Overview of PLMD/RLS
Periodic limb movement disorder (PLMD) and restless legs syndrome (RLS) are distinct disorders, but they often occur simultaneously. PLMD and RLS are types of parasomnia (i.e., conditions that occur during sleep and create a disruptive event). They also are called (nocturnal) myoclonus, which describes frequent or involuntary muscle spasms.
Periodic limb movement was formally described first in the 1950s, and, by the 1970s, it was listed as a potential cause of insomnia. In addition to producing similar symptoms, PLMD and RLS are treated similarly.
Periodic limb movement disorder affects people only during sleep. The condition is characterized by behavior ranging from shallow, continual movement of the ankle or toes, to wild and strenuous kicking and flailing of the legs and arms. Furthermore, abdominal, oral, and nasal movement sometimes accompanies PLMD. Movement of the legs is more typical than movement of the arms in cases of PLMD. Movements typically occur for 0.5 to 10 seconds, in intervals separated by five to 90 seconds.
In 1979, the Association of Sleep Disorder Centers (ASDC) set the parameters for determining the presence of PLMD:
- A formal diagnosis of nocturnal myoclonus requires three periods during the night, lasting from a few minutes to an hour or more, each containing at least 30 movements followed by partial arousal or awakening. (ASDC 1979)
Today, these parameters are a bit more relaxed, and PLMD usually includes any repetitive, involuntary movement during the night. These limb movements usually occur in deep stage two sleep, but often cause arousal. Thus, PLMD can cause poor sleep, which may lead to sleep maintenance insomnia and/or excessive daytime sleepiness.
The incidence of PLMD increases with age. It is estimated to occur in 5% of people age 30 to 50 and in 44% of people over the age of 65. As many as 12.2% of patients suffering from insomnia and 3.5% of patients suffering from excessive daytime sleepiness may experience PLMD.
Restless legs syndrome was described as early as the 16th century but was not studied until the 1940s. People with RLS complain of an irresistible urge to move their legs while at rest. A person with RLS will experience a vague, uncomfortable feeling while at rest that is only relieved by moving the legs.
Symptoms of RLS may be present all day long, making it difficult for an individual to sit motionless. Or they may be present only in the late evening. Late evening symptoms can lead to sleep onset insomnia, which tends to compound the effects of RLS. Pregnancy, uremia, and post-surgery conditions have also been known to increase the incidence of RLS. And, surprisingly, fever seems to decrease it.
Although one study found RLS to be most prevalent in middle-aged females, its incidence increases with age.
Restless legs syndrome is estimated to affect 5% of the population. Approximately 80% of people with RLS have PLMD, though most people with PLMD do not experience RLS.