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.