What is myoclonus?

Myoclonus is a symptom of a disease in which a muscle or group of muscles suddenly starts twitching or jerking. Some common examples of myoclonus are:

  • Hiccups
  • A sudden violent jerking or twitching motion when one is startled; or,
  • The twitching of an arm or leg as you begin to fall asleep

These types of muscle contractions (tightening) are normal. However, when muscle jerks or twitches happen frequently or affect more than one area of the body, they may cause problems with ordinary activities, such as walking, talking, or eating.

There are two basic types of myoclonus:

  • Positive myoclonus is the contraction of muscles.
  • Negative myoclonus is the sudden involuntary (uncontrolled) relaxation of a muscle or group of muscles. If a person is walking or standing, the sudden loss of muscle tone can cause him or her to fall.

Myoclonus may be categorized according to the part of the brain or central nervous system that is involved, or according to its cause.

What causes myoclonus?

Exactly why myoclonus occurs is not fully understood. In general, a myoclonic episode occurs when an abnormal electrical impulse is sent to a muscle or groups of muscles. The impulse usually comes from a location in the central nervous system, such as the brain’s cortex, the brainstem, or nerves. In some cases, the cause is damage to a peripheral nerve or nerves (those outside the central nervous system).

There are many underlying disorders or conditions that are associated with myoclonus, including:

  • Epilepsy
  • Head or spinal cord injuries
  • Brain tumors
  • Kidney or liver failure
  • Chemical or drug poisoning
  • A long period of time without oxygen (hypoxia)
  • Nervous system disorders
  • Genetic disorders

What are the types of myoclonus?

The different types of myoclonus are:

  1. Essential myoclonus is not caused by any underlying medical condition. This type of myoclonus remains stable (unchanging) over time.
  2. Opsoclonus myoclonus (Dancing Eyes-Dancing Feet Syndrome): a rare neurological condition whose symptoms include sudden, brief muscle spasms and rapid, irregular eye movements called opsoclonus. It may occur in children who have tumors, or as a result of a viral infection. Patients may also have trouble speaking. Other symptoms include poor muscle tone, irritability, or lethargy (feeling tired).
  3. Action myoclonus: This type can come on when the person moves or even tries to move. Action myoclonus is the most disabling type, because the muscle spasms can affect the arms, legs, face, and voice.
  4. Stimulus sensitive myoclonus: This type is triggered by outside stimuli, such as noise, lights, or movement.