Quantification of muscle tone in patients with REM sleep behavior disorder

Introduction: REM sleep behaviour disorder (RBD) is characterised by excessive tone of the chin muscle and limb movement during sleep. In the past quantification of increased muscle tone in REM sleep has been performed visually using no stringent criteria. The aim of this study was to develop an automatic analysis allowing the quantification of muscle activity and its amplitude for all sleep stages, with a focus on REM sleep in patients with RBD.

Methods: 48 patients (27 male, 21 female) with RBD were included in the analysis. 21 had idiopathic RBD, 28 had narcolepsy plus RBD. 25 patients without confirmed sleep disorder served as controls. The amplitude of the EMG was generated from the difference of the upper and lower envelope of the mentalis muscle recordings. By smoothing the amplitude curve a threshold curve was defined. Any muscle activity beyond the threshold curve was defined as motor activity. The means of the motor activity per second were summarised statistically and calculated for each sleep stage. Due to variable distribution of REM sleep the latter was assigned to respective quartiles of the recorded night. Muscle activity was defined according to a histogram as short (<0.5s) and long (>0.5s) lasting activity.

Results: No difference in the distribution of REM sleep/quartile and mean muscle tone throughout the sleep cycle could be found within the RBD groups and controls. Muscle activity was in the range of 200ms. No clusters or regular distribution of muscle activity were found. Long muscle activity in the group with manifest clinical RBD was significantly higher than in controls, whereas it was nonsignificantly higher in subclinical RBD. The correlation between the frequency of long muscle activity in REM sleep and age was highly significant only for patients with idiopathic RBD.

Discussion: Automatic analysis of muscle activity in sleep is a reliable, easy method which may easily be used in the evaluation for REM sleep behaviour disorder creating indices of muscle activity similar to the indices for sleep apnea or PLMS. Together with the overt behaviour the analyses provides an important tool to get a deeper insight into the pathophysiology of RBD. Long movements seem to represent the motor disinhibition in REM sleep more distinct than short movements. The positive correlation of age and increased motor activity in REM sleep in idiopathic RBD highlights the idea of age dependant motor disinhibition as a continuum of a neurodegenerative disorder, which in narcolepsy patients with RBD only seems to happen as a single temporal event at onset of the disorder.