Our purpose was to clarify the radiological appearance of the EOMs in comparison with matched healthy individuals studied concurrently and, by correlation with clinical measures, investigate the feasibility and validity of quantitative MRI indices as functionally relevant markers of mitochondrial disease severity. We therefore conducted a peri-orbital MRI study in a genetically homogenous CPEO patient group with single deletions of mtDNA. We aimed to define in patients with CPEO the qualitative appearance of EOM on T1w and short-tau inversion recovery (STIR) MRI images, EOM spin-spin relaxation times (T2s) and cross-sectional areas, in comparison with controls, and to correlate MRI findings with clinical measures including ROEM.