SPAST and primary progressive multiple sclerosis: To further assess the temporal dynamics of sNfL and sGFAP, we performed a pair-matching procedure of SPG4 and PPMS patients as detailed in the Section 4.4 and calculated the ratio (biomarker levelPPMS patient: biomarker levelSPG4 patient, e.g., 20 pg/mL: 10 pg/mL = 2) of sNfL and sGFAP levels for each matched pair.