Myoclonus refers to sudden, brief involuntary twitching or jerking of a muscle or group of muscles. It describes a clinical sign and is not itself a disease. The twitching cannot be stopped or controlled by the person experiencing it.

People with myoclonus often describe their signs and symptoms just as jerks (myoclonic jerks), shakes, or spasms that are: Sudden, brief, involuntary shock-like, variable in intensity and frequency, localized to one part of the body or all over the body, and sometimes severe enough to interfere with eating, speaking or walking.

Myoclonic twitches or jerks usually are caused by sudden muscle contractions (tightening), called positive myoclonus, or by muscle relaxation, called negative myoclonus. Myoclonic jerks may occur alone or in sequence, in a pattern of movement or without a pattern. They may occur infrequently or many times per minute. Myoclonus sometimes happens in response to an external event or when a person attempts to make a movement.

When myoclonus is more widespread, it may involve persistent, shock-like contractions in a group of muscles. In some cases, myoclonus begins in one region of the body and spreads to muscles in other areas. More severe cases of myoclonus can affect movement and severely limit a person’s ability to eat, talk, or walk. These types of myoclonus are called pathologic myoclonus and can be one of many signs indicating a wide variety of underlying disorders in the brain or nerves, secondary to certain medical conditions, or can be a reaction to certain types of medication.

Myoclonus can occur by itself or as one of several symptoms associated with a wide variety of nervous system disorders such as autoimmune encephalitis. Treating the underlying cause will help control your myoclonus symptoms.

Studies suggest that several locations in the brain are involved in myoclonus. The cerebral cortex is the most common origin for myoclonus. Another location is in the brain stem close to structures that are responsible for the startle response—an automatic reaction to an unexpected stimulus involving rapid muscle contraction.

The specific mechanisms underlying myoclonus are not yet fully understood.