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Dauvilliers et al. (2007) have recently measured the prevalence of periodic leg movements (PLMs) during NREM and REM sleep and assessed the functional impact in narcolepsy patients. The authors stated that only uncontrolled studies had previously reported the presence of PLMs in patients with narcolepsy. However, an earlier controlled study (age, male-to-female ratio and body mass index matched normal controls) has already reported similar findings in 47 newly diagnosed narcolepsy patients (Bahammam, 2007). We reported a prevalence of PLMs index ≥5 h−1 in 66% of narcolepsy patients and have shown that narcolepsy patients with PLMs have a higher arousal index and increased PLM arousals (during REM and NREM sleep) compared with narcolepsy patients without PLMs. Additionally, we observed a significant correlation between PLM index and arousal index, stage shifts, and stage 1 and a negative correlation with average sleep latency on MSLT. It is important for the reader to realize that studies addressing the prevalence and effects of PLMs on sleep architecture in narcoleptics are limited and have given conflicting results. Earlier studies, which defined PLMs as ≥100 leg movements per night, showed relatively lower prevalence (Wittig et al., 1983). More recent studies, which used a PLMs index of ≥5 h−1, showed higher prevalence rates (Montplaisir et al., 2000). Additionally, conflicting results have been reported on the effects of PLMs on sleep architecture in narcolepsy patients, with some researchers concluding that PLMs disturb sleep (Harsh et al., 2000) and others concluding that they do not (Montplaisir and Godbout, 1986). Using a large cohort of 530 patients, Harsh et al. (2000) found that narcolepsy patients with PLM index ≥5 h−1 have poorer sleep, with greater arousal, awakening and awake >2 min indices, enhanced stage 1 and reduced sleep efficiency (Harsh et al., 2000). However, the group with PLMs index ≥5 h−1 were significantly older than the group with PLMs index <5 h−1, which may partially account for the elevated PLMs and the disturbed sleep architecture (Harsh et al., 2000). Another study found that narcolepsy patients with PLMs were significantly older than those without PLMs (Montplaisir and Godbout, 1986). When the groups were matched for age, however, the only difference was an increase in shifts from stages 2 to 1 or waking in the PLMs group (Montplaisir and Godbout, 1986). Hence it is important to control for the age when studying the prevalence and effects of PLMs in narcolepsy patients. In our study and Dauvilliers et al.’s study, age-matched narcoleptics with and without PLMs were compared (Bahammam, 2007; Dauvilliers et al., 2007). Age-matched narcoleptics with PLMs had higher arousals than those without PLMs (Bahammam, 2007; Dauvilliers et al., 2007).

At present, the contribution of PLMs to the perceived sleep quality and daytime sleepiness in narcolepsy patients is not clear. The observation that PLMs arousals were significantly higher in the PLMs group suggests that reducing the frequency of PLMs may improve sleep architecture. The pathophysiology of narcolepsy may involve an abnormal cholinergic–dopaminergic interaction (Boutrel and Koob, 2004). Pharmacological agents that decrease dopaminergic release, such as gamma-hydroxybutyrate and neuroleptic-D2-receptor antagonists, have been shown to worsen PLMs (Montplaisir et al., 1991). Additional studies, using large numbers of individuals, are needed to assess the impact of PLMs on sleep architecture in narcoleptics, the effects of different PLMs treatments on sleep architecture and daytime vigilance in narcoleptics and the common pathophysiological mechanisms underlying both disorders.

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