Aim: Periodic leg movements in sleep (PLMS) are generally evaluated by the number of events per hour during sleep, but this is an unstable measure with marked nightly variability and also fails to assess the basic periodicity that essentially characterizes these movements. The inter-movement interval (IMI) evaluates a putative biological process producing the period of PLMS. By contrast, the actual number of PLMS reflects the expression of this biological process that would likely be affected by multiple factors, particularly those disrupting sleep. Thus, this study tests the hypothesis that measurement of IMI duration should be more stable over nights in comparison to any measure based on counting the number of movements. IMI approximates a log-normal distribution. This study, therefore, tests the hypothesis that the means of the log IMI are more stable from night 1 (N1) to night 2 (N2) sleep recordings compared with measures of the number of PLMS. Methods: PLMS/h and IMI were measured for two consecutive nights of full sleep recordings for 29 restless legs syndrome (RLS) patients not being treated and 22 healthy controls without RLS. N1-N2 difference between nights was measured as percent of average for the nights. Results: Mean log IMI showed little nightly variability (mean ± SD for RLS: 3.6% ± 3.7, controls: 7.1% ± 7.0) significantly less p < 0.001) in comparison to PLMS/h (mean ± SD for RLS: 43.2 ± 37.1, controls: 63.7 ± 40.8). The IMI nightly variability was also significantly better than that for the periodicity index. IMI also varied considerably between individuals. Conclusion: Mean log IMI is a remarkably stable measure across nights within a subject and shows differences between subjects that may have clinical and biological significance. Because of this consistency, the mean log IMI should be considered as one standard measure of PLMS alongside the PLMS index.
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