Clinical characteristics: MERRF (myoclonic epilepsy with ragged red fibers) is a multisystem disorder characterized by myoclonus (often the first symptom) followed by generalized epilepsy, ataxia, weakness, exercise intolerance, and dementia. Onset can occur from childhood to adulthood, occurring after normal early development. Common findings are ptosis, hearing loss, short stature, optic atrophy, cardiomyopathy, cardiac dysrhythmias such as Wolff-Parkinson-White syndrome, and peripheral neuropathy. Pigmentary retinopathy, optic neuropathy, diabetes mellitus, and lipomatosis have been observed.

Diagnosis/testing: A clinical diagnosis of MERRF can be established in a proband with the following four "canonic" features: myoclonus, generalized epilepsy, ataxia, and ragged red fibers (RRF) in the muscle biopsy. A molecular diagnosis is established in a proband with suggestive findings and a pathogenic variant in one of the genes associated with MERRF. The m.8344A>G pathogenic variant in the mitochondrial gene MT-TK is present in more than 80% of affected individuals with typical findings. Pathogenic variants in MT-TF, MT-TH, MT-TI, MT-TL1, MT-TP, MT-TS1, and MT-TS2 have also been described in a subset of individuals with MERRF.

Management: Treatment of manifestations: Ubiquinol, carnitine, alpha lipoic acid, vitamin E, vitamin B complex, and creatine may be of benefit to some individuals; traditional anticonvulsant therapy per neurologist for seizures; levetiracetam or clonazepam for myoclonus; physical therapy to improve any impaired motor function; aerobic exercise; standard pharmacologic therapy for cardiac symptoms; hearing aids or cochlear implants for hearing loss; diabetes mellitus treatment per endocrinologist.

Prevention of primary manifestations: Coenzyme Q₁₀ (50-200 mg 2-3x/day), L-carnitine (1000 mg 2-3x/day), alpha lipoic acid, vitamin E, vitamin B supplements, and creatine, often used to improve mitochondrial function, have been of modest benefit in some individuals. Doses for children should be adjusted appropriately.

Surveillance: Routine evaluations every six to 12 months initially; annual neurologic, ophthalmologic, cardiology (electrocardiogram and echocardiogram), and endocrinologic evaluations (fasting blood sugar and TSH); audiology evaluations every two to three years.

Agents/circumstances to avoid: Mitochondrial toxins (e.g., aminoglycoside antibiotics, linezolid, cigarettes, alcohol); valproic acid should be avoided in the treatment of seizures.

Pregnancy management: During pregnancy, affected or at-risk women should be monitored for diabetes mellitus and respiratory insufficiency, which may require therapeutic interventions.

Genetic counseling: MERRF is caused by pathogenic variants in mtDNA and is transmitted by maternal inheritance. The father of a proband is not at risk of having the mtDNA pathogenic variant. The mother of a proband usually has the mtDNA pathogenic variant and may or may not have symptoms. A male with a mtDNA pathogenic variant cannot transmit the pathogenic variant to any of his offspring. A female with a mtDNA pathogenic variant (whether symptomatic or asymptomatic) transmits the pathogenic variant to all of her offspring. Prenatal testing and preimplantation genetic testing for MERRF are possible if a mtDNA pathogenic variant has been detected in the mother. However, because the mutational load in embryonic and fetal tissues sampled (i.e., amniocytes and chorionic villi) may not correspond to that of all fetal tissues and because the mutational load in tissues sampled prenatally may shift in utero or after birth as a result of random mitotic segregation, prediction of the phenotype from prenatal studies is not possible.