NEFL and Parkinson disease: In PD patients with more severe and bilateral disease (ie, Hoehn &Yahr score of ≥ 2; n = 48) at baseline, levels of the same CSF biomarkers increased significantly over 2 years, that is, α‐syn, t43 = −2.232, P = .031; tau, z = −2.257, P = .024; P‐tau, t35 = −2.135, P = .040; NFL, z = −3.185, P = .001; and YKL‐40, z = −3.241, P = .001.