Rethinking the lexicon of REM sleep behavior disorder: a two-axis proposal.

REM sleep behavior disorder (RBD) is a parasomnia occurring during REM sleep, with different etiologies. The current international lexicon identifies a clinically oriented definition, which is 'clinically isolated RBD', when a clear causative disorder is not identified, and a pathophysiology-based definition, termed 'secondary RBD', when a known cause of RBD is evident. However, these two terms reflect different aspects of the disease and may be misleading and confusing, especially when defining disorders biologically, e.g., with biomarkers. This viewpoint proposes a new RBD lexicon based on evidence from the literature. The proposal envisions a two-axis framework integrating a pathophysiological and a clinical axis to classify polysomnography-confirmed RBD more coherently than the current, possibly misleading terminology. We suggest that this framework may improve patient description in both clinical and research settings, thereby enhancing patient characterization and clinical transparency.