MSD Manual

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Alex Rajput

, MD, University of Saskatchewan;

Eric Noyes

, MD, University of Saskatchewan

Reviewed/Revised Feb 2024
Topic Resources

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 medications to relieve symptoms.

Classification of myoclonus

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 medications (see table ). The most common causes are

  • Hypoxia

  • Medication 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:

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 electromyography); 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 ). 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.


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 due to severe closed head trauma or hypoxic-ischemic brain damage may worsen when patients make 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.

Treatment of Myoclonus

  • Correcting the metabolic disturbance or other cause if possible

  • Stopping or reducing the dose of the causative agent

  • Using pharmacotherapy to relieve symptoms

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

Site of origin for myoclonus helps guide symptomatic 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. Doses of clonazepam or valproate may need to be lower in older adults. In some cases, a combination of medications 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 medications 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 medications.

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

NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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