NEFL and Parkinson disease: Further multivariate regression analysis showed that serum NFL was an independent contributor to motor symptom and cognition decline severity in PD patients (H-Y stage: t = 8.75, VIF = 2.93, p < 0.001; UPDRS III score: t = 3.17, VIF = 2.92, P = 0.002; MMSE score: t = − 4.81, VIF = 1.73, p < 0.001), while adjusting for potentially confounding demographic and clinical variables, including age, sex, education, BMI, age of onset, and disease duration.