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Background The m.3243A>G mtDNA variant is the most common pathogenic mitochondrial DNA (mtDNA) mutation and has an extremely varied clinical phenotype. The retina and optic nerve are among the most metabolically active tissues, making them particularly vulnerable to mitochondrial dysfunction. We aimed to investigate the relationship between RNFL thinning and CNS involvement in m.3243A>G related disease.Methods Patients with confirmed m.3243A>G mutation were recruited from the Newcastle Mitochondrial cohort. We performed optical coherence tomography to capture high resolution cross-sectional images of the layers of the retina and measure their thickness. We divided patients into two groups based on OCT data: those with normal RNFL and those with temporal RNFL thinning. We compared demographics, age-corrected heteroplasmy levels, clinical rating scale (NMDAS) total scores and CNS clinical involvement between the two groups.Results We found that there was no significant relationship between m.3243A>G heteroplasmy level or NMDAS scaled score and RNFL thinning. However, the group of patients with a thinned RNFL were more likely to have CNS involvement.Conclusion Our data demonstrate that RNFL thinning was associated with CNS involvement in our cohort of m.3243A>G patients and may have potential to serve as a biomarker for m.3243A>G associated CNS involvement.hatem.jouda1@nhs.net