Restless Legs Syndrome and Periodic Limb Movement Disorder

Joseph W. Anderson

Restless legs syndrome (RLS), sometimes called Willis-Ekbom disease, and periodic limb movement disorder (PLMD) are associated with each other; however, they remain as two separate clinical entities. RLS is a neurosensory motor disorder that occurs during wakefulness and may significantly impact sleep onset in the first half of the night as patients often must stretch, move, or walk to provide relief, resulting in a significant disruption to sleep quality. RLS is reported by about 10% of North American and northern European adults, with 2% to 3% having moderate-to-severe RLS (1). Periodic limb movements of sleep (PLMS) are physiologic signals that represent rhythmic movement of the lower extremity that are measured during sleep, which may or may not have clinical relevance. Periodic limb movements can also present in the upper extremities during sleep.

RLS is diagnosed through interview and appropriate questionnaires. A polysomnogram is not a requirement for diagnosis. In contrast, PLMD requires not only a patient interview confirming clinical sleep disturbances but also monitoring via a polysomnogram of specific limb movement activity. By convention, periodic electromyographic (EMG) activity recorded from bilateral anterior tibialis muscles defines PLMS. Movements of the upper limbs may also be sampled, if clinically indicated. Other techniques such as accelerometry of the toe, foot, or leg have also been used to measure movements in sleep. PLMS can be monitored in numerous muscle groups, including extensors and flexors of the toe, foot, knee, hip, finger, hand, elbow, and shoulder. The EMG of the anterior tibialis muscle provides a robust signal, and it was chosen by earlier researchers for measurement of limb movement activity, thereby becoming the standard.

The recording technologist must recognize that movements during sleep arise from multiple factors and that clinical relevance of a movement is based on the degree of arousal, which is usually defined by a 3-second or longer abrupt shift in electroencephalograph (EEG) frequency, although new research has additionally focused on cardiovascular measures of autonomic arousal from PLMS.

BRIEF HISTORY OF RLS AND PLMD

The first known medical description of RLS and PLMD was by Sir Thomas Willis in 1672 (“…arms and legs, leapings and contractions of the tendons ensued … [making the bed into] a place of the greatest torture”) (2). In 1945, Karl-Axel Ekbom (1907 to 1977) provided a detailed and comprehensive report of this condition in his doctoral thesis and a subsequent publication (3).

The first published report of periodic movements in sleep was by Symonds in 1953 (4). He used the term nocturnal myoclonus in describing the movements, but the term is best avoided because the characteristic
movement of myoclonus is different from that of PLMS. Lugaresi and colleagues (5) in Italy described the association between PLMD and RLS in 1965. Coleman (6), working at Stanford University, described the technique for recording and scoring of PLMS in 1982 that continues to be utilized (with modification) today.

Various investigators have shown that PLMS predominate during the first half of the night and tend to increase in frequency over each decade of life, with children under 10 years rarely having PLMS and many adults of age 60 or older having more than 15 limb movements per hour of sleep (7) Zucconi et al. (8) at the National Institutes of Health demonstrated that PLMS arise from regulatory motor neurons in the spine.

The reader should be aware that generalized body movements and limb movements during sleep can have multiple causes, including sleep-disordered breathing. In particular, limb movements presenting 0.5 seconds before or after increased upper airway resistance and subsequent respiratory effort-related arousals may be misinterpreted as PLMS. Conversely, PLMS, or other sleep-related movements, may cause changes in breathing patterns or create artifacts in the respiratory channels that can resemble sleep-disordered breathing. A careful examination of all relevant polysomnographic (PSG) data is essential for differentiating PLMS from other sleep-related events. Current consensus identifies PLMS as a relatively infrequent cause of insomnia or hypersomnia, the clinical symptoms that are required to arrive at a diagnosis of PLMD.

OVERVIEW OF MOVEMENTS IN SLEEP

Motor activity in sleep is common, as any arousing event may cause movement. Movement represents the primary reason for abrupt changes in any PSG signal. Isolated movements of the arms and legs typically occur 50 to 100 times during 7 to 8 hours of sleep. Axial changes of body position occur five to seven times during an average night and will often cluster in the 10 minutes before or after rapid eye movement (REM) sleep. During REM sleep, muscle twitching is common and may be recorded as brief fasciculations in the limb EMG channels.

Generalized movements in sleep may be exacerbated by a wide variety of sleep-related disorders such as physical pain or discomfort, psychological disturbances, and environmental factors. In contrast to these, PLMS exhibit a stereotypical pattern of independent limb movements that meet specific criteria as described in the Movement Rules section of the AASM (American Academy of Sleep Medicine) Manual for the Scoring of Sleep and Associated Events: Rules, Terminology and Technical Specifications, version 2.5 (9).

RLS often causes patients to shift and move as they attempt to relax into sleep. The confinement of the recording environment in the sleep center may heighten the sensation of restlessness and the urge to move the legs. Normally, patients will be able to report to the technologist that their legs are making it “difficult to settle.” Some patients show heightened anxiety as RLS symptoms increase.

It occasionally proves to be a challenge for the technologist to decide whether anxiety is the principal reason for difficulty with sleep onset or whether the restriction of movement in the RLS patient is worsening their anxiety. It is common to observe periodic limb movements of wakefulness (PLMW) appearing in a regular fashion in the anterior tibialis recording as the RLS patient falls asleep.

In some sleep centers, an average of five or more independent periodic leg movements per hour of sleep can be seen in 25% to 40% of adult patients undergoing clinical PSG. Only one-tenth to one-fifth of these PLMS lead to EEG-defined arousals of 3 or more seconds. Frequently, limb movements with arousals are secondary to the termination of breathing-related events.

DEFINITION OF RLS AND PLMS

The International Classification of Sleep Disorders, third edition lists three clinical criteria for the diagnosis of RLS (10). Testing to establish the diagnosis is not required. The patient must provide a history of an urge to move the legs that may or may not include an abnormal sensation in the affected limb(s), (1) begins or worsens at rest, is partially or completely relieved by movement, and occurs predominantly or exclusively in the evening or night; (2) the symptoms are not related to another condition; and (3) they must cause distress, sleep disruption, or functional impairment. All three criteria must be met, and differentiation from similar disorders such as leg cramps, peripheral neuropathy, and habitual foot tapping among others is important.

To assist in the recall of the primary criteria for the diagnosis of RLS, the acronym URGE is suggested:

U—urge to move

R—rest worsens the urge

G—gyration (movement) relieves the urge

E—evening or night worsening of the urge

PLMS are defined as a burst of EMG activity in the anterior tibialis muscle with a duration of at least 0.5 seconds but not longer than 10 seconds. Quantitatively, the burst should be 8 µV greater than the resting baseline EMG amplitude. Periodicity is defined when there are four or more consecutive movements that occur with intermovement intervals of between 5 and 90 seconds. The movement onset is defined as the starting point of the movement that produces an 8 µV increase above resting EMG, and the movement offset is defined as the decline of EMG to less than 2 µV of resting EMG. Leg movements occurring in both legs that are separated by less than 5 seconds between movement onsets are counted as a single leg movement (9).

Only gold members can continue reading. Log In or Register to continue