What is Nocturnal Myoclonus?

Nocturnal myoclonus, also known as periodic limb movement disorder or PLMD, is a form of myoclonus in which the symptoms of the disorder are experienced during sleep. Myoclonus is itself a disease symptom, rather than a diagnosis, which manifests as sudden muscular contractions or muscular relaxation. Muscular contraction or relaxation can occur in response to a trigger, or may occur randomly. Occasional involuntary muscle twitches and hiccups are examples of non-disordered myoclonus.

Many people who experience nocturnal myoclonus do not have accompanying negative side effects. They may simply move their limbs during sleep without being disturbed by the movement. In these cases the person is said to have periodic limb movements during sleep, or PLMS. This is not the same as PLMD, as PLMS is not a disorder in itself, and does not always require treatment. PLMS is, however, a symptom of PLMD.

Nocturnal myoclonus is classified as a sleep disorder, and can cause symptoms during the night as well as side effects during waking hours. The disorder can cause nighttime sleep disturbance, insomnia, and sleepiness during waking hours.People who experience multiple episodes of periodic limb movements during sleep over several nights, as well as sleep disturbance and side effects during waking hours, may be diagnosed with PLMD.

Diagnosis of PLMD is made on the basis of anecdotal evidence from a patient’s partner or other close relatives or friends, as well as the results of a polysomnography. The polysomnography, or sleep study, is a test which records the biological and physiological changes that occur when a person is asleep. The test is carried out during a patient’s normal sleeping hours and records brain function, eye movements, muscle activity, and heart rhythm.

An estimated 4% of people are affected by PLMS. The percentage increases with increased age, and women are more likely to be affected than men. Up to 11% of elderly women may be affected. There is also evidence that PLMS is related to restless leg syndrome. One study has found that up to 80% of people with restless leg syndrome also had PLMS.

Risk factors for nocturnal myoclonus, particularly in the absence of restless leg syndrome, are varied. These include drinking coffee, snoring, stress, the use of hypnotics, and being a shift worker. Several risk factors are associated with restless leg syndrome as well as PLMD, including obstructive sleep apnea, musculoskeletal disease, heart disease, catalepsy, and mental disorder.

Treatments for nocturnal myoclonus are targeted to reducing incidences of leg jerking caused by muscular contraction or relaxation, and reducing incidence of being woken during sleep. Dopaminergic medications, which are drugs capable of stimulating dopamine receptors, are useful in treating both symptoms. Low doses of a medication called Clonazepam® can also help improve sleep quality.