MERRF syndrome, also known as Myoclonus Epilepsy with Ragged-Red Fibers is a very rare multisystem disorder, part of the mitochondrial encephalomyopathies cluster, due to mitochondrial abnormalities (mutations in mitochondrial deoxyribonucleic acid) that mainly affects the muscular and nervous systems and becomes symptomatic during childhood or adolescence, affecting patients in an unequal manner, even if related.
Myoclonus epilepsy with ragged-red fiber (MERRF) patients present with symptoms related to the central and peripheral nervous system, ocular and cardiovascular apparatus and subcutaneous tissues. The main trait of this disease is myoclonus, consisting of involuntary, brief, sudden spasms affecting extremities. Other peripheral nervous system involvement signs include muscle weakness, as an expression of myopathy, progressive muscular spasticity, peripheral neuropathy, optic nerve atrophy or progressive external ophthalmoplegia and hearing loss. Central nervous system involvement causes seizures, ataxia, depression  and dementia, as well as the presence of pyramidal signs on evaluation.
The cardiovascular system is involved with patients developing a form of dilative cardiomyopathy after a period of normal development . Affected individuals also have heart rhythm abnormalities, a type of preexcitation syndrome known as Wolff-Parkinson-White syndrome, premature ventricular contractions, incomplete left bundle branch block and other, unrelated findings such as a short stature, lipomas, abdominal pain, recurrent vomiting, fatigue, exercise intolerance, dyspnea (caused by lactic acid accumulation) and pigmentary retinopathy. Cardiac involvement may progress to subclinical left ventricular dysfunction or overt heart failure. Cases of stroke   or stroke-like episodes  have also been described. This condition may also be associated with other diseases, such as atypical Charcot-Marie-Tooth and Leigh syndrome, diabetes mellitus and migraine headaches .
The diagnosis of MERRF is indicated by clinical examination and confirmed by molecular genetic testing and microscopic examination of the muscle biopsy specimen with modified Gomori trichrome or succinate dehydrogenase staining. This last method will show the presence of cytochrome c oxidase negative muscle fibers and ragged red fibers, while genetic tests will detect mitochondrial deoxyribonucleic acid mutations in white blood cells, urine, saliva, hair follicles, skin or muscle.
Blood workup reveals increased lactate and pyruvate concentrations in the blood and cerebrospinal fluid , which also has high protein concentration . Creatine kinase levels may be slightly increased , but this is not a specific disease characteristic.
Brain imaging techniques like magnetic resonance imaging are able to diagnose stroke, brainstem atrophy , cerebellar atrophy  and basal ganglia calcification , while magnetic resonance spectroscopy detects brain lactate levels.
The electrocardiogram highlights rhythm and conduction abnormalities, if present. The electroencephalogram usually reveals generalized spike and wave discharges or focal epileptiform waves. The electromyogram and nerve conduction velocity may have either a myogenic or neurogenic aspect.
Genetic analysis may reveal MT-TL1, MT-TK, MT-TP, MT-TF or MT-TI mutations. Combined respiratory chain defects  and cytochrome c oxidase deficiency  have also been described.