Improved computation of the atonia index in normal controls and patients with REM sleep behavior disorder

Abstract

Objective The aim of this study was to evaluate the effects of a simple method of noise reduction before the calculation of the REM sleep atonia index (AI) on a large number of recordings from different normal controls and patient groups. Subjects and methods Eighty-nine subjects were included: 25 young controls, 10 aged controls, 31 untreated patients with idiopathic REM sleep behavior disorder (iRBD), 8 treated patients with iRBD, 10 patients with multiple system atrophy (MSA) and 5 patients with obstructive sleep apnea syndrome (OSAS). The average amplitude of the rectified submentalis muscle EMG signal was then obtained for all 1-s mini epochs of REM sleep. The new correction method was implemented by subtracting from each mini epoch the minimum value found in a moving window including the 60 mini epochs surrounding it. Results Two arbitrary thresholds were established at AI < 0.8 and 0.8 < AI < 0.9; all young controls presented AI > 0.9; this was not true for aged controls, 3 of whom presented 0.8 < AI < 0.9 but none had AI < 0.8; on the contrary 74.4% of all iRBD showed AI < 0.9, with 38.5% of the whole group having AI < 0.8 and only 25.6% with AI > 0.9. All MSA patients showed AI < 0.8. Conclusions After the introduction of this new method for noise reduction, REM sleep AI index values lower than 0.8 were strongly indicative of altered (reduced) chin EMG atonia during REM sleep; values of AI between 0.8 and 0.9 indicated a less evident involvement of atonia, and values above 0.9 characterized the majority of normal recordings.