Table 1. Major polysomnographic parameters for PLMS.
| Traditional measures | Description | Measurement notes/proposed measures | Strengths | Limitations |
|---|---|---|---|---|
| PLMI | The number of PLMS per hour of sleep time (total number of PLMS/total sleep time, PLMS/h) (16). The criteria for the diagnosis of PLMD is PLMI >15/h, in adults and PLMI >5/h in children (16). The above PLMI cut-off for the diagnosis of PLMD in adults has been questioned as arbitrary (27) | Measurement notes: AASM criteria: ≥4 consecutive LM, each lasting 0.5–5 s, with 8 μV above resting EMG, and an inter-movement interval >5 s and ≤90 s (16). WASM criteria: ≥4 consecutive LM, each lasting 0.5–10 s (−15 s for bilateral movements), with 8 µV above resting EMG, and an inter-movement interval >10 s and ≤90 s. The PLM series is stopped when a LM with an interval of <10 s intervenes (28). PSG also calculates and reports PLMW, as PLMW per hours per wake time (PLMW/h) (16) |
Easy to produce. Widely used in clinical practice and research studies. Linked to CVD (29) | High night-to-night variability (30). Fails to characterise the basic periodicity of the movements which is a cardinal feature of the condition (30). Fails to correlate with the clinical symptoms (31) |
| PLMIar | The number of PLMS that are associated with an arousal per hour of sleep | Measurement notes: an arousal and a leg movement are associated with each other when there is a maximum of 0.5 s duration between them irrespective of which one comes first (16) | Easy to produce. Linked to increased risk of CVD in patients with PLMS (32,33) | Same with PLMI. Scoring needs caution when there is comorbid OSA due to possible interaction. Please also refer to the main text (28,34) |
| PI | Ratio of the number of LM that fulfill criteria for PLMS to the total number of LM (PLMS/LM) (28). Same ratio can be generated for PLM during wakefulness, PLMW/LMW (28) | Proposed measures: it ranges from 0 (no periodicity, to 1 (all intervals within the predetermined length, i.e., >10 s to ≤90 s for the WASM and >5 s to ≤90 s for the AASM group (16,28) | PI has a significantly lower night-to-night variability compared to the PLMI (17). Data support a clinical importance with highest PI in RLS with daytime symptoms compared to controls, narcolepsy REM behavior disorder, and Parkinson’s disease (28) |
Night-to-night variability captured does not capture the intrinsic periodicity of PLMS (28,34). A minimum amount of LM overnight of approximately 10/h is required for accurate computation of PI. This limits the measurement of PI in controls for comparison reasons (17) |
| IMI | IMI is defined as the time between onsets of 2 consecutive LMs that are part of PLMS (28) | Proposed measures: time between events ranges from >10 s to ≤90 s for the WASM and >5 s to ≤90 s for the AASM group (16,28). The IMI approximates a log-normal distribution and considers putative biological process that causes periodicity in PLMS (28) | IMI night-to-night variability is significantly lower than that of the PLMI and PI. Log IMI is superior to IMI with significantly lower night-to-night variability (4–8% change) in RLS and controls (30). Log IMI is independent of the number of LM and current disease state. It is proposed as a marker to characterise disease state and sleep status along with the PLMI (30). IMI may have the potential to express intrinsic or inherent periodicity of PLMS | Limited studies available |
PLMS, periodic limb movements during sleep; PLMD, periodic limb movement disorder; LM, leg movement; PLMI, periodic limb movement index; PLMIar, periodic limb movement arousal index; PI, periodicity index; IMI, inter-movement intervals; PLMW, periodic limb movement during wake; AASM, American Academy of Sleep Medicine; WASM, World Association of Sleep Medicine; EMG, electromyogram; PSG, polysomnographic; CVD, cardiovascular disease; RLS, restless legs syndrome; REM, rapid eye movement; OSA, obstructive sleep apnea.