve symptoms.

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