Neurofilament light chain (NFL) reflects axonal damage and shows higher CSF concentrations relating to white matter lesions in conditions including multiple sclerosis and Alzheimer’s.25,26 CSF NFL is higher in people with PD with established cognitive impairment,27 and plasma NFL is increased in people with PD who later developed PD-MCI or PD dementia28,29(p). Here, NEFL is linked to Parkinson disease.