Polysomnographic studies across a broad spectrum of alpha-synucleinopathies (including idiopathic Parkinson disease [PD], dementia with Lewy bodies [DLB], and idiopathic rapid eye movement [REM] sleep behavior disorder [RBD]) point to absence of REM sleep atonia as a common feature in these conditions.1–5 A key aspect of these observations is that RBD typically precedes the explicit diagnosis of PD or DLB, often by periods of several decades,5–7 and may represent an exceptionally early marker of synucleinopathic degeneration. Perhaps less well established is whether signs of RBD may also be prognostic of disease features once a diagnosis of Parkinsonism has been established. Disease progression in PD remains an incompletely understood phenomenon, although certain subtypes (eg, tremorous) usually are considered to herald a slower disease course than others (eg, bradykinetic/rigid).8,9 Additionally, early stage PD typically manifests asymmetrically, whereas later stage disease typically involves both body sides.8,10 In this cross-sectional study, we sought to evaluate whether, within a group of idiopathic PD patients, specific neurophysiological markers derived from overnight polysomnography (PSG) were associated with clinical indices typically associated with more advanced disease or more progressive disease course. Observations of RBD in parkinsonism have typically, but not exclusively, been derived from clinicians' judgments, based on history and occasionally supplemented by standardized questionnaires. However, measurements of phasic muscle activity recorded from surface electromyographic (EMG) electrodes have indicated large and robust effects in distinguishing the sleep of PD and idiopathic RBD patients who were not yet parkinsonian,11–15 with significantly higher rates of phasic discharges noted relative to controls. Very recently, Iranzo et al16 have shown that elevated muscle activity in sleep becomes worse in idiopathic RBD patients over a mean interval of 5 years in the absence of overt parkinsonism. In this study, we investigated the extent to which such quantified activity may be related to clinical features of PD, including symmetric (SYM) versus predominantly asymmetric (ASYM) involvement, and tremor-predominant versus akinetic-rigid PD phenotype. We hypothesized that (1) patients with bilateral involvement would demonstrate higher rates of phasic muscle activity in sleep and (2) akinetic-rigid patients would exhibit greater motor activation in sleep relative to tremor-predominant patients.
