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.
(See also Overview of Movement and Cerebellar Disorders.)
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 medications (see table Some Causes of Myoclonus). 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:
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 electroencephalogram (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 electroencephalogram abnormalities or giant somatosensory evoked potentials, and photic visual stimuli are not a trigger. The category of subcortical myoclonus includes essential myoclonus, myoclonus-dystonia (DYT11; associated with the SGCE gene), 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.
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 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.
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 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.
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.
