OPA1 and hereditary optic atrophy: Muscle biopsies from individuals diagnosed with OPA1-associated isolated or syndromic optic atrophy revealed mitochondrial myopathic features, including multiple deletions in the mtDNA, COX-deficient muscle fibers (approximately 5% and 1.3%, respectively) and numerous ragged red fibers (subsarcolemmal accumulation of abnormal mitochondria), even in the absence of a clinical myopathy.47