SPAST and Parkinson disease: As sNfL levels were 20% higher in patients compared to controls, the relative increase in SPG4 is similar to a previously published mixed HSP cohort11 and Friedreich's ataxia,20 but lower than in Alzheimer's disease (AD),21 Parkinson's disease (PD),22 and spinocerebellar ataxia type 3 (SCA3)23 as further examples of slowly progressive neurodegenerative diseases.