Late-onset hereditary myopathy with abnormal mitochondria and progressive dementia.

A family with autosomal dominant late-onset progressive dementia and myopathy is described. Electron microscopy of muscle revealed abnormal mitochondria in the proband. Thus, the disease may be classified as a "mitochondrial encephalomyopathy". The cases are unique because dementia was a dominating feature and because the symptoms developed late in life. These cases may represent a new subgroup of the mitochondrial encephalomyopathies.