[Mitochondrial myopathy and leukoencephalopathy in twins of different sexes]

Two 30-year old twins, one male, the other female, were followed up for 20 years for predominantly proximal muscular deficit without increase of muscle enzymes. The lactic acid level was elevated at rest and further increased during exercise. Muscle biopsy revealed mitochondrial abnormalities. Encephalopathy was also present. The female patient had been treated, at the age of 10 years, for myoclonic attacks which regressed when she was over 18 years. None of the two patients had dementia. CT and MRI showed very extensive and symmetrical lesions of the white matter which did not involve the basal ganglia. These two cases are interesting on three scores: (1) clinically, the woman exhibited symptoms of the MELAS syndrome (without cerebral vascular accidents) and symptoms of the MERRF syndrome, which suggests the existence of borderline cases; (2) genetically, our cases were in favour of a so-called "maternal" heredity (boys are affected in all cases): here both sexes were involved but the phenotype varied; (3) biochemically, we found no enzyme activity deficit likely to explain the clinical features. The significance of a selective increase of cytochrome c oxidase in both mother and daughter is unclear.