NEFL and Parkinson disease: In the PD patients with long disease duration ( > 5 years; n = 26), there were significantly increased levels of CSF biomarkers in α‐syn (z = −2.192, P = .028), tau (z = −2.437, P = .015), NFL (z = −2.210, P = .027), and YKL‐40 (z = −2.222, P = .026) after 2 years (Figure 2 and Supplementary Figure S2).