Autonomic characteristics of periodic limb movements: comparison of whole-night and stage N2 linear and non-linear heart rate variability

PURPOSE: This study aimed to investigate autonomic alterations associated with periodic limb movements during sleep (PLMS) by comparing linear and non-linear heart rate variability (HRV) parameters across whole-night recordings and stage N2 non-rapid eye movement (NREM) sleep (N2). METHODS: From 8082 polysomnographic (PSG) recordings, we identified 21 patients with PLMS and 28 age- and sex-matched controls. Linear and non-linear HRV indices were analyzed for whole-night recordings and the longest N2 segment. Periodic limb movement (PLM) indices and arousal-related parameters were also evaluated. RESULTS: Compared with controls, patients with PLMS showed significantly higher standard deviation of normal-to-normal intervals (SDNN) and root mean square of successive differences (RMSSD), increased low-frequency (LF) power, and reduced approximate entropy (ApEn) and sample entropy (SampEn) across the whole night, together with a lower Stress Index (SI). No significant group differences were observed in high-frequency (HF) power or in the composite sympathetic/parasympathetic nervous system (SNS/PNS) indices. During stage N2, the PLM group exhibited significantly greater SDNN, Poincaré plot long-axis standard deviation (SD2), detrended fluctuation analysis alpha-1 exponent (DFA α1), and very-low-frequency (VLF) power, along with lower ApEn values. A significant increase in the SD2/SD1 ratio was also observed specifically during N2, whereas the LF/HF ratio showed only a non-significant upward trend. PLMS counts, indices, and arousal-related parameters were markedly elevated during stage N2. CONCLUSIONS: PLMS are characterized by increased autonomic variability (SDNN, SD2, DFA α1) but reduced complexity (ApEn, SampEn), particularly during stage N2. Stage-specific HRV assessment may provide novel insights into the cardiovascular implications of PLMS.