NEFL and primary progressive multiple sclerosis: Moreover, when considering our previous work on CSF-NFL, PPMS patients presented significantly lower levels not only compared to ALS patients but also compared to patients with a slow neurodegenerative disease like frontotemporal dementia (FTD) (21), emphasizing the differences between primary neurodegeneration (slow: FTD, rapid: ALS) and inflammatory driven neuro-axonal injury (PPMS).