Intervening Leg Movements Disrupt PLMS Sequences.
Periodic leg movements during sleep (PLMS) occur within a subject as a series with a remarkably stable period defined by the intermovement interval (IMI). Sometimes a non-PLMS movement occurs intervening between two PLMS. PLMS scoring rules totally ignore these intervening leg movements (iLM). This implicitly assumes an iLM results from a process sufficiently independent from the periodic process producing PLMS that it does not affect the periodicity of the surrounding PLMS. This study for the first time tests this basic assumption and explores characteristics of iLM as a potentially significant class of leg movements during sleep. Leg movements were analyzed from two nights of polysomnography recordings from 27 RLS patients and 22 controls using the validated MATPLM1.1 program. All periods (IMI) between PLMS containing an iLM were compared to the local PLMS period defined as the immediately preceding PLMS IMI using pairwise two-sided Wilcoxon sign-rank tests. Similarly, iLM were tested to see if they started a new PLMS series by having the same period as the subsequent PLMS. The periods (IMIs) containing iLM were longer than the previous periods in RLS subjects, but not controls (p < .05). The periods beginning with the iLM were shorter than the subsequent periods in both RLS and controls (p < .05). iLM as a separate type of LM distort PLMS periodicity and do not restart PLMS series. iLM end PLMS series.