The influence of different definition criteria on the PLM index

Different criteria for the scoring of periodic leg movements (PLM) have been recently proposed. We investigated to what extent changes in PLM criteria for leg movement duration, intermovement interval and combination of bilateral leg movements (LM) influence the PLM index.

The nocturnal polysomnographies of 40 consecutive patients (20 males, 20 females, mean age 52+/-16 years) with sleep-wake complaints but without severe sleep-related breathing disorders (AHI<20) were evaluated. All patients showed a minimum of 100 LMs during the night. For each night eight PLM indices during sleep (PLMS) and eight PLM indices during wakefulness (PLMW) were computed by systematically varying the following criteria: LM duration (0.5-5s vs. 0.5-10s), intermovement interval (5-90s vs. 10-90s), and separation criteria for LMs occurring in both legs (<5s onset to onset vs. <0.5s offset to onset). Data were analyzed using linear mixed models.

The two different intermovement intervals and the leg movement durations both had a statistically significant influence on the PLMS and PLMN indices. These variations were highly systematic but numerically small for the PLMS index while they were substantially larger for the PLMW index. The separation criteria or possible two-way interactions between the criteria had no influence on the PLM indices.

Different criteria had a negligible influence on the PLM index during sleep. Across-study or sleep-laboratory comparability can be assumed within our parameter set. This does not apply to the PLM index during wakefulness.