Actigraph Assessment of Periodic Leg Movements and Restless Legs Syndrome

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Chapter 19 Actigraph Assessment of Periodic Leg Movements and Restless Legs Syndrome

As noted in Chapter 17 on periodic leg movements in sleep (PLMS) and waking (PLMW) the interpretation of the significance of these movements has changed since they were first reported by Symonds.¹ An examination of the electromyelograph (EMG) activation (lasting 0.5 to 10 seconds) seen with these movements has dispelled the initial concept that they were a form of myoclonus, which has a much briefer muscle activation. However, the supposed myoclonic nature of these movements led to the use of clonazepam as a treatment.² The second clinical interpretation of PLMS focused on the conspicuous arousals seen with these movements; it was postulated that they caused significant sleep disruption with consequent daytime sleepiness.³ Evaluations of those patients with PLMS who are without any other significant sleep or neurologic disorder, however, have failed to find any relation between periodic limb movements (PLM) and either insomnia or daytime sleepiness.⁴^–⁸ One of the early studies even showed an unexpected relation between increased PLMS and decreased inadvertent napping in the daytime.⁹ This dilemma of no apparent relation between these events conspicuously disturbing sleep and sleep-related complaints, however, appears to reflect the multiple causes of PLMS that were not well evaluated in prior studies. An examination of the multiple conditions associated with PLMS showed that many of these involve dopaminergic abnormalities, such as rapid eye movement (REM) behavior disorder,¹⁰ narcolepsy,¹¹ Parkinson’s disease,¹² and restless legs syndrome (RLS).¹³ Thus, PLMS can be considered a motor expression of the dopaminergic abnormalities associated with these disorders.⁴ In this view, the frequency and perhaps intensity of the PLMS would reflect the severity of this aspect of the underlying disease process. The effects of the severity of the underlying disease may produce significant sleep disruption for multiple reasons with PLMS only a minor component of this process. Thus, in most of these diseases, there is no established relation between subjective report of poor sleep and the frequency of PLMS. The one exception is RLS where at least on the first night of recording one study reported a significant correlation between subjective report of PLMS/hr and sleep complaint⁶ and two independent studies have reported a correlation between subjective report of disease severity and the rate of PLMS.^14,¹⁵ Because there has been no reported “first-night” effect with increased rate of PLMS.¹¹ this relation with RLS would be expected to continue for all nights recorded, but the extreme variability of PLMS per hour in RLS patients makes it likely that some single nights might not show this relationship (e.g., no relation to sleep compliant was found on the second PSG night in one study⁶). This emphasizes the importance of having multiple nights of recording for RLS patients. Overall, there appears to be a relation between disease severity and PLMS for RLS not reported in the other disorders with increased rates of PLMS.

PLM can, therefore, provide an objective measure of disease severity that is particularly important for RLS. Because PLM usually involve an actual movement of the leg, it becomes possible to use piezoelectric accelerometers in battery-operated ambulatory monitors to record these movements over several nights. The technology to do this has been developed and is presented here in the full context of PLM measurement, particularly for RLS. In addition, the leg activity meters provide a gross assessment of day and night activity, and the relative ratio of activity in these different time periods may also prove useful for RLS evaluation. First, we consider the PLM measurement issues. These determine to a large extent the requirements for leg activity monitoring. Then, we consider the application of these monitors for evaluating RLS and the potential for future developments using this technology.

Problems With Periodic Leg Movement Measurement on a Polysomnogram

The series of leg movements making up a PLM are usually measured by the electrical activity of the anterior tibialis EMG. Electrodes placed on the skin at the midline of this muscle provide the basic measurement of this physiologic event. Each leg is recorded separately. The new World Association of Sleep Medicine (WASM) standards for measuring and scoring these movements ¹⁶ provide the basic standardization for the physiologic recordings and scoring and officially recognize leg activity monitoring as an alternative method for measuring leg movements in sleep. The motivation for even considering activity monitoring to evaluate PLM stems largely from the following five significant problems that occur when these movements are limited to the EMG measurements in a sleep laboratory polysomnogram (PSG).

First, the surface EMG used is not a calibrated signal and probably varies with placement and even impedance changes during the night’s recording. This largely invalidates quantitative measurement of the amplitude of the EMG event. The current standards fail to provide any measure of magnitude of the PLM movement aside from its duration.

Second, the night-to-night variability is very large in many cases; in particular, this is true of PLM associated with RLS, a disease process that itself appears to have marked daily variation in severity.^11,¹⁷ This variability increases sample sizes required for studies involving only 1 or 2 nights of PSG measurement across many subjects, but it becomes particularly significant when addressing the disease state of a given individual. Multiple nights, probably 3 to 5, are need to find an estimate of the range and variation in PLM for a given individual.¹⁸

Third, the EMG usually involves only the anterior tibialis leg muscle. As noted in the chapter on periodic limb movements (Chapter 17), PLM events involve activation of several other muscles producing somewhat mixed and complicated leg movements (e.g., as reported by Provini and colleagues ¹⁹). EMG of only one muscle, albeit probably the most significant one, fails to capture the full duration and the magnitude of the actual movement event. Thus, the standard PSG procedure probably somewhat underreports the actual movement event.

Fourth, the artifacts of arousals at the end of sleep-disordered breathing (SDB) events produce EMG signals virtually identical to those defined as PLM. Moreover, the periodic nature of SDB overlaps that for PLM, and thus these artifactual leg movements may meet the PLM criteria. Separating SDB artifacts from primary PLM requires either ensuring that the patient has little or no significant SDB or simultaneous recording of the breathing with the PLM. Those PLM occurring at or very near to the termination of the SDB event should not be counted as PLM. They represent artifact.

Fifth, the recordings require the expense and inconvenience of the subject going to a sleep laboratory or, for ambulatory recordings, arranging for someone to put on the electrodes for the EMG recording. This creates an expensive process with significant barriers to access.