Mitochondrial myopathies. A clinico-pathological study of cases with and without extra-ocular muscle involvement.

The clinical and pathological features of 28 patients with mitochondrial myopathy were reviewed. The cases were divided into a group with involvement of the extra-ocular muscles alone or with limb muscle involvement, and a group with a facioscapulohumeral syndrome or generalised weakness without extra-ocular muscle involvement. Cardiac and central nervous system manifestations occurred particularly in the first group which included six patients with multisystemic features and two with the complete Kearns-Sayre syndrome. Diabetes mellitus occurred in the second group only. Quantitative histology on limb muscle biopsies showed a higher proportion of fibres with abnormal mitochondrial aggregates in the second group. No one type of mitochondrial inclusion or other ultrastructural change was specific for either group of cases. The findings illustrate the clinical heterogeneity of cases of mitochondrial myopathy and the lack of specificity of any of the myopathological changes for different subgroups of patients.