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SLEEP DISORDERS CENTER
Periodic Limb Movement
Disorder
Periodic limb movement disorder (PLMD), 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 limbs. The movements of PLMD occur most often when a person
is asleep and are involuntary (that is, not consciously controlled). People
with periodic limb movements are often not even aware of these movements,
although on rare occasions they may notice the involuntary movements 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 implies, 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 appears to also prevent periodic limb movements.
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 express difficulty falling
asleep, while another may have trouble staying asleep. Yet another patient
may experience excessive daytime sleepiness. These problems occur for the
same reason but involve a difference of 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 nighttime 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.
Periodic limb movements may cause several other
problems, some of which affect the bed partner and some of which affect the
patient. A bed partner may complain that he or she is being kicked at night
or that the bed covers are twisted or knocked off the bed. A patient may
complain that the frequent leg movements are wearing the hair off the
outside of the legs.
Periodic limb movements are not the same as hypnic
jerks (night muscle spasms), which are the occasional jerks of the body that
usually occur just as a person is falling asleep. These jerks are normal
and do not cause a problem with nighttime sleep or daytime alertness.
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