Periodic Limb Movements in Sleep

Definition and Prevalence
Periodic limb movements in sleep (PLMS) is a disorder of unknown etiology, in which patients involuntarily kick their limbs (most often it is their legs) in short, clustered episodes lasting between 0.5 and 5 s and occurring about every 20 to 40 s. The kicks are often accompanied by arousals. These episodes occur repeatedly throughout the night. The myoclonus index (MI) represents the number of kicks with arousals per hour of sleep.

The prevalence of PLMS seems to increase with age. Ancoli-Israel et al. reported that 45% of randomly selected elderly adults aged 65 years and older had PLMS, compared to 5% to 6% of the younger adult population.

Consequences of the Disorder
As with SDB, the nighttime arousals are often too short to be recalled. However, patients with PLMS may complain of insomnia, as they may have difficulty falling asleep as well as settling back to sleep following these episodes. PLMS occur most often in the first half of the night, during sleep stages 1 and 2. Sleep is fragmented, with reduced amounts of stages 3 and 4 and REM.

Clinical Presentation
In addition to complaining of difficulty falling asleep, patients may also complain of excessive daytime sleepiness, as they suffer from sleep fragmentation throughout the night. They may also note that the bedding is disorganized or jumbled when they wake up in the morning. Bed partners often complain of the leg kicks disturbing their sleep as well, and often it is important to obtain information from the bed partner in diagnosing and assessing the disorder.

Risk Factors
A related disorder that occurs during the relaxed, awake state often just before sleep onset is restless leg syndrome (RLS). Patients report unpleasant sensations in their legs and irresistible movement of the legs. The disagreeable, sometimes painful leg sensations are alleviated by rubbing or squeezing the legs or simply by walking. The prevalence of RLS is not well defined. Most patients with RLS also suffer from PLMS, suggesting that these disorders may be related. Furthermore, many patients with PLMS also suffer from other sleep disorders, including SDB and REM sleep behavior disorder.

Diagnosis and Differential Diagnosis
PLMS is diagnosed in a full night sleep recording in the sleep clinic, which includes the recording of the anterior tibialis muscles to establish the MI. The muscular jerks are often accompanied by EEG signs of arousal, which may appear following the leg jerks. As with SDB, ambulatory equipment is available to record sleep in the comfort of one's own home. PLMS is diagnosed with a MI greater than 5.