Secondary ''incidental'' REM sleep behavior disorder: do we ever think of it?

Most secondary forms of REM sleep behavior disorder are associated with neurodegenerative diseases belonging to the α-synucleinopathies or with narcolepsy. However, RBD may also occur in subacute- or acute-onset conditions involving the central nervous system, irrespective of subjects' age and sex, and with or without relapse at follow-up. These conditions include structural brain lesions (vascular, demyelinating, tumoral, iatrogenic, etc.), CNS diseases (encephalitis, Guillain-Barré syndrome, etc.), forms induced by drug consumption or alcohol withdrawal, and possibly post-traumatic stress disorder. This review focuses on these forms of RBD, which are referred to as 'acute' as they occur as incidental phenomena within the context of other subacute- or acute-onset disorders. In these cases, RBD does not appear as a 'classical' clinical feature of the underlying condition, but rather as an intercurrent, somewhat unexpected phenomenon that deserves consideration in routine clinical practice, in order to avoid misdiagnoses and mistreatments.