GFAP and optic nerve disorder: To identify differences that could point to distinct degenerative processes, we investigated clinical features, optical coherence tomography (OCT) findings, laboratory biomarkers [serum neurofilaments light chain (sNfL), serum glial fibrillary acidic protein (sGFAP) and serum growth differentiation factor-15 (sGDF15)] in a cohort of patients with these two heritable optic neuropathies in the chronic phase.