NEFL and Parkinson disease: The prospective assessment and validation of PD-CI and a deeper understanding of the interaction of multiple pathogenic factors will be achieved by modern fluid biomarkers (reduced αSyn and Aβ-42 levels, increased p-tau 181, glial fibrillary acidic protein/GFAP/and NfL) as well as in post-mortem brain tissue [452,453,454,455,456,457,458] that will also enable a differentiation between PD-CI and AD.