Also known as nocturnal myoclonus, involves disruptive movements of the legs and arms. Although RLS and PLMD both affect the limbs - and both affect a person’s ability to sleep at night and function normally during the day - they are two different disorders. The movements of RLS occur most often when a person is awake and are a voluntary response to uncomfortable or painful feelings in the legs.The movements of PLMD occur most often when a person is asleep and are involuntary (not consciously controlled). People with periodic limb movements are often not aware of these movements, although on rare occasions they may notice the involuntary movement of PLMD while they are still awake. Most people with RLS have periodic limb movements; however, patients with PLMD often do not have RLS.

Periodic limb movements usually occur in the legs but occasionally may also affect the arms. As the name inplies, the movements occur at periodic (regular) intervals, usually every 30 seconds. They typically consist of a rhythmic extension of the big toe, together with an upward bending of the ankle, knee or hip. The movements usually do not occur continuously throughout the night but, instead, cluster in the first half of the night during non-REM sleep. During REM sleep (when people are usually dreaming) the movements are much less common. The muscles are normally paralyzed during REM sleep to prevent a person from acting out the dream. This paralysis also appears to prevent periodic limb movement.

When the leg movements occur five or more times during each hour of sleep, they are serious enough to be considered the medical disorder known as periodic limb movement disorder. PLMD may result in various complaints about sleep. One patient may have trouble falling asleep, while another may express a difficulty in staying asleep. Yet another patient may experience excessive daytime sleepiness. These problems occur for the same reason but involve a different timing and perception. People who are awakened by leg movements that occur immediately after they fall asleep may awaken before they even realize they have fallen asleep. When this happens, the person may complain of difficulty falling asleep. A person who reports difficulty staying asleep may instead experience leg movements throughout the night that are accompanied by microarousals, very brief awakenings that can create an overall feeling of a poor night’s sleep. This person may have only a vague complaint of restless or unrefreshing sleep. Another person may not even be aware of any nightime disturbance, but the many brief awakenings do disturb sleep and cause excessive daytime sleepiness. This person will have no complaint about sleep at night but will fall asleep while reading, watching TV, working , or driving.

*Information taken from ASDA patient info brochure.