A Portable Monitor for the Measurement of Periodic Limb Movements in Restless Legs Syndrome: Validity and Reliability
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Purpose: A key objective feature in Restless Legs Syndrome (RLS) is the presence of Periodic Limb Movements (PLMs). The gold standard for documenting PLMs is through polysomnogram (PSG), an overnight sleep study in a sleep laboratory, which is expensive and often inaccessible. This work explored the ability of a portable sleep monitor (PM) to reliably record surface EMG signals, to reliably record limb movements overnight in comparison with PSG, and examined intra- and inter-rater reliability for scoring the PM recordings. Methods: The PM’s surface EMG channel was tested against a standard EMG amplifier by recording bilateral tibialis anterior muscle activity in five healthy participants. It was also tested for recording PLMs simultaneously with polysomnography at Kingston General Hospital with 40 participants referred for screening of sleep disorders. PLMs were scored using standard criteria according to the American Academy of Sleep Medicine Scoring Manual (2007) Analyses: Comparison between the two methods of surface EMG recording was through counts of muscle activity bursts. Comparison of overnight PLM counts was through t-test, Pearson’s r, Intraclass Correlation Coefficient (ICC) and Bland-Altman plots. Intra-rater reliability and inter-rater reliability between two analysts was examined by ICC. Results: Examination of the PM surface EMG recordings demonstrated an exact match of muscle activity counts between the PM and standard EMG recordings. In the PSG study, mean difference between the two PLMI values was +4.8 ± 11.1, t (34) = 2.1, p = 0.04, which was statistically significant and demonstrated systematic over-reporting by the portable monitor. The two PLMI values were strongly correlated, giving a Pearson’s r = .87, p < 0.001. ICC for absolute agreement was 0.87, (95% CI, 0.76 – 0.93), p < 0.001. Bland-Altman analysis gave 95% limits of agreement between the two PLM Indices as +27.9 (95% CI +33.0 to 20.2) to – 19.3 (95%CI -10.6 to -23.4). Conclusions: These data suggest there may be sufficient agreement between PLMI collected by PM and polysomnography to support the use of the PM for measuring PLMs. Further testing should address test retest reliability and examine the performance of the PM in a wider patient population.