[A case of late onset mitochondrial neuromyopathy]

A 68-year-old woman was admitted with dysesthesia on both soles, hands and buttocks. She first noticed the dysesthesia in the left first toe two years before admission. One year before admission, serum creatine kinase and lactate dehydrogenase levels were elevated but she had no clinical symptoms suggesting myopathy. Nerve conduction study showed no apparent abnormalities, and an electromyogram showed mild myogenic change. Computed tomography of the muscle showed fatty degeneration of the trunk, major gluteus muscle, and biceps femoris muscle and atrophic change of the biceps femoris muscle. Muscle biopsy revealed a slight variation in fiber size, the presence of cytochrome c oxidase (CCO)-negative fibers by CCO staining, ragged-red fibers by Gomori trichrome staining, and mild denervation fiber by neuron specific enolase (NSE) staining. Analysis of her mitochondrial DNA (mtDNA) revealed a large deletion of mtDNA (approximately 8.5 Kb), and mitochondrial neuromyophy was diagnosed. The frequency of mtDNA deletion increases with aging. Although her mtDNA abnormality was compatible with chronic progressive external ophthalmoplegia (CPEO), we speculate that the aging process may be contributed to the mtDNA abnormality, which would be related to the late onset of her symptoms. As the phenotype of this mitochondrial disorder shows a wide variation and mtDNA abnormality is more frequent among elderly people, we should consider the possibility of mitochondrial disorders even in elderly people.