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.
The cause of myoclonus is corrected if possible, but if the cause cannot be corrected, certain antiseizure medications or clonazepam (a mild sedative) may lessen symptoms.
(See also Overview of Movement Disorders Overview of Movement Disorders Every body movement, from raising a hand to smiling, involves a complex interaction between the central nervous system (brain and spinal cord), nerves, and muscles. Damage to or malfunction... read more .)
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 Encephalitis Encephalitis is inflammation of the brain that occurs when a virus directly infects the brain or when a virus, vaccine, or something else triggers inflammation. The spinal cord may also be involved... read more due to herpes simplex)
Brain damage after cardiac arrest Cardiac Arrest and CPR Cardiac arrest is when the heart stops pumping blood and oxygen to the brain and other organs and tissues. Sometimes a person can be revived after cardiac arrest, particularly if treatment is... read more (when the heart's pumping stops suddenly)
Metabolic disorders (such as a high blood sugar level or low blood sugar level Hypoglycemia Hypoglycemia is abnormally low levels of sugar (glucose) in the blood. Hypoglycemia is most often caused by medications taken to control diabetes. Much less common causes of hypoglycemia include... read more or low levels of calcium Hypocalcemia (Low Level of Calcium in the Blood) In hypocalcemia, the calcium level in blood is too low. A low calcium level may result from a problem with the parathyroid glands, as well as from diet, kidney disorders, or certain medications... read more , magnesium Hypomagnesemia (Low Level of Magnesium in the Blood) In hypomagnesemia, the level of magnesium in blood is too low. (See also Overview of Electrolytes and Overview of Magnesium's Role in the Body.) Magnesium is one of the body's electrolytes,... read more , or sodium Hyponatremia (Low Level of Sodium in the Blood) In hyponatremia, the level of sodium in blood is too low. A low sodium level has many causes, including consumption of too many fluids, kidney failure, heart failure, cirrhosis, and use of diuretics... read more )
Myoclonus can occur after a person takes high doses of certain medications such as the following:
Some antidepressants (such as amitriptyline)
Some antibiotics (such as penicillin and cephalosporins)
Levodopa (used for Parkinson disease)
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 Creutzfeldt-Jakob Disease (CJD) Creutzfeldt-Jakob disease is a prion disease characterized by progressive deterioration of mental function, leading to dementia, involuntary jerking of muscles (myoclonus), and staggering when... read more (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.
Did You Know...
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 Electroencephalography Diagnostic procedures may be needed to confirm a diagnosis suggested by the medical history and neurologic examination. Imaging tests commonly used to diagnose nervous system (neurologic) disorders... read more 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.
If the cause cannot be corrected, certain antiseizure medications (such as valproate and levetiracetam—see table ) or clonazepam (a mild sedative) may lessen symptoms. Choice of medication depends on where the brain is damaged. In some cases, a combination of medications is necessary.