In this study, we report the case of a 48-year-old woman diagnosed with LHON (m.11778G>A/MT-ND4) and type 2 diabetes mellitus who experienced visual field improvement following metformin treatment after 26 months of progressive vision loss unresponsive to idebenone, nicotinamide adenine dinucleotide (NAD+), and hormone replacement therapy (HRT). The gene discussed is MT-ND4; the disease is Leber hereditary optic neuropathy.