NEFL and Parkinson disease: NfL concentration is also, for instance, increased by ischemic small vessel disease43 and Alzheimer disease.44 Comorbid brain pathology such as ischemic small vessel disease or Alzheimer disease pathology could contribute to increased mortality in PD and DTI findings and may relate to balance and gait impairment in PD.45,46 However, the finding that increased cNfL is associated with lower striatal 123I-FP-CIT uptake, higher scores in validated PD severity scales, and hyposmia indicates that the increase of cNfL is at least partially specific for PD neurodegeneration.