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Periodic limb movements in sleep (PLMS) is a disorder in which the limbs (primarily the legs) twitch or jerk every 20 to 40 seconds during sleep. This disorder was previously called nocturnal myoclonus. Each movement lasts between half a second to five seconds and can involve the big toe, ankle, knee, and sometimes the hip.

Some patients also experience jerking of the arms. Most jerking movements cause a brief awakening; as if every time you fell asleep, someone shook you just enough to wake you again. PLMS is not to be confused with hypnic jerks (sensation of falling), which only occur at sleep onset and are considered normal.

How Is a Diagnosis of Periodic Limb Movement Movements in Sleep Made?

As part of the normal clinical workup, leg movements are recorded during sleep. Sensors are placed over the tibialis muscle on the calf of each leg. The number of times you kick your legs during each hour of sleep is counted. This is called the myoclonus index. If you kick your legs at least 5 times for every hour of sleep, with each kick causing an awakening, then the diagnosis of PLMS is made.

The kicks generally occur in the lighter stages of sleep, for example, stage 1 or stage 2 sleep. Therefore people who kick often have difficulty falling asleep, since each time they begin to enter stage 1 or stage 2 sleep, their leg movements woke them up. It is less common to see kicks during REM sleep or during the deeper stages of sleep.

The number of kicks varies from night to night, so sometimes two lights of recording are necessary. The kicks can occur in one leg at a time, in both legs together, or alternate from one leg to the other. Although PLMS is a sleep disorder of its own, it is not unusual to also find it in patients who have sleep disordered breathing or narcolepsy. A periodic limb movement in sleep also has been associated with diabetes, renal disease, anemia, uremia, chronic lung disease, leukemia, and arthritis. Certain medications can make PLMS worse, such as certain tricyclic antidepressants (a class of drug used to treat depression).

What Symptoms Are Associated with Periodic Limb Movements in Sleep?

Patients with PLMS often complain of sleep onset insomnia because as soon as they relax enough to fall asleep, their legs begin to jerk, thus waking them again. Many patients are unaware that they kick but are aware of difficulty falling asleep. Patients who wake up numerous times during the night might also experience excessive daytime sleepiness.

Some patients with PLMS also complain of feeling restless at night, whereas some complain of having extremely cold feet. Individuals with PLMS are reported to sleep about an hour less per night than people without periodic limb movements in sleep.