Myoclonus is an involuntary contraction or relaxation of part of a muscle, an entire muscle or a group of muscles. In other words, myoclonus is a brief, involuntary twitching of a muscle or a group of muscles in a coarse, repetitive, involuntary, and rhythmic manner at rates up to 60 times per minute (sometimes even occurring during sleep). Positive myoclonus is characterized by muscular contraction, and negative myoclonus by sudden muscular relaxation. They may be the symptom of disease, especially if recurring. It describes a medical sign and, generally, is not a diagnosis of a disease.
The abnormal contractions are due to nervous dysfunction. Myoclonus most commonly affect groups of muscles involved in chewing and/or any of the skeletal muscles of the limbs.
Since recurring myoclonus is the sign of another underlying condition, the symptoms vary depending on the underlying condition. Most apparent symptom is the involuntary, continuous, coarse, and rhythmic contractions of a muscle, portion of a muscle, or group of muscles.