MERRF (Orphanet_551, an Orphanet rare-disease nomenclature term) is a mitochondrial encephalomyopathy characterized clinically by a mixed seizure disorder, myoclonus, progressive ataxia, spasticity, and a mild myopathy. Dysarthria, optic atrophy, growth retardation, deafness, and dementia may also occur. This condition tends to present in childhood and to be transmitted via maternal lineage. Muscle biopsies reveal ragged-red fibers and respiratory chain enzymatic defects. (From Adams et al., Principles of Neurology, 6th ed, p986). Also known as: Fukuhara syndrome, MERRF syndrome, Myoclonus epilepsy associated with ragged-red fibers, myoclonic epilepsy- ragged red fibers.