In some people, myoclonic jerks occur in only a part of the body, such as the legs, with all the muscles in that part being involved in each jerk. Reticular reflex myoclonus can be triggered by either a voluntary movement or an external stimulus. Stimulus-sensitive myoclonus is triggered by a variety of external events, including noise, movement, and light. Surprise may increase the sensitivity of the patient. Sleep myoclonus occurs during the initial phases of sleep, especially at the moment of dropping off to sleep.
Some forms appear to be stimulus-sensitive.
Some persons with sleep myoclonus are rarely troubled by, or need treatment for, the condition. However, myoclonus may be a symptom in more complex and disturbing sleep disorders, such as restless legs syndrome, and may require treatment by a doctor. Studies suggest that several locations in the brain are involved in myoclonus. One such location, for example, is in the brainstem 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.