Does idiopathic REM sleep behavior disorder (iRBD) really exist? What are the potential markers of neurodegeneration in iRBD?

REM sleep behavior disorder (RBD) may be idiopathic or associated with other neurologic disorders. A strong association between RBD and α-synucleinopathies has recently been observed, with the parasomnia often heralding the clinical onset of the neurodegenerative disease. The idiopathic form accounts for up to 60% of the cases reported in the three largest series of patients with RBD. Some clinical follow-up studies revealed that a large proportion of these patients will eventually develop a parkinsonian syndrome or a dementia of the Lewy bodies type in the years following the RBD diagnosis. The estimated 10-year risk of neurodegenerative disease for idiopathic RBD is about 40%. Moreover, it has been reported that the median interval between RBD and subsequent neurologic syndrome is 25 years. Several studies have looked at neurophysiologic and neuropsychological functions in idiopathic RBD and have found evidence of CNS dysfunction during both wakefulness and sleep in a variable proportion of these patients, challenging the concept of idiopathic RBD. Identifying subjects with a high risk of developing a neurodegenerative process may be crucial to develop early intervention strategies. Prospective studies in idiopathic RBD showed that potential markers of neurodegeneration include: (1) marked EEG slowing on spectral analysis; (2) decreased striatal 123I-FP-CIT binding and substantia nigra hyperechogenicity; (3) impaired olfactory function; (4) impaired color vision.