Correlates of PLMs variability over multiple nights and impact upon RLS diagnosis.

Objective: Night-to-night variability of periodic leg movements (PLMs) in restless legs syndrome (RLS) was examined to define the range of intra-subject values, impact upon diagnosing RLS, and clinical correlates.
Methods: Twenty RLS patients were monitored for 10-15 nights using a validated, tri-axial accelerometer worn on the ankle.
Results: The mean difference in PLMs index (PLMI) between the lowest and highest night was 25.1/h (range: 3.9-73.8). Inter-subject differences accounted for nearly five times the variance in PLMI relative to between nights within an individual. Based on a single night of recording, PLMI criterion thresholds of 5, 10, and 15/h were exceeded on approximately 70.1%, 51.9% and 34.1% of individual nights among these patients. Based on five randomly sampled nights of recordings, the likelihood that such thresholds were met on at least a single night increased to 91.2%, 80.8% and 62.7%, respectively. Women exhibited greater variability.
Conclusions: Variability in PLMs within RLS subjects was substantial, yet individuals' characteristic PLM level represented a quantitative trait. Variability was unrelated to age or scores on scales of RLS severity, sleepiness, functional status, and mood. A larger number of recording nights increased the likelihood that any criterion was reached.