Optic nerve lesion length is a biomarker of visual disability in the pre-chronic phase of Leber's hereditary optic neuropathy

•A hyperintense signal may be not uncommon on T2-weighted images in Leber’s hereditary optic neuropathy (LHON).•Our results argued in favour of a higher logMAR value measurement of visual acuity when the length of T2-STIR hyperintensities increased in the pre-chronic stages of LHON.•A significant positive relationship was also suggested between median deviation and optic nerve lesion lengths in the pre-chronic stages of LHON.•The ON lesion length could not offer its value for the definition and evaluation of the therapeutic window