Myoclonus is random, involuntary jerking involving single muscles or muscle groups. Myoclonus is not necessarily pathological; it includes the muscle jerk or startle motion sometimes experienced whilst falling asleep.
Multifocal myoclonus is associated with brain damage, neurological degenerative disease, and viral infections involving the central nervous system.
In the palliative care setting, myoclonus may be associated with hepatic or renal failure, severe hyponatremia, and is seen as part of the terminal restlessness syndrome.
Multifocal myoclonus may occur with any opioid analgesic and is most frequently seen with meperidine (pethidine).
Treatment is directed at the cause, where possible. Opioid-related myoclonus is treated by dose reduction or by switching to an alternative opioid. Clonazepam, midazolam, and gabapentin are reported to be effective.

Woodruff R. Palliative medicine evidence-based symptomatic and supportive care for patients with advanced cancer. 4th ed. Oxford University Press, 2004. p. 340.