MT-ND4 and optic atrophy: Although sNfL and sGFAP are not specific to optic nerve atrophy or visual pathway degeneration and may indicate neurodegeneration in other areas of the central nervous system (a possibility encompassed in the well-described ADOA+ and LHON+ phenotypes), this is unlikely in our patient cohort, who were asymptomatic for non-visual neurological symptoms and had normal brain MRI findings apart from optic atrophy.