NEFL and Parkinson disease: Consistent with a previous report, higher serum NFL levels were associated with older age in healthy controls, patients with PD and patients with ET (control: r = 0.30, 95% CI 0.05–0.52, p = 0.02; PD: r = 0.25, 95% CI 0.07–0.42, p = 0.002; ET: r = 0.30, 95% CI 0.13–0.46, p = 0.006; Pearson correlation analysis).