Progressive external ophthalmoplegia (PEO) is a common presentation and frequently the defining clinical feature of mitochondrial respiratory-chain disease. Patients may develop additional muscular, neurological or systemic features, leading to a variety of syndromes that range from isolated chronic PEO to those with multisystem involvement such as Kearns-Sayre syndrome or mitochondrial neurogastrointestinal encephalomyopathy (Chinnery and Shoubridge, 2010) These must be distinguished from other non-mitochondrial disorders causing ptosis and ophthalmoplegia such as myasthenia gravis, oculopharyngeal muscular dystrophy, oculopharyngodistal myopathy or MYH2-related myopathy. PEO is associated with both primary and secondary mitochondrial DNA defects (Hirano and DiMauro, 2001) The former include single, large-scale mitochondrial DNA deletions, which are usually sporadic in occurrence and have a low transmission risk, and maternally-inherited mutations in mitochondrial tRNA or protein-coding genes (e.g. MT-TL1, MT-TI and MTND4) (Sweeney et al, 1993; Raffelsberger et al, 2001; Pulkes et al, 2003; Smits et al, 2007; Nesbitt et al, 2013). PEO has been reported to occur in 12% of patients with the most frequent point mutation, the m.3243A4G transition in MT-TL1 [tRNALeu(UUA/G)] (Nesbitt et al, 2013)