While the pre-planned primary CSF substudy analysis was negative, showing no difference in CSF NfL and NfH between any treatment arms,22 this current exploratory analysis aims to look at CSF NfH measured with a contemporary immunoassay,23,24 as a predictor of disability, brain atrophy and MRI disease activity in a cohort of people with SPMS. Here, NEFH is linked to Brain atrophy.