Tonic-dominant sleep bruxism in multiple system atrophy: A polysomnographic comparison with Parkinsonian-related disorders.

Sleep bruxism (SB), classified into phasic, tonic, and mixed electromyographic (EMG) patterns, is increasingly recognized as a heterogeneous phenomenon in neurodegenerative disorders. However, its specific characteristics in multiple system atrophy (MSA) relative to Parkinsonian-related disorders (PRD) remain unclear. We retrospectively analyzed overnight polysomnography (PSG) with bilateral masseter EMG in patients with MSA (n = 8) and PRD (n = 7). SB episodes were classified as phasic, tonic, or mixed according to standard criteria. Sleep architecture and bruxism parameters were assessed, and the proportion of tonic activity (tonic rate) was calculated. Patients with MSA demonstrated a predominance of tonic and mixed bruxism patterns. The median number of tonic bruxism events was significantly higher in MSA than in PRD (35 [25-37] vs. 8 [2-24] events; p = 0.015, r = 0.63). In contrast, PRD showed greater interindividual variability with relatively more phasic activity. REM sleep-associated bruxism episodes were more frequent in MSA and were predominantly accompanied by tonic or mixed activity. Although not reaching statistical significance, the tonic rate tended to be higher in MSA than in PRD (22.8% [12.1-27.6] vs. 9.5% [3.1-13.5]; p = 0.064). SB in MSA is characterized by tonic- and mixed-dominant EMG activity and significantly increased tonic bruxism events, suggesting sustained jaw muscle activation during sleep. PSG-based characterization of sleep-related jaw-muscle activity may provide additional insight into disease-specific motor dysregulation during sleep.