Myoclonus is muscle twitching that often occurs as people fall asleep. The sudden twitch startles many people out of sleep.
There is a sudden, involuntary jerking of a muscle or group of muscles. People cannot control their twitching. It never happens during stage R sleep, when the leg muscles are paralyzed. The kicking lasts from several minutes to hours. Sometimes the person knows about it; sometimes not – the subject only learns about the twitching when someone else notices or from kicking over bedding material. Note that hypnic jerks are not myoclonus; pretty much everyone occasionally has hypnic jerks when falling asleep.
It is most common in leg muscles and to a lesser extent in arms. It happens most often in light sleep. Some experts refer to this phenomenon as “periodic limb movements of sleep (PLMS)” and stress that they are not a true myoclonus the way some neurological disorders produce a myoclonus.
Myoclonic twitches or jerks usually are caused by positive myoclonus refers to sudden muscle contractions, negative myoclonus to muscle relaxation.
Myoclonic jerks may occur alone or in sequence, in a pattern or without pattern. They may occur infrequently or many times each minute. Myoclonus sometimes occurs in response to an external event or when a person attempts to make a movement. The twitching cannot be controlled by the person experiencing it.
Most people experience sleep myoclonus at one time or another. Hiccups are also a form of normal myoclonus. Fragmentary myoclonus is when certain parts of the body twitch; such as the digits or face muscles.
The large majority of sleep myoclonus cases do not need treatment. In bad cases, doctors do have some options.
Treatment of myoclonus may include anti-seizure drugs. Many of these drugs, which include barbiturates, clonazepam, phenytoin, primidone, and sodium valproate, are also used to treat epilepsy. Even with treatment, muscle contractions may persist. If myoclonus is due to a side effect of medication, discontinuing the drug is the best treatment option.
The first drug of choice is usually clonazepam, which as a tranquilizer has serious side effects. Because myoclonus has complex origins, researchers feel that no one drug works and that multiple drugs may be necessary.
Epidemiology studies of periodic limb movements
A high prevalence of periodic limb movements was found in elderly women according to an observational study from 2006 in the Journal of Clinical Sleep Medicine. This study examined 455 elderly women (mean age 82.9 years) from the community. The results showed 66% were found to have periodic limb movements of sleep index (PLMI) over 5 per hour, while 52% had PMLI over 15 per hour. This study also measured the number of periodic leg movements causing arousal (PMLA). The results showed 124 women (27%) had over 5 arousals per hour, while 26 women (6%) had over 15 arousals per hour. The researchers also found women with higher PMLA also suffered from lower sleep efficiency, higher percentages of sleep stages 1 and 2 and lower percentages of stages 3-4 and REM .
Another meta-study of people in several countries estimated the prevalence at 3.9%.
Another study assessed the frequency of periodic limb movements of sleep in patients with a history of stroke. The relevance of limb movements to stroke has implications for secondary stroke prevention and stroke recovery. A recent study from 2010 in the Journal of Clinical Sleep Medicine, found a higher prevalence of periodic limb movements of sleep in 40 patients with a history of stroke compared to a control group. 19 patients (47.5%) with a history of stroke and only 5 (12.5%) control patients were found to have periodic limb movements of sleep index over 5 per hour. The researchers concluded the stroke patients had both a higher prevalence and greater severity of periodic limb movements of sleep compared to the control group.
A similar study found a 19% prevalence of periodic limb movements of sleep in a sample of congestive heart failure outpatients
Restless leg syndrome is less common, affecting only 5% of the population. The main characteristic of RLS is a crawly feeling in the legs when the person is lying down. Both myoclonus and RLS can cause insomnia. Both are more common in men than women, and both tend to run in families. Neither of these parasomnias are understood at a deep biochemical level but they are thought to be related and derive from a dopaminergic dysfunction.
Other neurologic conditions may mimic the muscle contractions of myoclonus. But these are more accurately referred to as:
- Tics — a repetitive movement that’s difficult, if not impossible, to control – can be transient or chronic.
- Chorea — involuntary, complex body movements
- Dyskinesia — difficulty in performing voluntary movements