SNCA and REM sleep behavior disorder: Subsequent spread of pathology, including to the cortex or autonomic nervous system, is associated with cognitive impairment or autonomic dysfunction, respectively.7 Cell death and neuroinflammation occur in regions with a higher pathological burden.1 Given the established genetic link to SNCA (the gene encoding α-synuclein) in Parkinson’s disease, rapid eye movement sleep behaviour disorder and Lewy body dementia,8-12 there has been a suggestion to shift from a clinical to a biological definition of Parkinson’s disease (and dementia with Lewy bodies).