MERRF is a disorder characterized by myoclonus, which is often the first symptom, followed by generalized epilepsy, ataxia, weakness, and dementia. Symptoms usually first appear in childhood after normal early development but symptoms can also first occur in adulthood. Symptoms vary widely from individual to individual and the course may be slowly progressive or rapidly downhill. Other common findings include hearing loss, short stature, optic atrophy, and cardiomyopathy with Wolff-Parkinson-White (WPW) syndrome. The diagnosis is based on clinical features and a muscle biopsy finding of ragged red fibers (RRF) . The condition is caused by mutations - that can be detected with genetic testing in 90% of affected individuals - in the mitochondrial gene called MT-TK. The seizure disorder is treated with conventional anticonvulsant therapy. Coenzyme Q10 and L-carnitine are often used in hopes of improving mitochondrial function. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.