REM sleep without atonia thresholds are associated with reduced cardiac MIBG uptake but not with DAT-SPECT binding in probable REM sleep behavior

Background: Quantitative thresholds for REM sleep without atonia (RWA) vary across populations, and their relationships with neuroimaging markers of Lewy body disease remain incompletely understood. Objectives: To examine whether previously proposed RWA thresholds for Japanese patients are associated with cardiac sympathetic and striatal dopaminergic imaging markers. Methods: This multicenter study included 91 Japanese patients with probable RBD (isolated or PD-associated). Assessments included the RBD Questionnaire-Japanese version (RBDQ-JP), video-PSG, cardiac I-123-MIBG scintigraphy, and Dopamine transporter (DAT) imaging (I-123-FP-CIT-SPECT). RWA was quantified using three methods: (1) the Sleep Innsbruck Barcelona (SINBAR) criteria, combining any submental activity and phasic activity in the flexor digitorum superficialis (FDS); (2) the AASM tonic criteria, assessing submental tonic activity; and (3) the AASM phasic criteria, assessing FDS phasic activity. Results: 97.8% (45/46) of patients meeting either the SINBAR >= 11.3% or AASM phasic >= 9.4% RWA threshold exhibited reduced cardiac MIBG uptake (H/M ratio <2.2), although the SINBAR threshold was substantially lower than the Western standard of 27.2%. No significant correlation existed between the specific binding ratio (SBR) and either %RWA or MDS-UPDRS-III scores. Notably, RWA and RBDQ-JP scores were only weakly correlated (r(s) = 0.14-0.37); among patients exceeding the RWA threshold, 19.7% scored below the RBDQ-JP cutoff of 19.5. Conclusions: The Japanese SINBAR and AASM phasic RWA thresholds are strongly associated with reduced cardiac MIBG uptake, but not with striatal DAT binding. These findings support an association between elevated RWA and cardiac sympathetic denervation, while suggesting that optimal thresholds vary across populations.