Myoclonus

Myoclonus refers to unintentional, quick, lightning-like jerks (contractions) of muscles. Although not life-threatening, myoclonus can interfere with daily activities.

Myoclonus may occur normally. For example, it often occurs when a leg jerks as a person is falling asleep. But it may result from a disorder, such as liver or kidney failure. Myoclonus may occur after cardiac arrest—when the heart's pumping stops suddenly—or after taking high doses of certain drugs such as levodopa or bismuth. It may be caused by seizure disorders, Parkinson's disease, Alzheimer's disease, other dementias, head injuries, or heatstroke.

Myoclonus may affect only one hand, a group of muscles in the upper arm or leg, or a group of facial muscles. Hiccups are a type of myoclonus that involves only the diaphragm, the muscle that separates the chest from the abdomen. Myoclonus may also affect many muscles at the same time.

One type of myoclonus is called flapping tremor (asterixis). Flapping tremor occurs when a person stretches out the arms and extends the hands. One or both hands quickly drop, then return to their original position. That is, they flap. Flapping occurs because the muscle tone is lost briefly, then returns. Flapping tremor commonly results from liver failure and so has been called liver flap. However, it may also result from kidney failure or brain damage that results from lack of oxygen (for example, because blood flow is blocked) or from a low blood sugar level (hypoglycemia).

Myoclonus may vary in severity and frequency. It may be triggered by sudden noise, movement, or light. If severe, myoclonus can interfere with walking, talking, and eating. Thus, severe myoclonus can worsen an older person's quality of life and lead to undernutrition.

Diagnosis and Treatment

The diagnosis is based on symptoms and the results of a physical examination. Certain tests may be done to identify the cause of myoclonus. They include electroencephalography (EEG—which records electrical activity in the brain), electromyography (EMG—which records electrical activity in muscles), magnetic resonance imaging (MRI), and computed tomography (CT). For flapping tremor, blood tests are done to evaluate liver and kidney function.

If possible, the disorder causing myoclonus is treated. For flapping tremor, the underlying liver or kidney disorder is treated if possible. The tremor may then resolve. If myoclonus is severe, the same drugs used to prevent seizures (anticonvulsants), such as clonazepam or valproate, are sometimes helpful.

See the sidebar Hiccups: Spasms of the Diaphragm.