Periodic leg movements (PLM): their relationship to sleep stages

We investigated the characteristics of periodic leg movements (PLM) during nocturnal sleep and wakefulness in 13 drug-free patients presenting with the restless legs syndrome (RLS, n = 9) or with isolated PLM (n = 4). Eight-hour polygraphic sleep recordings included the electromyogram (EMG) of both tibialis anterior muscles. Scoring of leg movements was done according to established criteria for periodic movements in sleep, but movements occurring during episodes of wakefulness were scored as well. Twelve out of 13 patients had PLM during wakefulness, including three subjects not affected by RLS. The frequency of periodic movements in sleep (PMS) per hour of total sleep time was significantly lower than the frequency of PLM (including movements during wakefulness) per hour of polygraphic recording. Movement indices based on PMS alone underestimated the relative frequency of PLM particularly in patients with high amounts of wakefulness (> 20%). All features of PLM clearly differed between sleep stages. Relative frequency of movements, their duration and their arousing effect decreased along the nonrapid eye movement (NREM) sleep stages, whereas the intermovement interval increased. During rapid eye movement (REM) sleep the duration of movements was shortest and the intermovement interval was longest. The results presented suggest that the processes underlying PLM are most active at the transition from wakefulness to sleep and considerably attenuated during deep NREM sleep and even more during REM sleep. We suggest including movements during wakefulness in routine PLM scoring to get a more complete picture of the disturbance.