Myoclonus refers to quick, lightning-like jerks (contractions) of a muscle or a group of muscles.
Myoclonus may occur normally (for example, jerking of a leg when a person is falling asleep), but it may result from a disorder, such as liver failure, a head injury, low blood sugar, or Parkinson disease or from use of certain medications.
Muscles may jerk quickly or slowly, and jerking may be rhythmic or not.
Doctors diagnose myoclonus based on symptoms and do blood tests, electromyography, and/or magnetic resonance imaging to identify the cause.
(See also Overview of Movement Disorders.)
Myoclonus may involve only one hand, a group of muscles in the upper arm or leg, or a group of facial muscles. Or it may involve many muscles at the same time.
Myoclonus may be classified in many different ways:
By cause: Whether it occurs normally, is caused by a disorder, medication, or substance or has no identifiable cause
By severity: How much of the brain is damaged or how severe the symptoms are
By location: Where the brain is damaged
By triggers: Whether it is triggered by a stimulus (such as bright light or a sudden noise) or occurs on its own (spontaneously)
Classifying the type of myoclonus may help doctors identify the cause and choose appropriate treatments.
Causes of Myoclonus
Myoclonus may occur normally, often when a person is falling asleep. For example, as people start to doze off, they may jerk suddenly and awaken (as if startled), or muscles in part of the body may twitch. Abnormal myoclonus tends to occur more randomly and/or be more severe or frequent.
However, in some cases myoclonus may result from a disorder, such as the following:
Brain damage due to a virus (such as encephalitis due to herpes simplex)
Brain damage after cardiac arrest (when the heart's pumping stops suddenly)
Metabolic disorders (such as a high blood sugar level or low blood sugar level or low levels of calcium, magnesium, or sodium)
Oxygen deprivation
Alzheimer disease (occasionally)
Certain seizure disorders (such as juvenile myoclonic epilepsy)
Myoclonus can occur after a person takes high doses of certain medications such as the following:
Antihistamines
Some antibiotics (such as penicillin and cephalosporins)
Bismuth
Opioids
Symptoms of Myoclonus
Myoclonus can be mild or severe. Muscles may jerk quickly or slowly, rhythmically or not. Myoclonus may occur once in a while or frequently. It may occur spontaneously or be triggered by a stimulus, such as a sudden noise, light, or a movement. For example, reaching for an object or taking a step may trigger jerks that disrupt the movement.
Myoclonus that occurs when people are suddenly startled (startle myoclonus) may be an early symptom of Creutzfeldt-Jacob disease (a rare disorder that causes the brain to degenerate).
If myoclonus is due to a metabolic disorder, the face, upper arms, and thighs are usually affected. If it persists, it may affect muscles throughout the body, sometimes leading to seizures.
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Diagnosis of Myoclonus
A doctor's evaluation
Blood tests and sometimes other tests to identify the cause
The diagnosis of myoclonus is based on symptoms.
Testing is usually done to identify the cause:
Blood tests are usually done to check kidney and liver function and to measure the level of sugar, calcium, magnesium, or sodium in the blood. Abnormal levels of these substances may indicate that the cause is a metabolic disorder.
Magnetic resonance imaging may be done to check for abnormalities in the brain, such as those caused by Alzheimer disease or Creutzfeldt-Jacob disease.
Electroencephalography may be done to check for myoclonus in people with a seizure disorder.
Treatment of Myoclonus
Correction of the cause if possible
Medications to lessen symptoms
The cause of myoclonus is corrected if possible. For example, medications that can cause myoclonus are stopped. A high or low blood sugar level is corrected, and kidney failure is treated with hemodialysis.
Medications Used to Treat Seizures
