Periodic Limb Movements During Sleep: Population Prevalence, Clinical Correlates, and Racial Differences

THERE HAS BEEN A GROWING INTEREST IN ETHNIC AND RACIAL DIVERSITY IN RELATION TO SLEEP PARAMETERS. THE STUDY OF DIFFERENCES IN SLEEP between racial groups may lead to an increased understanding of genetic and environmental contributions to sleep physiology and its disorders. Numerous studies of sleep have examined differences in sleep parameters and sleep related symptoms by gender and age, but not nearly as many have focused on racial differences. Nonetheless, several studies have demonstrated racial differences in relation to sleep variables. Differences in sleep habits1 as well as a higher prevalence of sleep disordered breathing (SDB) in African Americans compared to Caucasians2–4 has been reported. The heterogeneity of restless leg syndrome (RLS) prevalence estimates (0.1% to 13%) across countries with varying ethnic and racial distributions5–10 has led to studies investigating potential racial differences in this disorder. In one population-based survey in the United States, the prevalence of RLS was estimated at approximately 4% in both African Americans and Caucasians.11 In a study of older hemodialysis patients, Kutner and Bliwise found a lower prevalence of restless leg symptoms in African American patients12 and in a follow-up analysis reported reduced periodic limb movements during sleep (PLMS) in the African American sample.13 Thus, the influence of racial differences in RLS and/or PLMS is not well defined. Despite the wealth of data available on the prevalence of RLS among individuals of varying ethnic and racial backgrounds, few studies have examined potential racial differences in the closely related occurrence of PLMS. The term for periodic limb movements during sleep, originally called nocturnal myoclonus, was defined as a phenomenon characterized by repetitive flexions and extensions involving the toes, ankles, and occasionally the knee and hip. These frequent movements occur in one or both legs during sleep, and often produce brief arousals and, in some cases, full awakenings from sleep.14 Periodic limb movement disorder (PLMD) is defined by the International Classification of Sleep Disorders (ICSD) as periodic leg movements >15/h accompanied by a clinical sleep disturbance or a complaint of daytime fatigue.14 Although results have been mixed as to whether PLMD causes significant sleep disturbance or excessive daytime sleepiness,15–17 recent studies have demonstrated that nearly all PLMS are accompanied by autonomic arousal.18,19 In addition, evidence is emerging that PLMS is associated with increases in blood pressure.20,21 It has been demonstrated that RLS and PLMS are related to low iron stores as reflected by reduced serum ferritin levels.22–24 In fact, a recent study has demonstrated a distinct genetic sequence variant related to PLMS that also varies with serum ferritin levels, providing further evidence for the link between low iron stores and the development of PLMS.25 Given studies demonstrating higher iron stores in African Americans,26 these data provide a plausible rationale for racial differences in the prevalence of PLMS.23 There are no PSG studies examining the prevalence of PLMS in the general population and even fewer examining the prevalence of PLMS in African Americans. The reason for this discrepancy is likely due to the practical difficulty inherent in assessing large population-based samples in the laboratory; PLMS is a sign-based finding utilizing polysomnographic measures, whereas RLS is a symptom-based disorder.17 In an attempt to circumvent this limitation, one population-based survey obtained respondent reports of PLMS and estimated the prevalence of PLMD at approximately 3.9%. While no racial differences were examined in this study, PLMD was found to be associated with shift work, daily caffeine intake, and stress.27 The major limitation of this study was that PSG was not used to verify the presence of PLMS. Indeed, nearly all studies of PLMD have been carried out in self-selected clinical populations. In one of the few laboratory studies investigating potential racial differences in PLMS, O'Brien and colleagues2 found that African American children 5-7 years of age had a lower prevalence of PLMS than Caucasian children, despite absence of differences in sleep architecture. Further support for this racial difference comes from a study of PLMS in hemodialysis patients, which found a lower prevalence of PLMS in African Americans.13 It is important to determine the prevalence of PLMS in the general population, as well as to identify associated risk factors in order to determine possible pathophysiology and effective treatment.28 Selection bias inherent in clinical samples limits the ability to generalize previous findings.13,15,16,29–32 Thus, it is unclear if the racial distribution of adult PLMS varies, which is important in terms of allocation of health services and health promotion regarding PLMS. In addition, identification of a reduced or increased risk for PLMS in certain racial groups may inform future etiological hypotheses. Because of limited data available regarding the prevalence and morbidity of PSG-defined PLMS as well as a lack of population-based data on the racial distribution of this important aspect of sleep, the purpose of the present analysis was to (1) estimate the prevalence of PLMS >15 in the general population, (2) determine if there is a relationship between PLMS and race, and (3) investigate the relationship between PLMS and an objective measure of daytime sleepiness.