Assessment of Periodic Leg Movements Is Not an Essential Component of an Overnight Sleep Study

Mark W. Mahowald

Minnesota Regional Sleep Disorders Center, Hennepin County Medical Center, Minneapolis, MNDepartment of Neurology, University of Minnesota Medical School, Minneapolis, Minnesota

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  The emperor marched in the procession under the beautiful canopy, and all who saw him in the street and out of the windows exclaimed: "Indeed, the emperor's new suit is incomparable! What a long train he has! How well it fits him!" Nobody wished to let others know he saw nothing, for then he would have been unfit for his office or too stupid. Never emperor's clothes were more admired.

  "But he has nothing on at all," said a little child at last. "Good heavens! listen to the voice of an innocent child," said the father, and one whispered to the other what the child had said. "But he has nothing on at all," cried at last the whole people.

Hans Christian Andersen, The Emperor's New Clothes

Periodic leg movements in sleep (PLMs) are rhythmical extensions of the great toe and dorsiflexion of the ankle, knee, and hip (and probably represent spontaneously occurring Babinski signs or triple spinal flexion reflexesnormal phenomena during the lighter stages of non-REM [NREM] sleep due to enhanced spinal cord excitability) lasting 2-4 s with a frequency of one every 20-40 s (1). Arbitrarily, and without documented clinical correlation, a PLM index > 5/h is said to be "abnormal." For perspective on "abnormality," PLMs are present in 5-6% of all adults, and in 30-86% of adults aged 60 yr or older.

PLMs have been claimed to cause either insomnia or excessive daytime sleepiness; but if they are detected in asymptomatic individuals, they are ignored. Is it our good fortune to have it all three ways (causing either insomnia, or hypersomnia, or being asymptomatic)taking or leaving PLMs depending on the clinical symptoms (or lack thereof)?

Notably, PLMs are simply and only a polysomnographic observation, and are virtually never (possibly excepting patients with the restless legs syndrome) appreciated by the patient. On the other hand, PLMs may be a source of complaint to the patient's bedpartner(s). The primary source of confusion has arisen from the fact that 80% of patients with restless legs syndrome (a clinical complaint) will exhibit PLMs (a polysomnographic observation), but the converse is not true. Even in patients with restless legs syndrome with PLMs, the restless legs syndrome symptoms do not correlate with the frequency of PLMs (2). As a corollary, in patients with narcolepsy, the reduction of PLMs by bromocriptine does not improve the nocturnal sleep disruption in that disorder (3).

In support of the lack of a PLM disorder are a myriad of studies indicating virtually no predictable correlation with symptoms of insomnia or excessive daytime sleepiness as determined by sleep history, depression scales, objective or subjective sleep variables; and PLMs are not manifested in self-reported sleep-wake or mood disturbance (4, 5). Notably, there is no correlation between PLMs and excessive daytime sleepiness (2, 6). In patients with idiopathic central nervous system hypersomnia, there is no correlation between PLMS index and poor sleep efficiency or daytime sleepiness as measured by the multiple sleep latency test (7). In patients with insomnia, it has been concluded that because of the lack of differences in both the macro- and microstructure of sleep and the electroencephalogram (EEG) activity content regarding the association with movements, PLMs do not primarily cause sleep disturbance (8).

Mendelson demonstrated that there is no correlation between PLM arousal index and subjective complaint of disturbed sleep, objective measure of daytime sleepiness, or a sense of awakening refreshed in the morning (9). Because PLMs are not more prevalent in patients with insomnia or hypersomnia than in control subjects, Montplaisir and coworkers have concluded that ". . . the validity of PLM disorder as a distinct nosological entity is highly questionable and in our experience, a diagnosis of PLM disorder has no specific utility" (10).

Some believe that PLMs are significant if they are associated with arousals. (The common misperception that the leg movements cause the arousal is clearly not true; in one study, 49% the EEG arousals occurred before the leg movement, 31% occurred simultaneously with the leg movement, and in only 23% did the leg movement precede the EEG arousal [11].)

This raises two critical unanswered questions: (1) What exactly constitutes an arousal? and at what level of the nervous system (autonomic activation without EEG arousal, EEG arousal [as defined by which or whose criterion?], or full behavioral arousal)? and (2) at what frequency or index (number per hour of sleep) do these arousals (however defined) have clinical consequences such as insomnia or hypersomnia? What exactly determines either a clinically significant arousal or arousal index has never been scientifically established. The American Academy of Sleep Medicine arousal criteria are arbitrary and capriciouswith no validation as to whether these consensus-defined arousals, or at what frequency, result in subjective or objective sleep complaints (12). There is growing evidence that arousals measured by respiratory paradox or autonomic measures (increases in blood pressure or heart rate) without EEG evidence of arousal (by any definition) may be significant (13). It is most likely that PLMs are simply another manifestation of the cyclic alternating pattern, a microstructural sleep pattern of normal arousal instability during NREM sleep (20).

The similar prevalence of PLMs in various sleep disorders and the asymptomatic general population speaks against their diagnostic or functional significance. We cannot have it all three ways, and until rigorous scientific studies clothe him, the PLM disorder emperor will remain naked. And, lacking such a rigorous study, PLMs should not be monitored, lest one be tempted to use this currently meaningless information to obfuscate or sabotage the true diagnosis. Once a diagnosis of PLM disorder is made, clinical thinking ceases, and patients are exposed to gratuitous medication. To borrow a phrase from Neil Douglas in describing the upper airway resistance syndrome, PLM disorder is a "dustbin term for the diagnostically destitute" (21). Our field, with close collaboration between clinicians and basic scientists, is up to the challenge of confirming or refuting the true (and unlikely) significance of PLMs.

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