GFAP and optic neuritis: In contrast, the CSF GFAP levels of most participants (27/30, 90%) in the AQP4–NMOSD group were higher than the highest measured CSF GFAP level in the OND group, and the remaining 3 participants with AQP4–NMOSD experienced relatively mild clinical symptoms (2 with optic neuritis and 1 with sensory-only partial myelitis).