Maternally inherited mitochondrial myopathy and myoclonic epilepsy

A family is described with familial myoclonic epilepsy associated with mitochondrial myopathy. The disorder follows a maternal inheritance pattern consistent with a mitochondrial DNA (mtDNA) mutation. The large kindred permitted exclusion of autosomal dominant, recessive, and X‐linked patterns of transmission. Several characteristics of the inheriatnce and variability of expression within the pedigree are consistent with recently acquired knowledge about the genetics of human mtDNA. The clilnical spectrum of disease is compatible with a proportionality model of mutant and wild‐type mt DNAs. Muscle biopsies of affected patients showed an increased number of abnormal muscle mitochondria. Serum levels of pyruvate or and lactate were elevated. The most severely affected pateint had constant myoclonic jerking, dementia, ataxia, spasticity, hearing loss, and hopoventilation. Cerebral dysfuntion in patients with mild involvement waas marked by prominent photic driving seen on electroencephalograms and high‐amplitude visual and somatosensory evoked responses but no myoclonus, ataxia, or dementia. The individual clinical features of the disease worsen over time for all patients; however, mildly affected patients have not become moderately afftected and moderately affected patients have not become severely affected.