SNCA and Parkinson disease: As for the biological factors, some studies indicated that alterations in amyloid-β1–42 (Aβ42) or total α-synuclein levels in CSF were related to CI in patients with PD (Aarsland et al., 2021; Lin and Wu, 2015; Schrag et al., 2017), whereas other study found no associations (Chen et al., 2023).